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ORIGIN OF BIO-POTENTIALS
Bioelectric potentials are ionic voltages produced as a result of electrochemical activity
of certain special types of cells such as nerve cell or muscle cells.
Special types of cells like nerve and muscle cells in the body are encased in semipermeable membrane that permits some substance to pass through the membrane while others
are kept out.
The cells are surrounded by fluid. The fluid contains ions such as sodium, potassium,
chloride etc. The fluid outside the cell membrane is called as Extracellular fluid (ECF) and the
fluid inside the cell membrane is called as Intracellular fluid (ICF). ICF is rich in K+, Mg++,
phosphates and ECF is rich in Na +, Cl−.
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In normal condition when the semi-permeable
membranes are in polarised state, Sodium (Na+) ions
will be outside the membrane. Since the size of Na+
ions is more than the size of holes in semi-permeable
membrane, they cannot enter inside whereas other
ions like potassium (K+) and Chloride (Cl−) can
enter the membranes and exhibits resting potential.
Normal Semipermeable membrane
The sodium ions can enter the membrane when the
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holes of it are increased by stimulation (excitation)
After stimulation of membrane, all sodium
ions can enter inside by its increased diameter
ofpores or holes. It constitutes depolarisation
and gives action potential.
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Excited semipermeable membrane
The quantity of ECF and ICF for different ions is given below:
ECF
ICF
Na+
110
60
K+
20
120
Cl−
100
45
Resting Potential:
Fluids surrounding the cells of the body are conducting. These conductive solutions contain atoms
known as ions. Principal ions present are: Sodium-Na+, Potassium-K+ and Chlorlde-Cl−. The membrane
of excitable cells readily permits entry of K+ and Cl-, but effectively blocks Na+ Ions. According to
concentration and electric charge, various Ions seek a balance between inside and outside of cell. Due to
inability of Na+, to penetrate the membrane results two conditions:
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(a)
The Na+ ions inside the cell become much lower than in the Extracellular fluid
outside. (Sodium ions are +ve. It tends to make outside of cell more +ve than
inside).
Or
(b)
In an attempt to balance the electric charge, additional potassium ions, which are also
+ve, enter the cell causing a higher concentration of potassium on the inside than
on the outside. But, this charge balance cannot achieve, due to imbalance
concentrate of K+ ions.
Equilibrium is reached with a potential difference across the membrane, -ve on inside
and +ve on the outside. And this membrane potential is known as resting potentialof cell.This
potential is maintained until some disturbance upsets the equilibrium. The membrane potential
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is made from inside the cell w.r.to the body fluids. Therefore, the resting potential is -ve rating
from -60mV to -100mV. The figure below shows the cross section of cell with resting
potential and the state is said to be polarized state.
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Polarized cell with resting potential
Action potential:
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Due to some external energy or by the flow of ionic current, a section of cell membrane
changes its characteristics and begins to allow some of sodium ions to enter. This movement
of sodium ions into cell constitutes an ionic current flow that further reduces the balance of
membrane to sodium ions. The net result is avalanche effect and tries to balance with ions
outside. At the same time, K+ ions, in higherconcentration inside the cell during resting state,
try to leave cell, but are unable to move as fast as Na+ ions. The result is cell attains small +ve
potential on the inside due to imbalance ofK+ ions, known as action potential. The action
potential is nearly +20 mV.
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Depolarization of Cell
Depolarized cell with Action potential
When a cell is excited and displays an action potential, it is said to be "depolarized"
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and the process of changing from resting state to action potential is called as depolarization.
Once the rush of sodium ions through the cell membrane has stopped (a new state of equilibrium
is reached), the ionic currents that lowered the barrier to sodium ions are no longer present and the
membrane reverts back to its original, selectively permeable condition. Now passage of sodium ions from
the outside to inside of the cell is again blocked.However,it would take a long time for a resting potential
to develop again.
By an active process, called asodium pump, the sodium ions are quickly transported to the outside
of the cell, and the cell again becomes polarized and assumes its resting potential. This process is called
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Repolarization.The rate of pumping is directly proportional to the sodium concentration in the cell. It is
also believed that the operation of this pump is linked with the influx of potassium into the cell, as if a
cyclic process involving an exchange of sodium for potassium existed.
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The Figure below shows a typical action-potential waveform, beginning at the resting potential,
depolarization, and returning to the resting potential after repolarization. The time scale for the action
potential depends on the type of cell producing the potential. In nerve and muscle cells, repolarization
occurs so rapidly following depolarization that the action potential appears as a spike of as little as 1msec
total duration.
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Waveform of the Action potential
Heart muscle, on the other hand, repolarizes much more slowly, with the action potential for heart
muscle usually lasting from 150 to 300msec.
Regardless of the method by which a cell is excited or the intensity of the stimulus (provided it is
sufficient to activate the cell), the action potential is always the same for any given cell. This is known as
the all-or-nothing law. The net height of the action potential is defined as the difference between the
potential of the depolarized membrane at the peak of the action potential and the resting potential.
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Following the generation of an action potential, there is a brief period of time during which the
cell cannot respond to any new stimulus. This period is called the absolute refractory period, lasts about
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1msec in nerve cells. Following the absolute refractory period, there occurs a relative refractory period,
during which another action potential can be triggered, but a much stronger stimulation is required. In
nerve cells, the relative refractory period lasts several milliseconds.
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ELECTROCARDIOGRAPH (ECG)
Electrocardiography:
This is the technique by which the electrical activities of the heart are studied.
Electrocardiograph:
This is the instrument by which the electrical activities of the heart are recorded.
Electrocardiogram:
This is the record or graphical registration of electrical activities of the heart.
ECG waves and leads
ECG Wave:
The electrocardiogram is a graphical representation of the bio-electrical currents generated by
the myocardial cells. By placing a pair of electrodes on the body, an ECG voltage potential between them
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can be measured and recorded. This graphical representation (ECG) can be either printed on a paper or
displayed on a monitor. The device capable of recording and printing the ECG on paper is called
electrocardiograph. The device which displays the ECG on a screen is called monitor or cardiac
monitor.
Einthoven was the inventor of the first device capable of printing the ECG on paper. Einthoven
named the waves using five capital letters from the alphabet: P, Q, R, S, and T. The width of a wave on
the horizontal axis represents a measure of time. The height and depth of a wave represent a measure of
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voltage.
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An upward deflection of a wave is called positive deflection and a downward deflection is called
negative deflection. A typical representation of the ECG waves is as shown in the figure below.
The description of the five ECG waveforms is as follows:
P wave
: represents the depolarization impulse across the atria
Q, R and S waves
: all these three waves represent the ventricular depolarization
(the downward stroke followed by and upward stroke is called Q wave, the upward
stroke is called R wave and any downward sroke preceded by an upward stroke is
called S wave)
T wave
: represents the repolarization of the ventricles
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Table: Waves of normal ECG
S.
No
Wave/ Segment
Cause
1
P Wave
Depolarisation of atria
2
R Wave
Repolarisation of atria and Depolarisation of
(QRS Complex)
Amplitude
Duration
(mV)
(sec)
0.25
0.12 to 0.22
1.6
0.07 to 0.1
0.1 to 0.5
0.05 to0.15
<0.1
0.2
ventricles
3
T Wave
Repolarisation of ventricles
4
U Wave
Slow repolarisation of
intraventricular(Purkinje fibers) system
The ECG Lead Configuration:
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The 12 standard ECG leads are divided in to
i)
Bipolar limb leads or Standard leads or Einthoven lead system
1. Lead I
2. Lead II
3. Lead III
ii)
Augmented unipolar limb leador Wilson Lead System
4. aVR
5. aVL
6. aVF
iii)
i)
Unipolar chest leads
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Bipolar limb leads or Standard Leads or Einthoven lead system:
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In this lead system, the potentials are tapped from four locations of our body. They are
i) Right arm
ii) Left arm
iii) Right Leg iv) Left Leg
The Right Leg (RL) electrode acts as the reference electrode.
Lead I: It is a lead obtained between a negative electrode placed on the right arm and a positive electrode
placed on the left arm.
It gives voltage VI, the voltage drop from the left arm(LA) to the right arm(RA).
Lead II: It is a lead obtained between a negative electrode placed on the right arm and a positive
electrode placed on the left foot.
It gives voltage VII, the voltage drop from the left leg (LL) to the right arm (RA).
Lead III: It is a lead obtained between a negative electrode placed on the left arm and a positive
electrode placed on the left foot.
It gives voltage VIII, the voltage drop from the left leg (LL) to the left arm (LA).
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Einthoven Triangle:
The closed path RA to LA to LL and back to RA is called as Einthoven Triangle. The vector
sum of the projections on all the three sides is equal to zero. Applying KVL, the R wave amplitude of
lead II is equal to the sum of the R wave amplitudes of Lead I and Lead III.
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The R wave nominal voltage from different lead is as given below.
Lead I
-
0.53 mv (0.07 – 1.13)mv
Lead II
-
0.71 mv (0.18 – 1.68)mv
Lead III
-
0.38 mv (0.03 – 1.31)mv
By KVL,VII = VI + VIII
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ii)
Augmented Unipolar Limb Leads:
Augmented unipolar limb lead system was introduced by Wilson. The electrocardiogram is
recorded between a single electrode and the central terminal which has a potential corresponding to the
center of the body. Thus two equal and large resistors are connected to a pair of limb electrodes and the
center of this resistive network. The remaining limb electrode acts as exploratory single electrode. By
means of augmented ECG lead connections, a small increase in the ECG voltage can be realized. The
augmented lead connections are,

augmented voltage Right arm (aVR)

augmented voltage Left arm (aVL)

augmented voltage Foot arm (aVF)
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aVR: is a lead obtained between the average signal obtained from three negative electrodes (left arm, left
leg and right foot) and the signal obtained from a positive electrode placed on the right arm
aVL: is a lead obtained between the average signal obtained from three negative electrodes (right arm,
left foot and right foot) and the signal obtained from a positive electrode placed on the left arm
aVF: is a lead obtained between the average signal obtained from three negative electrodes (left arm,
right arm and and right foot) and the signal obtained from a positive electrode placed on the left foot
Unipolar Chest Lead:
In unipolar chest leads, the exploratory electrode is obtained from one of the chest
electrodes. The chest electrodes are placed at six different points on the chest close to the heart.
By connecting 3 equal large resistors to RA, RL, LL, a central terminal is obtained. This
lead system is known as Wilson lead system.
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V1: is a lead obtained between the reference negative electrode and a positive electrode placed on the
chest in the V1 position
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V2: is a lead obtained between the reference negative electrode and a positive electrode placed on the
chest in the V2 position
V3: is a lead obtained between the reference negative electrode and a positive electrode placed on the
chest in the V3 position
V4: is a lead obtained between the reference negative electrode and a positive electrode placed on the
chest in the V4 position
V5: is a lead obtained between the reference negative electrode and a positive electrode placed on the
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chest in the V5 position
V6: is a lead obtained between the reference negative electrode and a positive electrode placed on the
chest in the V6 position.
ECG Recorder
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The connecting wires for the patient electrodes originate at the end of a patient cable, the other
end of which plugs into the ECG recorder. The wires from the electrodes connect to the lead selector
switch, which also incorporates the resistors necessary for the unipolar leads.
A push buttonallows the insertion of a standardization voltage of 1mV to standardize or calibrate
the recorder.
Changing the setting of the lead selector switch introduces an artifact on the recorded trace. A
special contact on the lead selector switch turns off the amplifier momentarily whenever this switch is
moved and turns it on again after the artifact has passed.
From the lead selector switch the ECG signal goes to a preamplifier, a differential amplifier with
high common-mode rejection. It is ac-coupled to avoid problems with small dc voltages that may
originate from polarization of the electrodes.
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The preamplifier is followed by a dc amplifier called the pen amplifier, which provides the power to
drive the pen motorthat records the actual ECG trace. The input of the pen amplifier is usually accessible
separately, with a special auxiliary input jack at the rear or side of the ECG recorder. Thus, the ECG
recorder can be used to record the output of other devices.
A positioncontrol on the pen amplifier makes it possible to center the pen on the recording paper.
All modern ECG recorders use heat-sensitive paper, and the pen is actually an electrically heated stylus,
the temperature of which can be adjusted with a stylus heat control for optimal recording trace.
Beside the recording stylus, there is amarkerstylus that can be actuated by a pushbutton and allows
the operator to mark a coded indication of the lead being recorded at the margin of the electrocardiogram.
Normally, electrocardiograms are recorded at a paper speed of 25 mm/sec, but a faster speed of 50
mm/secis provided to allow better resolution of the QRS complex at very high heart rates or when a
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particular waveform detail is desired.
The power switchof an ECG recorder has three positions. In the ONposition the power to the
amplifier is turned on, but the paper drive is not running. In order to start the paper drive, the switch must
be placed in the RUNposition.
ELECTROENCEPHALOGRAPH (EEG)
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Electroencephalography:This is the technique by which the electrical activities of the brain are studied.
Electroencephalograph:This is the instrument by which the electrical activities of the brain are
recorded.
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Electroencephalogram:This is the record or graphical registration of electrical activities of the brain.
Significance of EEG:
EEG is useful in the diagnosis of neurological disorders and sleep disorders.
EEG is primarily used for diagnosis including the following
i)
Helps to detect and localize cerebral brain lesions.
ii)
Aid in studying epilepsy
iii)
Assist in diagnosing mental disorders
iv)
Assist in studying sleep patterns
v)
Allow observation and analysis of brain responses to sensory stimuli.
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EEG Electrodes
EEG electrodes transform ionic currents from cerebral tissue into electrical current used in EEG
preamplifier.
5 types of electrodes are used.
1. Scalp: silver pads, discs or cups, stainless steel rods, chlorided silver wires.
2. Sphenoidal: Alternating insulated silver and bare wire and chlorided tip insertedthrough muscle
tissue by a needle.
3. Nasopharyngeal: Silver rod with silver ball at the tip inserted through the nostril.
4. Electrocorticographic: Cotton wicks soaked in saline solution that rests on brain surface.
5. Intracerebral: sheaves of Teflon coated gold or platinum wires used to stimulate the brain.
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EEG Waves:
The electrical recordings from the surface of the brain or from the outer surface of the head
demonstrate continuous electrical activity in the brain. The intensities of the brain waves on the surface of
the scalp range from 0-300uV and their frequencies range from 0.5 Hz to 100Hz.
Brain waves are irregular and no general pattern can be discerned in the EEG. However at other
times distinct patterns will appear. In normal persons it is classified as alpha, beta, theta, gamma and delta
waves.
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i)
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Delta Waves:
Frequency
: 0.5 to 4 HZ
Occurrence
:These occur only once in every 2 or 3 seconds during deep sleep, in premature babies
and in very serious organic diseases. These can occur strictly in the cortex independently by the
activities in the lower regions of the brain.
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ii)
Theta Waves:
Frequency
: 4 to 8 Hz
Occurrence
:These are recorded from the parietal and temporal regions of the scalp of children. These
also occur during emotional stress in some adults particularly during disappointment and frustration.
iii)
Alpha Waves:
Frequency
: 8 to 13 Hz
Occurrence
: They are found in normal persons when they are awake in a quiet resting state. They
normally occur in the occipital region. During sleep they disappear.
iv)
Beta Waves:
Frequency: 13 to 30 Hz
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Occurrence:These are recorded from the parietal and frontal regions of scalp. These are divided into two
types as beta I which inhibited by the cerebral activity and beta II which is excited by the mental
activitylike tension.
Placement of Electrodes:
In EEG, electrodes are placed in standard positions on the skull in an arrangement 10-20 system, a
placement scheme devised by the International Federation of societies of EEG. The electrodes in this
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arrangement are placed as follows:
i.
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Draw a line on the skull from the nasion,the root of the nose, to the inion, ossification center
(bump) on the occipital lobe.
ii.
Draw a similar line from the left preauricular (ear) point to the right preauricular point.
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iii.
Mark the intersection of these two lines as Cz which is the mid point of the distance between the
nasion and inion (or) the distance between the auricular points.
iv.
Mark points at 10, 20, 20, 20, 20 and 10% of the total nasion - inion distance. These points are Fpz,
Fz, Cz, Pz and Oz·
v.
Mark points at 10, 20, 20, 20, 20 and 10% of the total distance between the preauricular points.
These points are T3, C3, Cz, C4 and T4. In these odd numbered points T3 and C3 are on the left
and even numbered points C4 and T4 are on the right.
vi.
Measure the distance between Fpz and Oz along the great circle passing through T3 and mark
points at 10, 20, 20, 20, 20 and 10% of this distance. These are the positions of Fp 1, F7, T3, T5 and
O1.
vii.
Repeat this procedure on the right side and mark the positions of Fp2, F8, T4, T6 and O2.
viii.
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Measure the distance between Fp1 and O1 along the circle passing through C3 and mark points at
25%intervals. These points give the positions of F3, C3 and P3.
ix.
x.
Repeat this procedure on the right side and mark the positions of F 4, C4 and P4.
Check that F7, F3, Fz, F4 and F8 are equidistant along the transverse circle passing through F7, Fz
and F8 and check that T5, P3, Pz, P4 and T6 are equidistant along the transverse circle passing
through T5, Pz and T6.
In theabove figure the positions of the scalp electrodes are indicated. Further there are
nasopharyngeal electrodes Pg1 and Pg2 and ear electrodes A1and A2.
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Before placing the electrodes, the scalp is cleaned, lightly abraded and electrode paste is applied
between the electrode and the skin. By means of this application of electrode paste, the contact
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impedance is less than 10KΩ. Generally disc like surface electrodes are used. In some cases, needle
electrodes are inserted in the scalp to pick up EEG.
EEG Recorder:
The modern 8 channel EEG recorder is as shown in the figure below. The patient cable consists of
21 electrodes and is connected to the eight channel selector. The electrodes are attached to the channel
selector in groups of eight called a montage of electrodes.
The right ear electrode acts as reference electrode for the right brain electrodes and the left ear
electrode acts as reference electrode for the left brain electrodes. The 50 Hz interference is reduced by
employing differential amplifiers as preamplifiers with more than 80 dB CMRR and by use of 50 Hz
notch filters. The effect of notch filter on signal distortion is not so much because important EEG signals
have frequencies below 30 Hz. Further if the room, in which EEGunit is placed, is covered with ferrous
metal screen, 50 Hz a.c. interference is greatly reduced.
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By cascading, the gain of the amplifier is increased to 106 so as to drive the recorder or imaging
CRT without any difficulty. The output voltage from the amplifier may either be applied directly to the
eight channel display through the filter bank or it may be stored as data on a tape recorder or in a
computer memory for further processing. The filter bank consists of appropriate filters to select different
types of brain waves.
Other facilities are also available to record evoked potentials from sensory parts of the brain such
that there are external stimuli like visual stimulus, audio stimulus and tactile (touch) stimulus. The time
delay between the stimulus and response can also be measured in the signal processing unit.
In the eight channel pen recorder there are 8 pens such that a pen for each channel. The normal
paper chart speed is 30 mm/second. There are also 60 mm/second for higher frequency recording and 15
mm/second to conserve paper during setup time.
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Electromyograph (EMG)
Electromyography is the science of recording and interpreting the electrical activity of muscle’s
action potential. Meanwhile recording of peripheral nerve’s action potential is called as
Electroneurography. The electrical activity of the underlying muscle can be measured by placing surface
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electrodes on the skin. To record the action potentials individual motor units, the needle electrode is
inserted into the muscle. Thus EMG indicates the amount of activity of a given muscle or a group of
muscles and not an individual nerve fiber.
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The action potentials occur both positive and negative polarities at a given pair electrodes; so they
may add or cancel each other. Thus EMG appears, very much like random noise wave form. The
contraction of a muscle produces action potentials. When there is stimulation to a nerve fiber, all the
muscle fibers contract simultaneously developing action potentials. In a relaxed muscle, there is no action
potential
Recording setup:
The setup for recording the EMG is as shown in the figure below.The surface electrodes or needle
electrodes pick-up the potentials produced by the contracting muscle fibers. The surface electrodes are
from Ag-AgCl and are in disc shape. The surface of the skin is cleaned and electrode paste is applied.
The electrodes are kept in place by means of elastic bands. So, the contact impedance is reduced below
10 kilo ohms.
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There are two conventional electrodes: bipolar and unipolar type electrodes. In the case of bipolar
electrode, the potential difference between two surface electrodes resting on the skin is measured. In the
case of unipolarelectrode, the reference surface electrode is placed on the skin and the needle electrode
which acts as active electrode, is inserted into the muscle.Because of the small contact area, these
unipolar electrodes have high impedances from 0.5 to 100 Mega ohms.
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EMG Recording Setup
With needle electrodes, it is possible to pick-up action potentials from selected nerves or muscles
and individual motor units.
In the case of coaxial electrode which consists of an insulated wire threaded through a
hyperdermic needle with an oblique tip for easy penetration. The surrounding steel jacket acts as
reference and the metallic wire acts as exploring electrode. The needle is inserted into the muscle. To
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record the action potentials from a single nerve, microelectrodes are used.
The amplitude of the EMG signals depends upon the type and placement of electrodes used and
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the degree of muscular exertions. That is, the surface electrode picks up many overlapping spikes and
produces an average voltage from various muscles and motor units. The needle electrode picks up the
voltage from a single muscle fiber. Generally EMG signals range from 0.1 to 0.5 mV. They may contain
frequency components from 20 Hz to 10 kHz, which are in the audio range. But using low pass filter, the
electromyographer restricts this frequency range from 20 Hz to 200 Hz for clinical purposes. The normal
frequency of EMG is about 60 Hz. Therefore the slow speed strip chart recorders are not useful and the
signals are displayed on a cathode ray oscilloscope and photographic recordings are made. Normally
there are two cathode ray tubes, one for viewing and other one for recording. A light sensitive paper
moves over the recording cathode ray tube and the image is produced on that paper. After developing it,
one can see the visible image. For, continuous recording, the paper speed is about 5 to 25 cm/second. For
short duration it is about 50 to 400 cm/second. The paper width is about 10 cm.
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The amplifier should have uniform frequency response in the frequency range from 10Hz to 1
kHz with high CMRR (100 dB) and input impedance greater than 10 Mega ohm. The signal is also
recorded in the tape recorder for future reference. Further the myographer can listen the sounds from the
loud speaker and from that he can diagnose the neuromuscular disorders.
Determination of conduction velocities in motor nerves:
The measurement of conduction velocity in motor nerves is used to indicate the location and the
type of nerve lesion. Here the nerve function is examined directly at various segments of the nerve by
means of stimulating it with a brief electric shock having a duration of 0.2 - 0.5 milliseconds and by
measuring the latencies, we can calculate the conduction velocity in that peripheral nerve. Latency is
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defined as the elapsed time between the stimulating impulse and the muscle's action potential.
The measurement procedure is as shown in the figure below. The EMG electrode and the
stimulating electrode are placed at two points on the skin separated by a known distance l1. A electrical
pulse is applied through the stimulating electrode. When the excitation reaches the muscle, the muscle
contracts with a short twitch. Since all the nerve fibers are stimulated at the same time and the
conduction velocity is normally the same in all nerve fibers, there is synchronous activation of the
muscle fiber. This action potential of the muscle is picked up by the EMG electrode and is displayed on
the oscilloscope along with the stimulating impulse. The elapsed time 't1' (latency) between the
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stimulating impulse and muscle's action potential is measured. Now the two electrodes are repositioned
with the distance separation as l2metres. Among the distances 11 and 12, 12< 11 .The latency is now
measured as ‘t2’ seconds.
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Determination of conduction velocity in a motor nerve
The conduction velocity, v = (l1 – l2) / (t1 – t2)
The conduction velocity in peripheral nerves is normally 50 m/s. When we have it below 40m/s, there is
some disorder in that nerve conduction.
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Skin-Surface Electrodes:
Surface Electrodes are used to measure bioelectric potential available from the surface of the skin.
These electrodes are placed in contact with the skin of the subject.
Human skin has high impedance when compared with voltage sources. The skin impedance may
vary from 0.5 kΩ for sweaty skin surface to more than 20kΩ for dry skin surface. Surface Electrodes are
used to record ECG, EEG & EMG signals. Large surface Electrodes are used for ECG measurement and
smaller surface electrodes are used for EEG and EMG measurements.
The different types of electrodes for recording various potential on the body surface are,
1. Metal plate Electrode.
2. Metal Disc Electrode.
3. Disposable foam-pad Electrode.
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4. Metallic suction cup Electrode.
5. Floating Metal Body Surface Electrode.
6. Flexible Body Surface Electrode.
1) Metal plate Electrode:
Many types of electrodes have been designed for surface acquisition of biomedical signals.
One of the oldest forms of ECG Electrode in clinical use is metal plate Electrode.

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Metal plate Electrode consists of a flat metal plate that has been into a cylindrical segment. To
attach the lead wire to ECG a terminal is placed on its outside surface near one end.

The electrode is connected to the limbs of the patient.

A post is placed near the center to connect a rubber strap to the electrode and hold it in place
on an arm or leg.
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
The electrode is made up of German silver (Nickel-Silver alloy).

These electrodes are 1 to 2 sq. inches.

A conductive gel or paste is applied on its concave surface before it is attached to the body to
reduce the impedance between the electrode and the skin.

The metal plate surface electrodes are of 2 types
i) Rectangular (3.5cm x 5cm)
ii) Circular (4.75cm dia)
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2) Metal Disc Electrode:
This electrode has a lead wire soldered or welded to the back surface.

The connection between the lead wire and electrode is protected by a layer of insulating material
such as epoxy or polyvinyl chloride.

This electrode is used for recording ECG or in cardiac monitoring for long term recordings.

The electrode is made up of silver with an electrolytically deposited layer of Agcl on its
contacting surface.

An electrolyte gel or paste is applied on its bottom surface before it is attached to the patient’s
chest wall to reduce the impedance between the electrode and the skin.

This electrode is fixed to patient’s body by a strip of surgical tape.

This electrode is also used for recording EEG and EMG.

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The metal disk electrode used for monitoring EEG or EMG is smaller in diameter than electrodes
used in recording ECG.

These electrodes are also manufactured using stainless steel, platinum or gold.
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3) Disposable foam-pad Electrode:-
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
It is a disposable electrode with adhesive already in place.

These electrodes can be readily applied to the patient.

Since these electrodes are disposable type, it is not reused so, the time spent for cleaning the
electrode is minimized.

This disposable electrode is constructed by a plastic foam material with a silver-plated disk on one
side attached to a silver-plated snap in the center of the electrode on the snap.

The silver plated disk serves as the electrode and may be coated with an Agcl layer.

A layer of electrolyte get covers the disk.

The electrode side of the foam is covered with an adhesive material.
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
A protective cover or strip of release paper is placed over this side of the electrode and foam.

The complete electrode assembly is packaged in a foil envelope so that the water component of
the gel will not evaporate away.

To apply the electrode to the patient, the technician has to clean the area of the skin on which the
electrode is to be placed.
Advantage:
No skilled technicians are required.

No special technique need to be learned such as using correct amount of gel or cutting strips of
adhesive tape to hold the electrode in place.
4) Suction Electrode:

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A modification of the metal-plate electrode that requires no traps or adhesives for holding it in
place is the suction electrode.


Suction electrodes are used for recording ECG.
Suction electrode consists of a hollow metallic cylindrical electrode that makes contact with the
skin at it base.

A terminal for the lead wire is attached to the metal cylinder and a rubber suction bulb is fitted
over its base.

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To apply the electrode to the patient, the technician has to clean the area of the skin on which the
electrode is to be placed.
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
Then apply the electrolyte gel on the cleaned surface.

Now, the bulb of the electrode is squeezed placed it on the surface of the skin and bulb is released
and applies suction against the skin, holding the electrode assembly in place.
Disadvantage:1. This electrode can be used only for short periods of time.
2. The suction and the pressure of the contact surface against the skin can cause irritation.
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3. The electrode is larger in size but the contacting area is very small.
4. This electrode has high source impedance when compared to metal plate electrodes.
5) Floating Electrodes:
A floating electrode is as shown in the figure below.


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The electrode looks like hat structure.
The electrode is constructed in such a way that the metal disk does not come in contact with the
skin. The metal disk is surrounded by electrolyte gel in the cavity.


The cavity does not move with respect to metal disk so, it will not produce any artifacts.
In practice, the electrode is filled with electrolyte gel and then attached to the skin surface by
means of a double sided adhesive tape ring.

The metal disk is made up of silver coated with Agcl.
6) Flexible Electrodes:
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As the name implies, flexible electrodes are flexible and can be applied on any irregular surface of
the body.The different types of flexible electrodes are –
i) Carbon-filled silicone rubber electrode:-
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A carbon filled silicone rubber compound in the form of a thin strip or disk is used as the active
element of an electrode.
A pin connector is pushed into the lead connector hole and the electrode is used in the same way
as a similar type of metal- plate electrode.
Flexible electrodes are used for detecting the ECG in premature infants.
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ii) Flexible thin-film neonatal electrode:This electrode consists of a 13μm thick Mylar film on which an Ag and Agcl film have been
deposited.
Flexible thin film Electrode
Cross sectional view of the thin film electrode
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Needle Electrode:
Basic needle electrode consists of a solid needle usually made of stainless steel with a
sharp point. The shank of the needle is insulated with a coating such as an Insulating varnishes.
Only the tip is left exposed. A lead wire is attached to the other end of the needle and the joint is
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encapsulated in a plastic hum to protect it. This electrode is frequently used in
electromyography as shown in fig (a). When placed in a particular muscle it obtains an EMG
from that muscle acutely and can then be removed.
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Shielded percutaneous electrode can be fabricated in the form shown in Fig. (b) which
consists of a small gage hypodermic needle that has been modified by running an insulated fine
wire down the center of its lumen and filling the remainder of the lumen with an insulating
material such as an epoxy resin. When the resin has set, the tip of the needle is filled to its
original bevel exposing on oblique cross section of the central wire which serves as the active
electrode. The needle itself is connected to ground through the shield of a coaxial cable, thereby
extending the coaxial structure to its very tip.
A multiple electrodes in a single needle can be formed as shown in the fig (c). There are many
different types of wire electrodes and schemes for introducing them through the skin. The principle can
be illustrated as shown in fig (d). A fine wire made up of stainless steel ranging in diameter from 25125um is insulated with an insulating varnish to within a few millimeters of the tip. This non insulated tip
is bent back on itself to form a J-shaped structure. The tip is introduced into the lumen of the needle as
shown in fig (d).
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The needle is inserted through the skin into the muscle at the desired location to the desired depth.
It is then slowly withdrawn leaving the electrode in place as shown in fig (e). The bent over portion of the
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wire serves as a barb holding the wire in place in the muscle and to remove the wire, the technician
applies a mild uniform force to straighten out the barb and pulls it out through the wire’s tract.
Microelectrode:
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Electrodes with tip sufficiently small to penetrate a single cell in order to obtain potential from
within the cell are called as microelectrode.The size of the microelectrode must be small with respect to
the cell dimensions to avoid cellular injury and cell damage thereby changing the cell’s behavior.
In addition to small in size, the electrode used for measuring intracellular potential must also be
strong enough so that it can penetrate the cell membrane and remain mechanically stable.
Microelectrodes have tip diameters ranging from approximately 0.05 to 10µm.
Microelectrodes are of two types1. Metal Microelectrode and
2. Micropipet Microelectrode
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Metal Microelectrode:
Metal microelectrodes are formed by electrolytically etching the tip of fine tungsten or stainless
steel wire to the desired size. Then the wire is coated with an insulating material almost to the tip.
Some electrolytic processing like chloriding the tip is also carried out to reduce the tip impedance.
The electrode-electrolyte interface takes place where the metal tip contacts the electrolyte either
inside or outside the cell.
The etched metal needle is then supported in a larger metallic shaft that can be insulated. This
shaft serves as a mechanical support for the microelectrode and lead wire is connected to it.
The microelectrode and the supporting shaft are insulated by a thin film of polymeric material or
varnish. Only the extreme tip of the electrode remains uninsulated.
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Supported metal microelectrodes:
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Some examples of supported metal microelectrode is as shown in the figure below. A glass tube is drawn
to a micropipette structure with its lumen filled with an appropriate metal. This type of microelectrode is
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prepared by first filling a glass tube with a metal that has a melting point near the softening point of the
glass.
Metal inside Glass
Glass inside Metal
The tube can then be heated to the softening point and pulled to form a narrow constriction.
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When it is broken at the constriction, two micropipets filled with metal are formed. In this type of
structure, the glass not only provides the mechanical support but also serves as the insulation. Only the
active tip is exposed.
Metals such as silver- solder alloy and platinum and silver alloys are used.
In new supported metal electrode, a solid rod or glass tube is drawn to form micropipet. A metal
film is deposited uniformly on this surface to a thickness of the order of tenths of a µm.
A polymeric insulation is coated over this. Only the tip with the metal film will be exposed.
The position of the metal microelectrode and its equivalent circuit diagram is as shown below.
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Position of Electrode
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Electrical Equivalent circuit of the above figure
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Approximate Equivalent of the above figure
RA
= Resistance of connecting wire,
RS
= Resistance of the shaft of the microelectrode.
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RFA, RWA, CWA = Impedance of microelectrode tip
RFA
= Resistance of intracellular fluid interface.
Rs
= Resistance of the shaft of the microelectrode
Cd
= Distributed capacitance between the metal and ECF
Rma, Cma
= Impedance of microelectrode tip – Intracellular fluid Interface
Ema
= metal electrode- electrolyte potential at the microelectrode tip.
Ri
= Resistance of intracellular fluid.
Emp
= variable cell membrane potential
Re
= Resistance of Extracellular fluid
Emb
= Reference electrode- electrolyte potential
Cmb, Rmb
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= Impedance of Reference electrode- ECF interface.
= Capacitance associated with lead wires.
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The actual equivalent circuit is shown in figure 2 and the simplified equivalent circuit is shown in
figure 3.
In the figure 3, the impedance of the reference electrode, series resistance contribution from the
intracellular and Extracellular fluid, distributed capacitances are neglected.
Since the area of the reference electrode is many times greater than the metal electrode’s tip, its
impedance is negligible.
When the electrode output is coupled with an amplifier, the low frequency components of the
bioelectric potential will be attenuated if the input impedance of the amplifier is not high. Thus when the
input impedance of the amplifier is not high it behaves as a high pass filter.
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Micropipet Microelectrode:
Glass micropipet microelectrodes are fabricated from glass capillaries. The center region of the
capillary is heated with a burner to the softening point, and then the capillary is rapidly stretched to
produce the constriction.
Section of fine bore glass capillary Capillary narrowed through heating and stretching
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Final structure of Glass pipete microelectrode
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The two halves of the stretched capillary structure are broken at the constriction to produce a
pipette structure that has a tip diameter of the order of 1µm. now the pipette is fabricated into the
electrode form as shown in the figure 3.
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The electrode is filled with an electrolyte solution (3m KCl). A cap containing a metal electrode is
then sealed to the pipette. The metal electrode contacts the electrolyte within the pipette.
The electrode used is silver wire prepared with an electrolytic AgCl surface. Platinum or Stainless
steel wires are also used.
The nonmetallic micropipet consists of a glass micropipet whose tip's diameter is about 1
micrometre. The micropipet is filled with an electrolyte usually 3 M KCI which is compatible with the
cellular fluids. A thin, flexible metal wire from chlorided silver, stainless steel or tungsten is inserted into
the stem of the micropipet. The friction between the wire and the stem of the micropipet and the fluid
surface tension hold the micropipet on the wire. The other end of the metal wire is mounted to a rigid
support and the other free end of it resting on the cell as shown in the figure below.
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Position of Electrode
Electrical Equivalent circuit of the above figure
EA – Potential between the metal wire and electrolyte filled in the micropipette
EB – Potential between the reference electrode and the extracellular fluid
EC – Variable cell membrane potential
ED – potential existing at the tip due to different electrolytes present in the pipet
and the cell.
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E = EA+EB+ED
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Approximate Equivalent of the above figure
RA denotes the resistance of the connecting wire, RFA, RWA and CWA constitute the impedance of
the electrode-electrolyte interface in the stem of the micropipet and RT is the resistance of the electrolyte
filling the tip of the micropipette.
RIN and REX are the resistances of the electrolyte inside the cell and the electrolyte outside the cell
respectively. RFB, RWB and CWBconstitute reference electrode - electrolyte interface impedance and RB is
the resistance of the wire connected with reference electrode. CD is the distributed capacitance existing
between the fluid in the pipet and the extracellular fluid.
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UNIT - II
BIO-CHEMICAL AND NON ELECTRICAL
PARAMETERS MEASUREMENT
1. FILTER PHOTOMETER (COLORIMETER):
The schematic diagram of a simple filter photometer is as shown in the figure
below. It is used to measure transmittance.
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Light from a halogen lamp is made to incident on a filter, F. It transmits only a
suitable wavelength range, at which the measurement is performed. The divergent
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transmitted light is converted into two parallel beams by an optical arrangement. One
beam on a reference selenium photoelectric cell, CR and other beam fall on a sample
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selenium photoelectric cell Cs after passing through sample in the cuvette. Without
the sample, the outputs from photoelectric cells are same. When the sample is placed
in the path, the output of the sample cell is reduced and hence the potentiometer is
adjusted that both the cells CR and Cs give the same output which is indicated by the
null deflection in the galvanometer 'G'. Since the potentiometer is calibrated in terms
of transmittance, the concentration of the given substance in the sample can be
determined.
2. AUTOANALYSER:
The autoanalyzer sequentially measures blood chemistry and displays this on a
graphic readout. As shown in Figure below, this is accomplished by mixing, reagent
reaction, and colorimetric measurement in a continuous stream. The system includes
the following elements.
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1. Sampler - aspirates samples, standards, and wash solutions to the autoanalyzer
system.
2. Proportioning pump and manifold - introduces (mixes) samples with reagents to
effect the proper chemical color reaction to be read by the colorimeter. It also
pumps fluids at precise flow rates to other modules, as proper color
development depends on reaction time and temperature.
3. Dialyzer - separates interfacing substances from the sample material by
permitting selective passage of sample components through a semipermeable
membrane.
4. Heating bath - heats fluids continuously to exact temperature (typically 37°C
incubation, equivalent to body temperature). Temperature is critical to color
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development.
5. Colorimeter - monitors the changes in optical density of the fluid stream flowing
through a tubular flow cell. Color intensities (optical densities) proportional to
substance concentrations are converted to equivalent electrical voltages
6. Recorder – Converts optical density electrical signal from the colorimeter into a
graphic display on a moving chart.
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The heart of the autoanalyzer system is the proportioning pump. This consists
of a peristaltic pump. Air segmentation in the mixing tube separates the sample /
reagent mixture from the cleaning fluid and other samples. As these air-separated
fluids traverse the coil of the mixing tube, effective mixing action is achieved.
One problem with automatic analyzer is certain identification of samples.
Patient data can be intermixed with that of other patients if care is not taken.
Sterilization is also needed for samples, glassware and equipment parts that are
contaminated with disease. Diseases such as hepatitis or other communicable
infections can be spread to equipment operators.
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3. RESPIRATORY MEASUREMENT:
Measurement of Respiration rate:
The transducers commonly used to measure respiration rate are,
i)
Thermistor placed in front of the nostril
ii)
Displacement transducer put across the chest
iii)
Impedance electrodes
 The respiratory signal from any one of these transducers is amplified and
the time interval is measured between two successive pulses.
 The measuring range is 0-50 respiration / minute.
 The methods used to measure respiration rate are,
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o Thermistor method
o Impedance Pneumography
o CO2 measurement of respiration rate
o Displacement method
1. Thermistor Method:
In this method a thermistor is placed in front of the nostrils by means of a
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suitable holding device to detect the difference in temperature between the inspired
and expired air.
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Since the inspired air passes through the lungs and respiratory tract, its
temperature is increased while coming out. This change in temperature is detected by
using thermistor.
Incase the difference in temperature of the outside air and expired air is small,
the thermistor can be initially heated to an appropriate temperature and the variation
of its resistance in synchronous with the respiration rate can be detected.
The thermistor is placed as part of a voltage dividing circuit or in a bridge circuit
whose unbalance signal can be amplified to obtain the respiration rate.
Occasionally, unconscious patients display a tendency for the uncontrolled tongue to
block the breathing system. Under such systems not a single milliliter of air is
inhaled, but the patient’s thorax is carrying out large air through frustral breathing.
The impedance pneumograph and switch methods will show the correct state.
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2. Displacement Method:
During each respiratory cycle, the thoracic volume changes. These changes can
be sensed by means of displacement transducer.
The transducer is held by an elastic band which goes around the chest.
The respiratory movement results in resistance changes of the strain gauge
element connected as one arm of a wheatstone bridge circuit. Bridge output varies
with chest expansion and yields signals corresponding to respiratory activity.
Changes in the chest circumference can also be detected by a rubber tube filled
with mercury. The tube is fastened firmly around the chest
During inspiration, the chest expands and the rubber tube increases in length
and the resistance of the mercury from one end of the tube to the other end changes.
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The change in resistance can be measured by sending a constant current
through it and measured in terms of change in voltage during each respiratory cycle.
3. Impedance Pneumography:
This is an indirect technique for the measurement of respiration rate.
Impedance Pneumograph is based on the fact that the impedance across the
chest changes during each respiratory cycle.
In this method, a low voltage 50 to 500KHz AC signal is applied to the chest of
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the patient through surface electrodes and the modulated signal is detected.
The signal is modulated by changes in the body impedance accompanying the
respiratory cycle.
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High value fixed resistors are connected in series with each electrode to create a
constant AC current source. The signal voltage applied to the differential AC amplifier
is the voltage drop across the resistance representing patient’s thoracic impedance.
Eo = I.(R ± ∆R)
Where,
Eo – Output potential in volts.
I
– Current through the chest in amps.
R
– Chest impedance without respiration in ohms.
∆R – Change of chest impedance caused by respiration in ohms.
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The current passed through the patient’s chest is very small. The signal Eo is
amplified and then demodulated by AM detector.
The Low Pass Filter removes the residual carrier signal and a DC amplifier
scales the output waveform to the level required by the display device.
4. CO2 method of respiration rate measurement:
Respiration rate can also be measured by continuously monitoring the CO2
contained in the subject’s alveolar air.
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The measurement is based on the absorption property of infrared rays by
certain gases. Suitable filters are used to determine the concentration of specific gases
(like CO, CO2, and NO2) present in the expired air.
Rare gases and diatomic gases do not absorb infrared rays.
When infrared rays are passed through the expired air containing a certain
amount of CO2 some of the radiations are absorbed by it. There is a proportional loss
of heat energy associated with the rays.
The detector converts this heat loss of the rays into an electrical signal. This
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signal is used to obtain the average respiration rate.
The arrangement for detection of CO2 in the expired air is shown in the above
figure.
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Two beams of equal intensity of infrared radiations from the infrared source fall
on one half of each of the condenser microphone assembly.
The infrared rays from the infrared source are chopped at 25 KHz by the
chopper motor. A disc is connected to the spindle of the chopper motor.
The detector has two identical portions separated by a thin flexible metal
diaphragm. One is called test side and the other is called as reference side.
The detector is filled with a sample of pure CO2. Because of absorption of CO2 in
the analysis cell, the beam falling on the test side of the detector is weaker than that
falling on the reference side.
The gas in the reference side would be heated more than that on the analysis
side. As a result, the diaphragm is pushed slightly to the analysis side of the detector.
The diaphragm forms one plate of the capacitor.
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The voltage developed across the diaphragm is amplified, shaped and suitably
integrated to the give the respiration rate.
Spirometer:
Conventional spirometer is shown in figure below. This instrument uses a bell
jar, suspended from above, in a tank of water. An air hose leads from a mouthpiece to
the space inside of the bell above the water level. A weight is suspended from the
string that holds the bell in such a way that it places a tension force on the string that
exactly balances the weight of the bell at atmospheric pressure.
When no one is breathing into the mouthpiece, the bell will be at rest with a
fixed volume above the water level. But when the subject exhales, the pressure inside
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the bell increases above atmospheric pressure causing the bell to rise. Similarly, when
the patient inhales, the pressure inside the bell decreases. The bell will rise when the
pressure increases and drop when the pressure decreases.
The change in bell pressure changes the volume bell, which also causes the
position of the counterweight to change. We may record the volume changes on a
piece of graph paper by attaching a pen to the counterweight or tension string.
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Bell-Jar mechanical Spirometer
The chart recorder is a rotary drum model called a kymograph. It rotates slowly
at speeds between 30 and 2000 mm/min.
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Some spirometers also offer an electrical output that is the electrical analog of
the respiration waveform. Most frequently the electrical output is generated by
connecting the pen and weight assembly to a linear potentiometer. If precise positive
and negative potentials are connected to the ends of the potentiometer, then the
electrical signal will represent the same data as the pen. When no one is breathing
into the mouthpiece, Eo will be zero. But when a patient is breathing into the tube, Eo
will take a value proportional to the volume and a polarity that indicates inspiration or
expiration.
4. TEMPERATURE MEASUREMENT:
Temperature measurement is one of the most essential commonly used
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parameter in the medical field.
The variation in the temperature is a direct result of the variation in blood
pressure. The metabolic rate and temperature have a close relation.
Body temperature is one of the indicators of a person being normal.
Basically two types of temperature measurement cam be obtained from the
human body
1. Systemic
2. Skin surface
Systemic Temperature:
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Systemic temperature is the temperature of the internal regions of the body.
This temperature is maintained by balancing the heat generated by the active
tissues of the body (muscles & Liver) and the heat lost by the body to the
environment.
Systemic temperature is measured by using temperature sensing devices placed
in mouth, under the armpits or in the rectum.
The normal oral (mouth) temperature of a healthy person is 37˚C. The normal
under arm temperature of a healthy person is 36˚C and The normal rectum
temperature of a healthy person is 38˚C.
The systemic body temperature can be measured more accurately at the
tympanic membrane in the ear.
Even for the healthy person, the temperature will not be constant. It will vary
about 1 to 1 ½ ˚C in the early morning compared to the late afternoon.
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The temperature control center for the body is located deep within the brain.
Here the temperature of the blood is monitored and its control functions are
coordinated.
If the surrounding temperature is warm, then the body is cooled by perspiration
due to secretion of the sweat glands and by increased circulation of blood near the
surface. The body acts as a radiator.
If the surrounding temperature becomes too low, then the body conserves heat
by reducing the blood flow near the surface to the minimum required for maintenance
of the cells. At the same time metabolism is increased.
Surface of Skin temperature:
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Surface or skin temperature is a result of a balance but here the balance is
between the heat supplied by blood circulation in the local area and the cooling of that
area by conduction, radiation, convection and evaporation. Thus skin temperature is a
function of the surface circulation, environmental temperature, air circulation around
the area from which the measurement is to be taken and perspiration.
To obtain a meaningful skin temperature measurement, it is usually necessary
to have the subject remain with no cloth covering the region of measurement in a
fairly cool ambient temperature.
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Measurement of systemic Body temperature:
1. Mercury Thermometer:
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Mercury thermometer is the standard method of temperature measurement.
Mercury thermometer is used where continuous recording of temperature is not
required.
Mercury thermometers are inexpensive, easy to use and sufficiently accurate.
2. Electronic Thermometer:
Now-a-days electronic thermometers are available as a replacement of mercury
thermometer. IT has disposable tip and requires only less time for reading and also
much easier to read the value.
Electronic thermometers are used where continuous recording and accuracy of
the temperature is necessary.
Two types of electronic temperature sensing devices are found in biomedical
applications. They are,
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1. Thermocouple
2. Thermistor
Thermistors are variable resistance devices formed into disks, Beads, Rods or
other desired shapes. They are manufactured from mixtures of oxides of various
elements such as nickel, copper, magnesium, manganese, cobalt, titanium and
aluminium.
After the mixture is compressed into shape, it is shaped at a high temperature
into a solid mass. The result is a resistor with a large temperature coefficient.
Most metals show an increase in resistance of about 0.3 to 0.5 percent per ˚C
temperature rise and the thermistors show decrease in resistance by 4 to 6 percent
per ˚C temperature rise.
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A Comparison of resistance Vs Temperature curves for copper, thermistor,
posistor is as shown below.
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Skin temperature Measurement:
Although the systemic skin temperature remains very constant throughout the
body, skin temperature can vary several degrees from one point to another. The range
is usually from about 30 to 35°C (85 to 95◦F). Exposure to ambient temperatures, the
covering of fat over capillary areas, and local blood circulation patterns are just a few
of the many factors that influence the distribution of temperatures over the surface of
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the body. Often, skin temperature measurements can be used to detect or locate
defects in the circulatory system by showing differences in the pattern from one side
of the body to the other.
Skin temperature measurements from specific locations on the body are
frequently made by using small, flat thermistor probes taped to the skin. The
simultaneous readings from a number of these probes provide a means of measuring
changes in the spatial characteristics of the circulatory pattern over a time interval or
with a given stimulus.
Although the effect is insignificant in most cases, the presence of the thermistor
on the skin slightly affects the temperature at that location. Other methods of
measuring skin temperature that draw less heat from the point of measurement are
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available. The most popular of these methods involve the measurement of infrared
radiation.
The human skin has been found to be an almost perfect emitter of infrared
radiation. That is, it is able to emit infrared energy in proportion to the surface
temperature at any location of the body. If a person is allowed to remain in a room at
about 21°C (70°F) without clothing over the area to be measured, a device sensitive to
infrared radiation can accurately read the surface temperature. Such a device, called
an infrared thermometer.
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Infrared thermometers in the physiological temperature range are available
commercially and can be used to locate breast cancer and other unseen sources of
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heat. They can also be used to detect areas of poor circulation and other sources of
coolness and to measure skin temperature changes that reflect the effects of
circulatory changes in the body.
An extension of this method of skin temperature measurement is the
Thermograph. This device is an infrared thermometer incorporated into a scanner so
that the entire surface of a body, or some portion of the body, is scanned in much the
same way that a television camera scans an image, but much slower. While the
scanner scans the body, the infrared energy is measured and used to modulate the
intensity of a light beam that produces a map of the infrared energy on photographic
paper. This presentation is called a thermogram.
The advantage of this method is that relatively warm and cool areas are
immediately evident. By calibrating the instrument against known temperature
sources, the picture can be read quantitatively.
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A similar device, called Thermovision, has a scanner that operates at a rate
sufficiently high to permit the image to be shown in real time on an oscilloscope. The
raster has about 100 vertical lines per frame, and the horizontal resolution is also
about 100 lines, which seems to be adequate for good representation. The intensity of
the measured infrared radiation is reproduced by Z-axis modulation of the
oscilloscope beam. One advantage of this system is that certain portion of the grey
scale can be enhanced to bring out specific features of the picture. Also, the image can
be changed so that warm spots appear dark instead of light.
5. MEASUREMENT OF PULSE RATE:
Whenever the heart muscle contracts, blood is ejected from the ventricles and a
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pulse of pressure is transmitted through the circulatory system. This pressure pulse
when traveling through the vessels causes vessel wall displacement which is
measurable at various points of peripheral circulatory system.
The pulse can be felt by placing the finger tip over the radial artery in the wrist
or some other locations where an artery seems just below the skin.
The pulse pressure and waveform are indicators for blood pressure and blood
flow. The instrument used to detect the arterial pulse and pulse pressure waveform is
called as plethysmograph.
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The pulse waveform travels at 5 to 15 m/sec depending up on the size and
rigidity of arterial walls.
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The methods used to detect volume (pulse) change due to blood flow are,
1. Electrical Impedance changes
2. Strain Gauge or microphone (mechanical)
3. Optical change (Changes in density)
Electrical Impedance changes:
Electrical Impedance method measures the impedance change between 2
electrodes caused by the change in blood volume between them.
The change in the impedance (0.1 ohm) may be as small as compared to the
total impedance (Several hundred ohms).
The impedance is measured by applying an alternating current between
electrodes attached to the body.
An alternating current (10 – 100 KHz) is used.
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Strain Gauge or microphone (mechanical):
The mechanical method involves the use of strain gauge connected to a rubber
band placed around the limb or finger.
Expansion in the band due to change in blood volume causes a change in
resistance of the strain gauge. (OR)
A sensitive crystal microphone is placed on the skin surface to pick up the
pulsation.
Optical change (Changes in density):
The most commonly used method to measure blood volume change is photo
electric method. In this method we have 2 types of method
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1. Transmittance method
2. Reflectance method
1. Transmittance method:
In transmittance method, a light emitting diode (LED) and photoresistor are
mounted in an enclosure that fits over the tip of the patient’s finger. The light is
transmitted through the finger tip of the subject’s finger and the resistance of the
photoresistor is determined by the amount of light reaching it. With each contraction
of the heart, blood is forced to the extremities and the amount of blood in the finger
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increases. It alters the optical density with the result that the light transmission
through the finger reduces and the resistance of the photoresistor increases
accordingly.
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The photoresistor is connected as part of a voltage divider circuit and produces
a voltage that varies with the amount of blood in the finger. This voltage that closely
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follows the pressure pulse and its waveshape can be displayed on an oscilloscope or
recorded on a strip-chart recorder.
2. Reflectance method:
The arrangement used in the reflectance method of photoelectric
plethysmography is shown in the figure below.
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The photoresistor is placed adjacent to the exciter lamp. Part of the light rays
emitted by the LED is reflected and scattered from the skin and the tissues and falls
on the photoresistor.
The quantity of light reflected depends upon the amount of blood filling the
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capillaries and, therefore, the voltage drop across the photoresistor, connected as a
voltage divider, will vary in proportion to the volume changes of the blood vessels.
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The LED phototransistor-photoplethysmograph transducer (Lee et al, 1975)
consists of a Ca-As infrared emitting diode and a phototransistor in a compact
package measuring 6.25 x 45 x 4.75 mm. The peak spectral emission of the LED is at
0.94m with a 0.707 peak bandwidth of 0.04 m. The phototransistor is sensitive to
radiation between 0.4 and 1.1m as shown above.
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For pulse rate measurement, a photoelectric transducer suitable for use on the
finger or ear lobe is used. The signal from the photocell is amplified and filtered (0.5 to
5 Hz passband) and the time interval between two successive pulses is measured. The
measuring range is 0-250 bpm. Careful placement and application of the device is
essential in order to prevent movement artifacts due to mechanical distortion of the
skin. The figure below shows the block diagram for processing the plethysmographic
signal detected from a photoelectric transducer.
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The circuit consists of two parts, a LED oscillator and driver, which produce
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300 Hz, 50 S infrared light pulses to the finger probe attached to the patient, and a
phototransistor that picks up the attenuated light. The electrical signal obtained from
the phototransistor is amplified and its peak value is sampled and filtered. An
automatic gain control circuit adjusts the amplifier gain to yield a constant
average pulse height at the output.
The ac component with a frequency in the heart rate range (0.8-5 Hz) is
further amplified to output the plethysmographic pulse rate form. This signal is
transmitted across the isolation barrier, demodulated, low-pass filtered and
transmitted to the analog multiplexer resident on the CPU board.
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MEASUREMENT OF BLOOD FLOW AND CARDIAC OUTPUT:
An adequate blood supply is necessary for all organs of the body; in fact, an
impaired supply of blood is the cause of various diseases. The ability to measure blood
flow in the vessel that supplies a particular organ would therefore be of great help in
diagnosing such diseases. Unfortunately, blood flow is a rather elusive variable that
cannot be measured easily.
The rate of flow of a liquid or gas in a pipe is expressed as the volume of the
substance that passes through the pipe in a given unit of time. Flow rates are
therefore usually expressed in liters per minute or milliliters per minute (cm3/min).
Methods used in industry for flow measurements of other liquids, like the
turbine flowmeter and the rotameter, are not very suitable for the measurement of
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blood flow because they require cutting the blood vessel. These methods also expose
the blood to sharp edges, which are conducive to blood-clot formation.
Practically all blood flow meters currently used in clinical and
research applications are based on one of the following physical principles:
1. Electromagnetic induction.
2. Ultrasound transmission or reflection.
3. Thermal convection.
4. Radiographic principles.
5. Indicator (dye or thermal) dilution
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Magnetic and ultrasonic blood flow meters actually measure the velocity of
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the bloodstream. Because these techniques require that a transducer surround an
excised blood vessel, they are mainly used during surgery. Ultrasound, however, can
be used transcutaneously to detect obstructions of blood vessels where quantitative
blood flow measurements are not required.
A plethysmograph, which actually indicates volume changes in body segments,
can be used to measure the flow of blood in the limbs.
Magnetic Blood Flow Meters:
Magnetic blood flow meters are based on the principle of magnetic induction. When an electrical conductor is moved through a magnetic field, a
voltage is induced in the conductor proportional to the velocity of its motion. The
same principle applies when the moving conductor is not a wire, but rather a
column of conductive fluid that flows through a tube located in the magnetic field.
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Magnetic Blood flow meter, Principle
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A permanent magnet or electromagnet positioned around the blood vessel
generates a magnetic field perpendicular to the direction of the blood flow. The
voltage induced in the moving blood column is measured with stationary
electrodes located on opposite sides of the blood vessel and perpendicular to the
direction of the magnetic field
The most commonly used types of implantable magnetic blood flow probes are
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shown in Figures below. The slip-on or C type is applied by squeezing an excised blood
vessel together and slipping it through the slot of the probe. In some transducer
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models the slot is then closed by inserting a keystone-shaped segment of plastic.
Contact is provided by two slightly protruding platinum disks that touch the wall of
the blood vessel. For proper operation, the orifice of the probe must fit tightly, around
the vessel. For this reason, probes of this type are manufactured in sets, with
diameters increasing in steps of 0.5 or 1 mm from about 2 to 20 mm.
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Sample of large and small lumen diameter blood flow probes
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Blood flow probe – clip-on type for use during surgery
In the cannula-type transducer, the blood flows through a plastic cannula
around which the magnet is arranged. The contacts penetrate the walls of the
cannula. This type of transducer requires that the blood vessel be cut and its ends
slipped over the cannula and secured with a suture. A similar type of transducer is
also used to measure the blood flow in extracorporeal devices, such as dialyzers.
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Magnetic blood flow meters actually measure the mean blood velocity. Because
the cross-sectional area at the place of velocity measurement is well defined with
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either type of transducer, these transducers can be calibrated directly in units of flow
The output voltage of a magnetic blood flow transducer is very small, typically in
the order of a few microvolts. In early blood flow meters, a constant magnetic field was
used, which caused difficulties with electrode polarization and amplifier drift. To
overcome these problems, all contemporary magnetic blood flow meters use
electromagnets that are driven by alternating currents. But, the change of the
magnetic field causes the transducer to act like a transformer and induces error
voltages that often exceed the signal levels by several orders of magnitude. Thus, for
recovering the signal in the presence of the error voltage, amplifiers with large
dynamic range and phase-sensitive or gated detectors have to be used. To minimize
the problem, several different waveforms have been advocated for the magnet current,
as shown in figure below. With a sinusoidal magnet current, the induced voltage is
also sinusoidal but is 90° out of phase with the flow signal.
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With a suitable circuit, similar to a bridge, the induced voltage can be partially
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balanced out. With the magnet current in the form of a square wave, the induced
voltage should be zero once the spikes from the polarity reversal have passed. In
practice, however, these spikes are often of extremely high amplitude, and the
circuitry response tends to extend their effect. A compromise is the use of a magnet
current having a trapezoidal waveform. None of the three waveforms used seems to
have demonstrated a definite superiority.
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The block diagram of a magnetic blood flow meter is shown in above figure. The
oscillator, which drives the magnet and provides a control signal for the gate, operates
at a frequency of between 60 and 400 Hz. The use of a gated detector makes the
polarity of the output signal reverse when the flow direction reverses. The frequency
response of this type of system is mual!y high enough to allow the recording of the
flow pulses, while the mean or average flow can be derived by use of a low-pass filter.
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Ultrasonic blood flow meter
In an ultrasonic blood flow meter, a beam of ultrasonic energy is used to
measure the velocity of flowing blood. This can be done in two different ways. In the
transit time ultrasonic flow meter, a pulsed beam is directed through a blood.
vessel at a shallow angle and its transit time is then measured. When the blood flows
in the direction of the energy transmission, the transit time is shortened. If it flows in
the opposite direction, the transit time is lengthened.
More common are ultrasonic flow meters based on the Doppler principle. An
oscillator, operating at a frequency of several megahertz, excites a piezoelectric
transducer (usually made of barium titanate). This transducer is coupled to the wall of
an exposed blood vessel and sends an ultrasonic beam with a frequency F into the
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flowing blood. A small part of the transmitted energy is scattered back and is received
by a second transducer arranged opposite the first one. Because the scattering occurs
mainly as a result of the moving blood cells, the reflected signal has a different
frequency due to the Doppler effect. Its frequency is either F + FD or F - FD, depending
on the direction of the flow.
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Ultrasonic blood flow meter, Doppler type
The Doppler component FD is directly proportional to the velocity of the flowing
blood. A fraction of the transmitted ultrasonic energy, however, reaches the second
transducer directly, with the frequency being unchanged. After amplification of the
composite signal, the Doppler frequency can be obtained at the output of a detector as
the difference between the direct and the scattered signal components.
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With blood velocities in the range normally encountered, the Doppler signal is
typically in the low audio frequency range. Because of the velocity profile of the
flowing blood, the Doppler signal is not a pure sine wave, but has more the form of
narrow-band noise. Therefore, from a loudspeaker or earphone, the Doppler signal of
the pulsating blood flow can be heard as a characteristic "swish-swish-." When the
transducers are placed in a suitable mount (which defines the area of the blood
vessel), a frequency meter used to measure the Doppler frequency can be calibrated
directly in flow-rate units. Unfortunately, Doppler flow meters of this simple design
cannot discriminate the direction of flow. More complicated circuits, however, which
use the insertion of two quadrature components of the carrier, are capable of
indicating the direction of flow.
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Transducers for ultrasonic flow meters can be implanted for chronic use. Some
commercially available flow meters of this type incorporate a telemetry system to
measure the blood flow in unrestrained animals.
Blood Flow measurement by Thermal convection:
A hot object in a colder-flowing medium is cooled by thermal convection. The
rate of cooling is proportional to the rate of the flow of the medium. This principle,
often used to measure gas flow, has also been applied to the measurement of blood
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velocity. In one application, a thermistor in the bloodstream is kept at a constant
temperature by a servo system. The electrical energy required to maintain this
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constant temperature is a measure of the flow rate. In another method an electric
heater is placed between two thermocouples or thermistors that are located some
distance apart along the axis of the vessel. The temperature difference between the
upstream and the downstream sensor is a measure of the blood velocity. A device of
the latter type is sometimes called a thermostromuhr (literally, from the German "heat
current clock"). Thermal convection methods for blood flow determination, although
among the oldest ones used for this purpose, have now been widely replaced by the
other methods described in this chapter.
Blood Flow Determination by Radiographic Methods:
Blood is not normally visible on an X-ray image because it has about the same
radio density as the surrounding tissue. By the injection of a contrast medium into a
blood vessel (e.g., an iodated organic compound), the circulation pattern can be made
locally visible. On a sequential record of the X-ray image (either photographic or on a
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videotape recording), the progress of the contrast medium can be followed,
obstructions can be detected, and the blood flow in certain blood vessels can be
estimated. This technique, known as cine (or video) angiography, can be used to
assess the extent of damage after a stroke or heart attack.
Another method is the injection of a radioactive isotope into the blood circulation,
which allows the detection of vascular obstructions (e.g., in the lung) with an imaging
device for nuclear radiation, such as a scanner or gamma camera.
Vascular obstructions in the lower extremities can sometimes be detected by
measuring differences in the skin temperature caused by the reduced circulation.
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DEFIBRILLTOR:
Introduction:
Defibrillator is an electronic device that creates a sustained myocardial depolarization
of a patient’s heart in order to stop ventricular fibrillation or atrial fibrillation.
The instrument for administering the electric shock is called as defibrillator.
Defibrillation is the application of electric shock to the area of the heart which makes
all the heart muscle fibers enter their refractory period together, after that normal heart action
may resume.
If the heart does not recover spontaneously after delivering the shock to the heart
using defibrillator then a pacemaker may be employed to restart the rhythmic contraction of
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the myocardium.
Ventricular fibrillation is dangerous when compared to arterial fibrillation.
Defibrillator types:
There are two types of defibrillators based on the electrodes placement.
a) Internal defibrillator (Surgical Type)
b) External defibrillator (Therapeutic Type)
(a) Internal defibrillator:
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
It is used when chest is opened.

Here large spoon shaped electrodes with insulated handle are used.

Electrodes in the form of fine wires of Teflon coated stainless steel are used.

There are AC and DC defibrillator methods but DC defibrillator is used today.

Since the electrode comes in direct contact with the heart, the contact impedance is
about 50 ohms.
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
In internal defibrillation, the heart requires excitation energy of about 15 to 50 J.

The duration of the shock is about 2.5 to 5 milliseconds.

The spoon shaped electrode is as shown below.
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(b) External defibrillator:
 It is used on the chest.
 Here paddle shaped electrodes are used.
 There are AC and DC defibrillator methods but DC defibrillator is used today.
 Since the electrodes are placed above the chest, the contact impedance on the chest is
about 100 ohms even after applying the gel.
 In external defibrillation, the heart requires excitation energy of about 50 to 400 J.
 The duration of the shock is about 1 to 5 milliseconds.
 The paddle shaped electrode is as shown below.
 The bottom of the electrode consists of a copper disc with 3 to 5 cm diameter for
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pediatric patient and 8 to 10 cm diameter for adult patients with a highly insulated handle.
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Mechanism:
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Fibrillation results from a rapid discharge of impulses from a single or multiple foci in
the atria or in the ventricles. The atria or the ventricles are unable to respond completely and
effectively to each stimulus.
Under conditions of atrial fibrillation, the ventricles can still function normally but
they respond with irregular rhythms to the non-synchronized bombardment of electrical
stimulation from the fibrillating atria and the circulation is still maintained although not as
efficiently.
The sensation produced by the fibrillating atria and irregular ventricular action can be
quite traumatic for the patient. Ventricular fibrillation is dangerous when the ventricles are
unable to pump the blood. Hence, resuscitative measures must be applied within 5 minutes or
less after the attack or irreversible brain damage and death will occur.
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Types of defibrillator based on operation or Voltage delivered:
There are six types of defibrillators based on the nature of the output voltage
delivered. They are,
1. AC defibrillator
2. DC defibrillator
3. Synchronized DC defibrillator
4. Square pulse DC defibrillator
5. Double square pulse DC defibrillator
6. Biphasic DC defibrillator
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1. A.C. defibrillator:
Although mechanical methods like chest massage for defibrillation have been tried for
years, the most successful method of defibrillation is the application of electric shock to the
area of the heart which makes all the heart muscle fibers enter their refractory period together
after which normal heart action may resume.
One of the earliest forms of an electrical defibrillator is the AC defibrillator, which
applies several cycles of alternating current to the heart from the power line through a step-up
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transformer with various tappings on the secondary side.
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AC defibrillator
An electronic timer circuit is connected to the primary of the transformer. This timer
connects the output to the electrodes for a pre-set time. The timing device may be a simple
capacitor and resistor network or a monostable multivibrator which is triggered by a foot
switch or a push button switch.
The duration of the counter shock may vary from 0.1 second to 1 second depending
upon the voltage to be applied.
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For reasons of safety, the secondary coil of the transformer should be isolated from
earth so that there is no shock risk to anyone touching an earthed object. For external
defibrillation, the voltages are in the range from 250V to 750V. For internal defibrillation, the
voltages are in the range from 60V to 250V.
Disadvantages:
1. Large currents are required in external defibrillation to produce uniform and
simultaneous contraction of the heart muscles fibers. This current not only causes
a violent contraction of the thoracic muscles but also results in occasional burning
of the skin under the electrodes.
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2. It produces atrium fibrillation while arresting the ventricular fibrillation.
2. Capacitive Discharge D.C. Defibrillators
The ventricular fibrillation is terminated by passing a high energy shock through
discharging a capacitor to the exposed heart, or to the chest of the patient. The capacitive
discharge type DC Defibrillator is as shown in the figure below.
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DC defibrillator
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The 230V AC main supply is connected to a variable autotransformer in the primary
circuit. The output of the autotransformer is fed as input to a step-up transformer to produce
high voltage with a rms value of about 8000 V.
A half-wave rectifier rectifies this high AC voltage to obtain DC voltage, which
charges the capacitor C.
The voltage to which C is charged is determined by the autotransformer in the
primary circuit.
A series resistance, Rs, limits the charging current to protect the components.
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An AC voltmeter across the primary is calibrated to indicate the energy stored in the
capacitor.
Five times the RC time constant circuit is required to reach 99% of a full charge-a
value it should reach in 10 seconds, which means that the time constant must be less than 2 s.
With the electrodes firmly placed at appropriate positions on the chest, the clinician or
technician discharges the capacitor by momentarily changing the switch S from position 1 to
position 2.
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The capacitor is discharged through the electrodes and the patient's torso represented
by a resistive load, and the inductor L.
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The inductor is used to shape the wave in order to eliminate a sharp, undesirable
current spike that would occur at the beginning of the discharge.
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The energy delivered to the patient is represented by the typical waveform shown in
figure above.
The area under the curve is proportional to the energy delivered.
The wave is monophasic and the peak value of the current is nearly 20 A.
Depending on the defibrillator energy setting, the amount of electrical energy
discharged by the capacitor may range between 100 and 400 watts or joules when the
electrodes are applied externally and the duration of the effective portion of the discharge is
approximately 5 m/s.
Once the discharge is completed, the switch automatically returns to position 1 and
the process can be repeated, if necessary.
When the electrodes are applied directly to the heart, about 50 to 100 joules only is
required for defibrillation.
The energy stored in the capacitor is given by the equation
W=
CV2
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Where, C is the capacitance and V is the voltage to which the capacitor is charged.
Capacitors used in the defibrillator range from 10 to 50µF. Thus, the voltage for a
maximum of 400 J ranges from 2 to 9 KV, depending on the size of the capacitor.
Delay-Line Capacitive Discharge DC Defibrillator
Even with DC defibrillation, there is a danger of damage to the myocardium and the
chest walls, because peak voltages as high as 6000 V may be used.
To reduce this risk, some defibrillators produce dual-peak waveforms of longer
duration (approximately 10 m/s) at a much lower voltage.
The circuit diagram of such a system is shown in figure below. The parallel combination of
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C1 and C2 stores the same energy as the single capacitor in the above figure. But its discharge
characteristic is more rectangular in shape (1onger duration of approximately 10 m/s) at a
much lower voltage, as shown in figure below.
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With this type of waveform, effective defibrillation can be achieved in adults with lower
levels of delivered energy - between 50 and 200 watts.
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3. Synchronised DC defibrillator:
Defibrillation is a risky procedure since if it is applied incorrectly; it could
induce fibrillations in a normal heart. There must be proper diagnosis for ventricular
fibrillation.
Simple DC defibrillator can arrest the ventricular fibrillation. But for termination of
ventricular tachycardia, atrial fibrillation and other arrhythmias it is essential to defibrillator
with synchronizer circuit.
There are two vulnerable zones in a normal cardiac cycle, T wave and U wave
segments. If the counter shock falls in the T segment then the ventricular fibrillation is
developed. If the counter shock falls in the U wave segment then atrial fibrillation is
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produced.
DC defibrillator circuit consisting of defibrillator, electrocardioscope and pacemaker
is as shown in the figure below.
The pacemaker is used in the case of emergency as a temporary pacing.
It includes diagnostic circuitry which is used to assess the fibrillation before
delivering the defibrillation pulse and synchroniser circuitry which is used to deliver the
defibrillation pulse at the correct time. So, as to eliminate the ventricular fibrillation or atrial
fibrillation without inducing them.
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Working:
1.
The electrocardiogram is obtained by means of an ECG unit, connected to the patient
who is going to receive defibrillation pulse.
2.
The switch is placed in the defibrillator mode if ventricular fibrillation is suspected.
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3.
The QRS detector in that mode consists of a threshold circuit that would pass a signal
as output if R wave is absent in the electrocardiogram. Other it would not give any
output if R Wave is present.
4.
Meanwhile the medical attendant energizes the switch to deliver a defibrillation pulse.
5.
The AND gate 'B' delivers on signal to the defibrillator only when the ‘R’ wave is
absent, provided the signal from the medical attendant is also present at one of the two
inputs of that AND gate.
6.
At the two inputs of AND gate 'B' if any one of the inputs is missing, then it would
not give any output. By this way the defibrillator is inhibited and would not deliver
the defibrillation pulse.
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7.
The fibrillation detector searches the ECG signal for frequency components above
150 Hz. If they are present, fibrillation is probable and the fibrillation detector gives
an output signal. A defibrillator pulse is delivered only if the fibrillation detector
produces an output at the same time that the attendant energizes the switch. This is
provided by the AND gate 'C'.
8.
Thus when the AND gate B and AND gate C are simultaneously triggering the
defibrillator, the defibrillation pulse is delivered.
9.
In the synchronization mode, the defibrillator is synchronised with the ECG unit.
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Suppose a patient is suffered by atrial fibrillation. First the doctor diagnoses it
correctly and then the treatment is initiated using this circuit.
10.
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The ECG signal in the instrument is given to QRS detector. Its output is used to time
the delivery of the defibrillation pulse with a delay of 30 milliseconds. At this time,
the ventricles will be in uniform state of depolarisation and the normal heart beat will
not be disturbed.
This delay of 30 milliseconds after the occurrence of R wave allows the
attendant to defibrillate atrium without inducing ventricular fibrillation.
4. Square wave defibrillator:
In this defibrillator, capacitor is discharged through the subject by turning on a series
silicon controlled rectifier (SCR). When sufficient energy has been delivered to the subject a
shunt SCR short circuits the capacitor and terminates the pulse.
The output can be controlled by varying the voltage on the capacitor or duration of
discharge.
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Here the defibrillation is obtained at low peak current and so there is no side effect.
Digital circuits can also produce a square pulse used for defibrillation.
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Analysis
In the figure above, Ro is the internal resistance of the defibrillator, RE is the electrode
- skin resistance and RT is the thorax resistance.
The Energy in the pulse is,
EP = VDIDTD
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Where, VD and ID are the instantaneous voltage and current available from the defibrillator
pulse respectively and TD is the duration of the pulse.
Total circuit resistance, R = RD + 2RE + RT
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Further, the energy in the pulse can also be written in terms of voltage and
resistance between the cable attached to the patient such that
2
VD
EP =
.TD = ID2 (2RE + RT)TD
2 R E  RT
The energy loss in the defibrillator
EDL = ID2RDTD
The energy loss in each electrode and skin,
EEL = ID2RETD
Energy delivered to the thorax,
ET = ID2RTTD
=
RT
EP
2 R E  RT
From the above equation we can know that the energy in the pulse is not delivered completely to
thorax. Similarly the energy delivered to the thorax can be expressed in the form of available
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energy from the capacitor discharge in the case of DC defibrillator whose output is assumed to a
square pulse.
Energy available from the capacitor,
EC = ID2RTD
=
EDL + 2EEL + ET
 ET = EC - EDL - 2EEL
(Or) ET =
RT
EC
2 R E  RT  R D
Thus the energy delivered to the thorax, ET is diminised from the available energy due to
effects of resistance of defibrillator and electrode-skin resistance.
Advantages:
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The advantages of square wave defibrillator are,
1. It requires low peak current
2. It requires no inductor
3. It is possible to use physically smaller electrolytic capacitors.
5. Double Square Pulse Defibrillator:
Double square pulse defibrillator is normally used after the open heart surgery.
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Conventional DC and AC defibrillators are producing myocardial injury with a
diminished ventricular function for a period of approximately 30 minutes following the
delivery of shock.
If the chest is opened, only lower energy electric shock should be given. Instead of
800 – 1500V, employed in DC capacitor discharge in the case of DC defibrillators,
Here 8-60 V double pulse is applied with a mean energy of 2.4 watt-second as shown
in figure above.
When the first pulse is delivered, some of the fibrillating cells will be excitable and
will be depolarised. However cells which are in refractory during the occurrence of first pulse
will continue to fibrillate.
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In order to, obtain a total defibrillation; the second pulse operates on latter group of
cells. The pulse amplitude and width together with the interval should be such that the cells
defibrillated by the first pulse will be refractory to the second pulse.
The timing of the second pulse should be such that those cells which were refractory
to the first pulse are now become excitable. Thus complete defibrillation can be obtained by
means of selecting proper pulse-space ratio.
Using double square pulse defibrillator, efficient and quick recovery of the heart to
beat in the normal manner without any side effect like burning of myocardium or inducement
of ventricular or atrial fibrillation.
The double square pulse with the required pulse-space ratio can be produced with the
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use of digital circuits similar to those digital pacemaker circuits.
6. Biphasic DC defibrillator
Biphasic DC defibrillator is similar to the double square pulse defibrillator such that it
delivers DC pulses alternatively in opposite directions. This type of waveform is found to be
more efficient for defibrillation of the ventricular muscles.
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PACEMAKER
INTRODUCTION:
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Pacemaker is an electrical pulse generator used for starting and/or maintaining the
normal heart beat. It generates artificial pacing impulses and delivers them to the heart.
The output of the pacemaker is applied either externally to the chest or internally to
the heart muscle.
In the case of cardiac stand still, the use of the pacemaker is temporary - just long
enough to start a normal heart rhythm. But in the case requiring long term pacing, the
pacemaker is surgically implanted in the body and its electrodes are in direct contact with the
heart.
In cardiac diseases, where the ventricular rate is too low, it can be increased to normal
rate by using pacemaker.
By fixing the artificial electronic pacemaker, the above defects in the heart can be
eliminated.
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Energy requirements to excite the heart muscle:
Like all muscle tissues, the heart muscle can be stimulated with an electric shock.
The minimum energy required to excite the heart muscle is about 10 µJ. For better
stimulation and safety purposes, a pulse of energy 100 µJ is applied on the heart muscle. i.e.,
a pulse of 5 V, 10 mA and 2 milli seconds duration is used.
Too high a pulse energy may provoke ventricular fibrillation. Ventricular fibrillation
is a dangerous condition. During that time, the ventricular muscle contracts so rapidly and
irregularly that the ventricles fail to fill the blood and circulatory arrest follows. The patient
loses consciousness in 10-15 seconds and the brain cells die within a few minutes from
oxygen deficiency in the brain. This is caused by a pulse of energy 400 µJ.
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Shape of pacemaker pulse
The above figure shows the shape of the pacemaker pulses. These pulses should have
the pulse to space ratio 1:10000 and that should be negatively going pulses to avoid the
ionization of the muscles.
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The pulse voltage is made variable to allow adjustments in the energy delivered by the
pacemaker to the heart during each pulse.
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During the pulse duration, the stimulus voltage drives energy into the heart muscles.
The pulse repetition rate is usually 70 pulses/min but many pacemakers are adjustable
in the range of 50-150 pulses/min. The duration of each pulse is between 1 and 2 milli
seconds.
Output pulses from the pacemaker appear at the pair of electrodes used for triggering
the heart.
The typical ranges of parameters of the pacemakers available today are,
S.No
1
2
3
4
5
6
7
Parameters
Pulse rate
Pulse width
Pulse amplitude
Battery capacity
Longevity
End-of-life indicator
Weight
-
Ranges
25 - 155 pulses per minute
0.1 - 2.3 milliseconds
2.5 - 10 volts
0.44 - 3.2 amp-hours
3.5 - 18 years
2 - 10% dropin pulse rate
33 - 98 grams
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Size
-
22 - 80 cm3
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Encapsulization
-
Silicon rubber, stainless steel, titanium
Methods of Stimulation:
There are two types of stimulation
1. External Stimulation and
2. Internal Stimulation
1. External Stimulation:
 External stimulation is employed to restart the normal rhythm of the heart in
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the case of cardiac stand still. Stand still can occur during open heart surgery
or whenever there is a sudden physical shock or accident.
 The paddle shaped electrodes are applied on the surface of the chest
 Currents in the range of 20 - 150 mA are employed.
2. Internal Stimulation:
 Internal stimulation is employed in cases requiring long term pacing because
of permanent damage.
 The electrodes are in the form of fine wires of teflon coated stainless steel are
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used. In some cases, during restarting of the heart after open heart surgery,
spoon like electrodes are used.
 The currents in the range of 2-15 mA are employed.
Classification of Pacemakers based on placement:
Based on the placement of the pacemaker, there are two types
1. External pacemaker and
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2. Internal (Implanted) pacemaker
3.
S.
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External Pacemaker
The pacemaker is placed outside the
body. It may be in the form of wrist
1
watch or in the pocket, from that one
wire will go into the heart through the
vein.
Internal Pacemaker
The pacemaker is miniaturized and is
surgically implanted beneath the skin near
the chest or abdomen with its output leads
are connected directly to the heart muscle.
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The electrodes are called endocardiac
electrodes and are applied to the heart by The electrodes are called myocardiac
means of an electrode catheter with electrodes and are in contact with the outer
2
electrode's tip situated in the apex of the wall of the myocardium.
right ventricle. These are in contact with
the inner surface of the heart chamber.
3
It does not need the open chest surgery
The battery can be easily replaced and
any defect or adjustment in the circuit
It requires a minor surgery to place the
circuit.
The battery can be replaced only by minor
surgery. Further any defect or adjustment in
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4
can be easily attended without getting
any help from a medical doctor.
During placement, swelling and pain do
5
not arise due to minimum foreign body
reaction.
Here there is no safety for the pacemaker
6
particularly in the case of children
carrying the pacemaker.
the circuit cannot be easily attended.
Doctor's' help is necessary to rectify the
defect in the circuit.
During placement swelling and pain arise
due to foreign body reaction.
Here there is a cent percent safety for the
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circuit from the external disturbances.
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Mostly these are used for temporary Mostly these are used for permanent heart
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heart irregularities
damages.
Different modes of Operation:
Based on the modes of operation of the pacemakers, they can be divided into five
types,
1. Ventricular asynchronous pacemaker (fixed rate pacemaker)
2. Ventricular synchronous pacemaker (Standby pacemaker) / (R-wave triggered)
3. Ventricular inhibited pacemaker (Demand pacemaker) / (R-wave inhibited)
4. Atrial synchronous pacemaker (P-wave synchronized)
5. Atrial sequential ventricular inhibited pacemaker
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1. Ventricular asynchronous pacemaker (fixed rate pacemaker)
It is the simplest pulse generator. This pacemaker is suitable for patients with either a
stable, total AV block, a slow atrial rate or atrial arrhythmia. It is basically a simple astable
multivibrator. It produces the pulses at fixed rate irrespective of the behavior of heart rhythm.
It consists of a square wave generator (first differential amplifier circuit) and a
positive edge triggered monostable multivibrator (second differential amplifier circuit with
diodes).
The period of the square wave generator is given by
T = − 2RC ln
1
1
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Where ‘’ is the feedback voltage fraction such that

R2
R1  R2
The period of the oscillator can be changed by changing ‘’ or the time constant RC.
The maximum output voltage is always equal to the modulus of the saturation voltage |Vsat|
of the voltage level detector.
The square wave generator is an astable multivibrator which periodically switches
between the output voltages |V sat| and -|Vsat|.
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The output of the square wave generator is coupled to the positive edge triggered
monostable multivibrator circuit.
A step at the trigger input will pass through the capacitor Cc and the diode will raise
the voltage at the lower node (non-inverting terminal) of the second differential amplifier.
The capacitor Cc is chosen so as to make five time constants equal to the pulse
duration TD. Otherwise the trigger would still be present after TD has passed and a second
pulse would be wrongly generated. Therefore TD is so chosen such that
 R R 
 R3 
TD = 5Cc  3 4  = − R5 Cm ln 

 R3  R 4 
 R3  R 4 
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Advantages:
1. It has the simplest mechanism and the longest battery life.
2. It is cheap.
3. It is least sensitive to outside interference.
Disadvantages:
1.
There may be competition between the natural heart beats and pacemaker
beats
2.
greater physical effort.
3.
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Using the fixed rate pacemaker, the heart rate cannot be increased to match
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Stimulation with a fixed impulse frequency results in the ventricles and
atria beating at different rates. This varies the stroke volume of the heart
causes some loss in the cardiac output.
4.
Possibility for ventricular fibrillation will be more.
2. Ventricular Synchronous pacemaker (Standby Pacemaker)
Ventricular synchronised pacemaker can be used only for patients with short periods
of AV block or bundle block.
This pacemaker does not compete with the normal heart activity. The block diagram
of ventricular synch pacemaker is as shown in the figure below.
A single transverse electrode placed in the right ventricle senses both R wave as well
as delivers the stimulation so, no separate sensing electrode is required.
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A R wave from an atrial generated ventricular contraction triggers the ventricular
synchronised pacemaker which provides an impulse falling in the lower part of the normal
QRS complex. This ensures that the pacemaker does not interfere with the sinus rhythm.
If atrial generated ventricular contractions are absent then the pacemaker provides impulses at
a basic frequency of 70 impulses/minute. Thus it provides impulses only when the atrial
generated ventricular contractions are absent.
Working:
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Using the sensing electrode, the heart rate is detected and is given to the timing circuit
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in the pacemaker. If the detected heart rate is below a certain minimum level, the fixed rate
pacemaker is turned on to pace the heart.
The lead used to detect the R wave is now used to stimulate the heart. If a natural
contraction occurs, the asynchronous pacer's timing circuit is reset so that it will time its next
pulse to detect heart beat. Otherwise the asynchronous pacemaker produces pulses at its
preset rate.
The pacemaker may detect noise and interpret as its ventricular excitation so to
eliminate this refractory period circuit or gate circuit is used.
In heart blocks, P waves occur at random times with respect to ventricular excitation.
However P and R waves have their principal energy in different frequency bands.
A high pass filter with a lower cut off frequency at 20 Hz almost completely
eliminates the P wave. The R wave is differentiated by such a filter and its peak to peak
amplitude is increased using an input amplifier.
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Advantages:
1. To arrest the ventricular fibrillation, this circuit can be used.
2. If the R-wave occurs with its normal value in amplitude and frequency then it
would not work. Therefore the power consumption is reduced
3. There is no chance of getting side effects due to competition between natural and
artificial pacemaker pulses.
4. When the R wave is appearing with lesser amplitude, the circuit amplifies it and
delivers it in proper form. If the R wave period is too low or too high, the
asynchronous pacer in the circuit is working up to the returning of the heart into
normal one.
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Disadvantages:
1. Atrial and ventricular contractions are not synchronized.
2. In the olden type, the circuit is more sensitive to external electromagnetic
interferences such as electric shavers, microwave ovens, car ignition systems, air
port security metal detectors, and so on. Therefore the patients could not work in
radio or T.V. stations.
3. Ventricular Inhibited Pacemaker (Demand Pacemaker)
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The R wave inhibited pacemaker allows the heart to pace at its normal rhythm when it
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is able to. However if the R wave is missing for a preset period of time, the pacemaker will
supply a stimulus. Therefore if the heart rate falls below a predetermined level then
pacemaker will turn on and provide the heart a stimulus. For this reason it is called as
demand pacemaker.
There is also a piezoelectric sensor shielded inside the pacemaker casing. When the
sensor is slightly stressed or bent by the patient's body activity, pacemaker can automatically
increase or decrease its rate. Thus it can match with the greater physical effort.
The sensing electrode picks up R wave. The refractory circuit provides a period of
time following an output pulse or a sensed R-wave during which the amplifier in the sensing
circuit will not respond to outside signals.
The sensing circuit detects the R wave and resets the oscillator.
The reversion circuit allows the amplifier to detect R wave in low level signal to noise
ratio. In the absence of R wave, it allows the oscillator in the timing circuit to deliver pulses
at its preset rate.
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The timing circuit consists of an RC network, a reference voltage source and a
comparator which determines the basic pulse rate of the pulse generator. The output of the
timing circuit is fed into pulse width circuit which is also a RC network.
The pulse width circuit determines the duration of the pulse delivered to the heart.
Then the output of the pulse width circuit is fed into the rate limiting circuit which limits the
pacing rate to a maximum of 120 pulses per minute.
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The output circuit provides a proper pulse to stimulate the heart. Thus the timing
circuit, pulse width circuit, rate limiting circuit and output circuit are used to produce the
desired pacemaker pulses to pace the heart.
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There is a special circuit called voltage monitor which senses the cell depletion and
signals the rate slow-down circuit and energy compensation circuit of this event.
The rate slow-down circuit shuts off some of the current to the basic timing network
to cause the rate to slow-down 8±3 beats per minute when cell depletion has occurred.
The energy - compensation circuit produces an increase in the pulse duration as the
battery voltage decreases to maintain constant stimulation energy to the heart.
4. Atrial Synchronous Pacemaker
This type of pacing is used for young patients with a mostly stable block. It can act as
a temporary pacemaker for the atrial fibrillation.
The atrial activity is picked up by a sensing electrode placed in a tissue close to the
dorsal wall of the atrium.
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The detected P wave is amplified and a delay of 0.12 second is provided by the AV
delay circuit. This is necessary corresponding to the actual delay in conducting the P wave to
the AV node in the heart. The signal is then used to trigger the resetable multivibrator.
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The output of the multivibrator is given to the amplifier which produces the desired
stimulus to be applied to the heart. The stimulus is delivered to the ventricle through the
ventricular electrode.
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If the rate of atrial excitation becomes too fast as in atrial fibrillation or too slow or
absent, a preset fixed rate pacemaker (resetable multivibrator) takes over until the abnormal
situation is over.
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Normally pacemaker pulse is so large that it would be detected by the atrial pick up
leads and cause the heart to beat. This problem has been eliminated by refractory period
control circuit. i.e., any signal detected on the atrial lead within 400 milliseconds of a paced
heart beat is ignored.
5. Atrial sequential ventricular inhibited pacemaker:
It has the capability of stimulating both the atria and ventricles and adopts its method
of stimulation to the patient’s needs.
If atrial function fails, this pacemaker will stimulate the atrium and then sense the
subsequent ventricular beat. If it is working properly it will discontinue its ventricular
stimulating function. However if atrial beat is not conducted to ventricle, the pacemaker on
sensing this will fire the ventricle at a preset interval of 0.12 second.
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UNIT IV
wPHYSICAL
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MEDICINE AND
.BIOTELEMETRY
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UNIT IV PHYSICAL MEDICINE AND BIOTELEMETRY
Diathermies- Shortwave, ultrasonic and microwave type and their applications, Surgical
Diathermy Telemetry principles, frequency selection, biotelemetry, radiopill, electrical
safety.
Radio pill
Radio pill when swallowed, will travel the GI tract (Gastrointestinal tract) and
simultaneously perform multiparameter in physiological analysis. After completing its
mission it will come out of the human body by normal bowel movement.
The pill is 10mm in diameter and 30mm long weighing around 5gm and records
parameters like temperature, pH, conductivity and dissolved oxygen in real time.
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The pill comprises an outer biocompatible capsule encasing micro sensors, a control
chip, radio transmitter and two silver-oxide cells.
INSIDE THE CAPSULE:
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The schematic diagram of the microelectronic pill is as shown in figure below. The
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outer casing of the pill is made by machining chemically resistant polyetheterketone, which is
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biocompatible. It is made up of two halves, which are joined together by screwing.
The pill houses a PCB chip carrier that acts as a common platform for attachment of,
1. sensors,
2. application- specific integrated circuit (ASIC),
3. radio transmitter and
4. batteries.
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Task of the sensors:
The device is provided with four micro sensors, namely
1. a silicon diode,
2. an ion-selective field effect transistor (ISFET),
3. a pair of direct- -contact gold electrodes and
4. a 3-electrode electrochemical cell.
1. Silicon diode:
The silicon diode is used to measure the body core temperature and also identify local
changes associated with tissue inflammation and ulcers.
2. ISFET:
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1. It is used to measure pH.
2. It is used to determine the presence of pathological conditions associated with
abnormal pH levels, particularly associated with pancreatic disease, hypertension,
inflammatory bowel disease, the activity of fermenting bacteria, the level of acid
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excretion, reflux to the oesophagus and the effect of GI-specific drugs on target
organs.
3. Gold electrodes:
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A pair of direct contact gold electrode is used to measure conductivity.
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The conductivity sensor is used to monitor the contents of the GI tract by measuring water
and salt absorption, bile secretion and the breakdown of organic components into charged
colloids.
4. 3- electrode electrochemical cell:
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The 3-electrode electrochemical cell is used to detect the level of dissolved oxygen in
solution.
The oxygen sensor measures the oxygen gradient from the proximal to the distal GI
tract. This enables a variety of syndromes to be investigated including the growth of aerobic
bacteria or bacterial infection.
Control chip:
The ASIC is the control unit that connects together other components of the
microsystem as shown in the figure below.
It contains an analogue signal conditioning module operating the sensors, 10-bit ADC
and DAC converters and a digital data processing module. An oscillator provides the clock
signal.
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The temperature circuitry biases the diode at constant current so a change in
temperature reflects a corresponding change in diode voltage.
The pH ISFET sensor is biased as a simple source and drain follower at constant
current with the drain-source voltage changing with the threshold voltage and pH.
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The conductivity circuit operates at direct current, measuring the resistance across the
electrode pair as an inverse function of solution
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conductivity. An incorporated potentiostat
circuit operates the amperometric oxygen sensor with a 10-bit DAC controlling the working
electrode potential with respect to the reference.
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The analogue signals have a full-scale dynamic range of 2.8V with the resolution
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determined by the ADC. These are sequenced through a multiplexer prior of being digitized
by the ADC. The bandwidth for each channel is limited by the sampling interval of 0.2msec.
The digital data processing module processes the digitized signals through the use of a
serial bit stream data compression algorithm, which decides when transmission is required by
comparing the most recent sample with the previous sampled data. The digital module is
clocked at 32KHz and employs a sleep mode to conserve power from the analogue module.
Radio transmitter:
The size of the transmitter is 8x5x3mm. The transmission range is one meter and the
modulation scheme frequency shift keying has a data rate of 1 kbps. The transmitter is
designed to operate at a transmission frequency of 40.01 MHz at 20C generating a signal of
10KHz bandwidth.
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Power consumption:
Two SR44 Ag2O batteries are used, which provide an operating time of more than 40
hours of the microsystem. The power consumption of the system is around 12.1mW and
current consumption is around 3.9mA at 3.1V supply.
The ASIC and sensor consume 5.3mW corresponding to 1.7mA of current and the
free running radio transmitter consumes 6.8mW at 2.2mA of current.
Range of measurement:
The microsystem can measure,
1. Temperature from 0 to 70C,
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2. pH from 1 to 13,
3. Dissolved oxygen up to 8.2mg/litre,
4. Conductivity from 0.05 to 10 ms.cm-1( s=siemens).
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Diathermy
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The term diathermy means “thorough heating” or producing deep heating directly in
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the tissues of the body. Applying heat to a particular area of the body increases the
temperature of the tissues and the flow of the blood in that area because of dilating the blood
vessels.
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There are several methods of raising the tissue temperature below burning levels.
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1. Conductive heating, which raises skin temperature but does not penetrate very deeply.
Eg: Hot compresses, Infrared heat lamp.
The externally applied sources of heat like hot towels, heat lamps and electric heating
pads often produce discomfort and skin burns before the heat has penetrated to the deep
tissues.
2. Diathermy, deep heat is produced without direct neuromuscular stimulation.
With diathermy technique, the subject‟s body becomes a part of the electrical circuit
and the heat is produced within the body and not transferred through the skin.
Another advantage of diathermy is that the treatment can be controlled precisely.
The amount of heat can be closely adjusted by means of circuit parameters.
Thus, Using higher frequency energy, deeper lying tissues Eg: muscles, bones,
internal organs etc.. can be provided with heat.
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Surgical Diathermy:
In therapeutic applications, high frequency currents are used. Apart from this, it can
also be used in operating rooms for surgery.
This diathermy is used for cutting, coagulation and blending in operating room.
In surgical diathermy a high frequency current of 300 – 3000 KHz is applied on the
skin surface. This frequency is much higher when compared to therapeutic diathermy
machines. When this high frequency current flows through sharp edge of a wire or a needle,
the concentration of current at point is higher. Now if this wire or electrode is placed on the
skin surface, the tissues are heated to such an extent that the cells which are immediately
under the electrode are boiled.
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The indifferent electrode establishes a large contact area with the patient. Therefore
the RF current is displayed and only a little heat is developed at the electrode tip.
Electrosurgical Unit (ESU):
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The Electrosurgical Unit (ESU) consists of
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1. Radio frequency Oscillator
2. Cutting Electrode
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3. Reference Electrode (Return Electrode)
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The ESU consists of a RF oscillator which produces a frequency of 300 – 3000 KHZ.
The edge of the active electrode is too dull to cut the tissues without the RF current produced
by oscillator.
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When the electrode is placed far away from the body, no current flows and no cutting
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action takes place. When the electrode is brought closer to the body, a spark will appear at the
tip of the electrode. The voltage on the electrode may range from 1000 – 10000 volts peak to
peak.
When the electrode touches the skin, no spark will be present. When high frequency
current is applied, it passes through individual cell membranes by capacitive coupling. At
these frequencies, large current flow into the cells causing it to vapourize and thereby causing
a rupture of the tissue close to the cutting electrode.
The return electrode must have large surface area to minimize the heating effect and
prevent surface burns. The ESU setup is as shown below.
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ESU Advantages:
1. It can cut faster
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2. It can Avoid blood bleeding
Active Electrode:
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Blade type electrode is used as active electrode. This electrode is 1mm in thickness
and 10mm in width. This electrode is used for cutting.
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In cutting mode, a sinusoidal steady state oscillating current is produced at the tip of
the electrode. The tip of the electrode is placed at a short distance from the skin; a sharply
directed spark appears at the tip of the electrode. The voltage will be same along the
electrode.
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In coagulation mode, the electrode produces a wide spark.
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The tip of the electrode is blunt so that the maximum voltage appears somewhere
other than at the tip and produces a spraying effect in the spark.
The spraying spark tends to cauterize the tissue and coagulate the blood.
Spherical, Bipolar and hemostat electrodes are used for cauterization and coagulation.
Spherical Electrode:
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The spark produced by the spherical tip electrode tends to broaden across the tissue
because there are several equidistant paths from the skin to the electrode surface.
Hemostat Electrode:
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Hemostat electrode is used to lift and clamp tissue especially a bleeding vessel.
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The current flows broadly through the grasped tissue causing coagulation effect.
Bipolar Electrode:
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A Bipolar electrode is arranged on the tip of a forceps. Each tip has separate electrical
path. One from ESU active head and another to the patient return.
When in use, these 2 electrodes contact tissues in close proximity to each other.
In this case, it is not necessary to have return electrode.
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Block Diagram:
The block diagram of ESU along with waveform is as shown below.
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Cut Mode:
Output power of cut mode is 400W in 500 ohm load voltage may range from 1000V
to 10,000V peak to peak.
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Coagulation mode:
The coagulation pulse train is a digital pulse that modulates the RF output according
to the selected duty cycle.
Pulse Train
Duty Cycle
Power (W)
Cut
100
400
Coag
33
132
Blend1
50
200
Blend2
75
300
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The cut setting vaporizes the cell.
The coagulation setting heats the tissue.
The Blending setting is used by the surgeon along with the power amplifier gain
control to create the degree and speed of coagulation desired.
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In order to measure the output power of ESU unit and its leakage current, an ESU
analyzer is used.
Electrosurgical Unit Safety hints:
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1. Do not use in the presence of combustible materials such as flammable anesthesia
agents or flammable disinfection agents.
2. Bowel gas contains explosive methane. So, use protective measures during
electro surgery involving the bowel or colon.
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3. Low frequency leakage current may be present in ESU active electrode,
presenting a microshock.
4. Place the return electrode as close as possible to the operative point.
5. Avoid placing patient electrode over scar tissue, Bony prominences, unshaved
hairy areas, metal implants or wet areas.
6. Place ESU patient electrode at right angles to a pacemaker to avoid ESU
interference.
7. Rigid metal dispersive electrodes should only be used on the buttocks, where the
weight of the body will maintain good contact.
8. Adhesive disposable electrodes should not be used where the weight of the body
would put uneven pressure on the electrode, causing hot spots and possible burns.
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Types of Diathermy
Three types of diathermy used for convention heating are,
1. Shortwave diathermy
2. Microwave diathermy
3. Ultrasonic diathermy
The first two involve electromagnetic effects and the third involves a
mechanical effect.
Shortwave Diathermy:
Here, the heating of the tissue is carried out at a high frequency of 27.12MHZ and a
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wavelength of 11m. When we use currents with very high frequencies, the motor or sensory
nerves are not stimulated and there is no contraction of the muscles. Thus, there is no
discomfort to the patient. In this method, the output of the oscillator is applied to the pair of
patient electrodes.
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Block Diagram of a shortwave Diathermy
The RF energy heats the tissues and promotes the healing of injured tissues and
inflammations. The power delivered bf the unit is about 500 W. There are several provisions
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to regulate the intensity of the current passed through the patient circuit. The electrodes or
pads are not directly in contact with the skin. Usually, layers of towels are interposed
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between the metal and the surface of the body. The pads are placed so that the portion of the
body to be treated is sandwiched between them. The pads form capacitor plates and the body
tissues between the pads act as a dielectric. Thus, the whole arrangement forms a capacitor.
When the RF current is applied to the pads, the dielectric loss of the capacitor produces heat
in the intervening tissues. This technique is called the condenser or capacitor method.
Further, there is also an inductive method, in which a flexible cable is coiled around
the arm or knee or any other portion of the patient's body, where plate electrodes are
inconvenient to use. When a RF current is passed though the cable, deep heating in the tissue
results from the electrostatic field set up between its ends and the superficial tissues are
heated by eddy currents set up by the magnetic field around the cable, as shown in Figure
below. This technique is known as Inductothermy.
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Methods of Applying Electrodes in Short-wave Diathermy Treatment. (a) Condenser Method,
(b) Inductive Method
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Instead of continuous RF waves, RF pulses of 65s (with an interval between pulses
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of 1600 s) are also used. This is called Diapulse shortwave diathermy. The rate of pulsations
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is adjustable in steps from 80 to 600 pulses per second with the peak power of the pulse
ranging from 293 to 975 W. By this method, the excess tissue fluid associated with cellular
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damage is reduced; the healing rate is enhanced and the depth of penetration is correctly
adjusted. There is no danger of bums or hyperthermic complications.
Microwave Diathermy
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Here, the frequency used is 2450 MHz, corresponding to a wavelength of 12.25 cm.
Heating of the tissues is produced due to absorption of the microwave energy. Better
therapeutic results are obtained by using microwave diathermy rather than short-wave
diathermy. Here, there is no pad-shaped electrode. Instead, the microwaves are transmitted
into the portion of the body to be treated directly from the director of the unit. Normally,
magnetrons are used to produce microwaves.
A delay of about 3 or 4 minutes is required for the warming of the magnetron. An
arrangement is made such that a lamp lights up after 4 minutes indicating that the magnetron
is ready to deliver its output to the director. Proper cooling of the magnetron is provided.
Further, the interference suppression filter in the circuit bypasses the high-frequency current
pick-up.
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Excessive dosage can cause skin bums and, in all cases, the sensation experienced by
the patient is the primary guide for application. The skin should be dry as these waves are
rapidly absorbed by water. The durations of irradiation generally range from 10 to 25
minutes.
Like all active devices, there are some hazards in using high-frequency energy.
Patients with implanted metal objects, such as pacemakers, should not receive treatment in
that area because of the possibility of overheating. Care should be exercised to restrict
treatment near the eyes and to avoid having the eyes to come in the line of sight of the
reflector.
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Ultrasonic Diathermy
Ultrasonic therapy is used where short-wave treatment has failed and in cases where
localization of the heat effect is desired. The ultrasonic diathermy units are very helpful in
curing the diseases of the peripheral nervous system like neuritis, of the skeletal muscle
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system like arthritis and of skin ulcers. The heating effect is produced because of the
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absorption of ultrasonic energy by the tissues. The effect of ultrasonics on the tissues is a
high speed vibration of micromassage. Micromassage is used in the treatment of soft tissue
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lesions. Ultrasonic massage is better than manual massage because of the greater depth of
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massage that can be obtained without any pain to the patient.
The simplified block diagram of an ultrasonic diathermy unit is as shown below.
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Block Diagram of an Ultrasonic Diathermy Unit
The RF oscillator produces a high-frequency alternating current which excites the
piezoelectric transducer. The transducer is made of barium titanate or lead zirconate titanate
crystal with 5-6 cm2 effective radiating area. The ultrasonic waves can be applied in the
continuous or pulsed mode. In the case of the pulsed mode, micromassage is obtained
effectively without any thermal effect.
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In front of the crystal, there is a metal face-plate which is made to vibrate by the
oscillations of the crystal and the ultrasonic waves are emitted from this plate. The amount of
ultrasonic energy absorbed by the tissues depends upon the frequency of the ultrasonic
waves. Normally, a frequency ranging from 800 KHz to 1 MHz is the suitable frequency for
ultrasonic therapy. The output power can be varied from 0 to 3 W/cm2. Thus, the intensity of
the ultrasonic waves is monitored in terms of electric power converted into acoustic power.
The treatment timer is an electrically-operated contact which can be set from 1 to 15 minutes.
Thus, it switches off the output power after the preset time.
The transducer probe is in direct contact with the body through a couplant (sonic gel).
In cases of large areas to be treated, the probe is moved up and down or circularly to obtain a
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uniform distribution of ultrasonic energy. If there is a wound or an uneven part like a joint,
the treatment is carried out in a warm water bath to avoid mechanical contact with the already
injured tissues. Then the probe is moved over the area at a distance or 1-2 cm from the area
under treatment.
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Physiological Effects of electric current on human body:
1.
The physiological effects of shock may range from discomfort to injury to death,
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if heart or respiratory system (Fails) is affected.
2.
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An Electrical shock may cause cell vapourization and tissue injury or it may
cause an unwanted cellular depolarization and its associated muscular
contraction.
3.
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Shock causes electrical accidents, involving a current pathway through the
person from one upper limb to the feet or to the opposite upper limb.
4.
Shock causes respiratory paralysis and pain.
5.
Shock causes ventricular fibrillation.
6.
Shock causes sustained myocardial contraction.
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The physiological effects due to passage of currents through the patients body are as
listed below.
Current
Type of current
Physiological effect
range (mA)
Threshold
1-5
Tingling sensation
Pain
5-8
Intense or painful sensation
Let-go
8-20
Paralysis
>20
Fibrillation
contraction
Respiratory paralysis and pain
80-1000
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Defibrillation
Threshold of involuntary muscle
Ventricular and heart fibrillation
Sustained myocardial contraction,
1000-10,000
temporary respiratory paralysis and
possible tissue burns
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MICROSHOCK AND MACROSHOCK:
MACROSHOCK:
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A physiological response to a current applied to the surface of the body that
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produces unwanted or unnecessary stimulation like muscle contractions or tissue injury is
called as macroshock.
MICROSHOCK:
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A physiological response to a current applied to the surface of the heart that results in
unwanted stimulation like muscle contractions or tissue injury is called microshock.
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Microshock is most often caused when currents in excess of 10 A flow through an
insulated catheter to the heart. The catheter may be an insulated, conductive-fluid-filled tube,
or a solid-wire pacemaker cable. The microshock results because the current density at the
heart can become high in the situation depicted there, in which the catheter is touching the
heart.
To produce macroshock, much larger current is required bec&use the current
distributes itself throughout the body, obviously the current density at the heart is much
lower in this case and more current is required to cause a shock. This accounts for the
thousand-to-one ratio of macroshock current to microshock current levels.
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MICROSHOCK HAZARDS:
Many devices have a metal chasis and cabinet that can be touched by the medical
attendants and patients. If they are not grounded, then an insulation failure or Short circuit
results and leads to macroshock or microshock.
If a patient is ill, unconscious anaesthetised or strapped on the rotating table, a
tingling effect on the skin reveals that the patient is receiving a shock. Such patients must be
isolated or insulated from the electrical circuit.
A) Leakage currents :
Most of the accidents occur due to improper grounding and leakage currents. The
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leakage current is an extraneous current flowing along a path other than those intended. It
could be due to resistive, inductive or capacitive couplings with the mains or some electronic
equipment.
The following example illustrates the damages of leakage current. A patient in an
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electrically operated bed is as shown in figure has a pacemaker with a bipolar catheter going
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to the right ventricle of the heart via the right vein. The pacemaker's case is connected to the
ground of the power cord. The three wire 2.5 metre power cord is now connected to a two
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wire 3 metre extension cord which is plugged into the three wire power outlet. The bed frame
is properly grounded to the power system.
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The patient's left hand is resting on the bed frame. Since the ground of the pacemaker
is floating, by capacitive coupling a total of 180 A of leakage current is present in the
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pacemaker. In this case leakage current is passed from the pacemaker through the catheter
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into the heart through the body core of the left hand and then to the ground via the bed frame.
The heart is under ventricular fibrillation. This dangerous accident arises because of the open
ground of the pacemaker by using a two wire extension cord. Here the leakage current flow is
due to
i. Ungrounded equipment
ii. Broken ground wire and
iii. Unequal ground potentials
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Microshock due to leakage current
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Broken ground connection on the electric bed allows a voltage to exist on the bed
frame due to capacitive coupling between the bed frame and power line. The pacemaker wire
is going into the heart of the patient. The heart activity is monitored by an ECG recorder.
Hence a leakage current flows from the motor of the bed frame to the medical attendants
hand and to the patient's heart through the catheter or pacemaker wire and then to the ground
of the ECG unit as shown in figure below.
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For example, the doctor is holding a pacemakers wire by his one hand and touching
the electrical bed frame by his other hand as shown below.
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Similarly if the patient is touching the table radio which is connected to the two pin
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socket and is working, then the leakage current flows from the radio to the monitor through
the heart of the patient if the monitor leads are at the heart and it is grounded.
If the patient is attached with an external pacemaker, ECG recorder and arterial
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pressure monitor, then the grounds of these equipment should be at the same potential.
Failure to meet this requirement will allow a current to pass through the patient if he is
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connected to two different equipment having different grounds. For example if a patient were
connected to a arterial pressure monitor transducer that is normally grounded and to an
electrocardiograph machine that has the usual right leg ground, a current due to difference in
ground potentials could flow through his heart. This current flow induces ventricular
fibrillation.
B) Static Electricity :
Static electricity may be dangerous to people and sensitive equipment having
integrated circuits.
The figure below shows an example for the accident from static electricity. Here a
vacuum cleaner is plugged into a wall power outlet as the same circuit as the ECG monitor
and the arterial pressure monitor. The frame of the vacuum cleaner is connected to ground.
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The motor in the cleaner due to dust collection and moisture may provide a leakage path from
line to the outer casing. If a fault current of 1A flows from the vacuum cleaner casing to
ground of the power board and the resistance to power board ground is 0.08 ohms (15 metres
of 12 gauge wire), then a voltage of 80 mV is developed. For a cardiac catheter saline
solution the resistance may be 500 ohms. 80mV across 500 ohms drives 160 mA current
which is very much greater than 10 A, the maximum safe current through the heart in the
case of microshock. The microshock arises due to static electricity due to dust collection in
the motor of the vacuum cleaner and by the saline catheter offering a low resistance path
thereby raising the current density at the heart. Even a potential difference of 5 mV across the
electronic instrument probes can be potentially lathal problem in the case of microshock.
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MACROSHOCK HAZARDS:
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Macroshock occurs more often with two-wire systems than with three-wire systems.
With two-wire equipment it is always dangerous to get between the hot H and neutral N
wires.
If the patient touches H and N wires simultaneously with two limbs, then the currents
are flowing directly through vital organs of circulation and respiration. Because N wires are
internally grounded, touching H and G wires can produce macroshock.
In the case of inexpensive AC/DC radios which are often brought to the hospital by
patients, the power-cord plug may be reversed so that the hot lead becomes attached to the
chassis. In this case, a macroshock hazard exists when the chassis and ground are touched
simultaneously.
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The figure below illustrates additional hazardous situations that result from faults that
may occur in the equipment.
In part (a), the H lead shorts to the patient lead P. Thus a macroshock results if the
patient touches ground or the chassis.
In part (b), the hot wire H and neutral N are reversed because the two-wire plug has
been reversed. A grounded patient is therefore shocked upon touching the chassis.
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In part (c) the H wire shorted to the chassis, causing the shock configuration shown as
the patient touches either neutral or ground and the chassis.
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In part (d) the neutral wire accidentally shorts to the equipment case, leading to a
shock situation of H to chassis or H to ground. If the H line faults to N, no shock occurs
unless the patient touches H or N and ground.
The primary defense against the hazards of a two-wire plug is to add a third
groundwire, as in 3-pin plug. This wire is usually connected to the chassis of the equipment
and ensures that it will not rise to a high voltage. Another method is to double-insulate the
chassis i.e., to place a layer of insulation between the circuit-board chassis and the equipment
case exposed to the user. To prevent a hot chassis on this type of equipment, two prong plugs
have prongs of different sizes so they cannot be reversed. The addition of a ground wire
protects against the hazard of high voltage on the chassis. Assuming the equipment is not
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faulty, the only macroshock danger occurs when a person gets between the hot lead and
ground, neutral, or chassis. Of course, when the third wire faults or breaks the hazards of the
two-wire equipment are once again present, except for plug reversal, which is prevented by
the shape of the three-prong plug.
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DEVICES TO PROTECT AGAINST ELECTRICAL HAZARDS:
Several devices are available to protect patients and health care workers from
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hazardous electrical currents. These range from devices to protect against high-voltage
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macroshock hazards to procedures that minimize the probability that a microshock will occur.
Ground Fault Interrupter (GFI):
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A ground fault interrupter (GFI) protects against a shock that occurs if a person
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touches the hot lead with one hand and the ground with the other. The GFI opens the power
lead if the hot lead current differs by more than approximately 2mA from the neutral lead
current for duration of longer than 0.2 second.
The GFI shown below consists of a magnetic coil on which the hot lead and the neutral lead
are wound with the same number of turns, but in opposite directions. When the system is
normal, IN is equal to IH, and the magnet flux,, in the coil due to these currents cancels.
Under this condition the sensing coil does not have a voltage induced in it.
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However, when the hot lead faults, or is touched by a person, the fault current IF is
shunted to ground. Then we have IN = IH - IF and IH is not equal to IN. Under this fault
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condition the corresponding fluxes in the coil are unequal, and a net flux exists in the coil.
This induces a voltage into the sensing amplifier.
If the current IF exceeds 2 mA for 0.2 second, the relay opens the line and prevents a
macroshock from injuring the person, as well as preventing further damage to the equipment.
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The GFI can be conveniently mounted in the power receptacle. It is required in wet areas.
ISOLATION TRANSFORMER:
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The isolation transformer provides a second means of protecting against an H-Iead to
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G-Iead macroshock. It also prevents sparks when the H lead touches ground, a particularly
important protection in an explosive or flammable environment, such as when flammable
anesthetics or excessive oxygen is present.
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The figure shown above clearly shows that a fault such as a short circuit from either
secondary lead of the transformer to ground will carry no current. Therefore, a secondary lead
to ground spark, or shock, is prevented. However, when the isolation transformer is in use,
and equipment is plugged into the secondary, the stray capacitance and input impedance of
the hardware tend to make a conductive path to ground. This reduces the isolation by
completing the circuit from either secondary lead to ground and then to the other secondary
lead.
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LINE ISOLATION MONITOR:
Line isolation monitor (LIM) puts relatively large impedance from either secondary
lead through an ammeter to ground of the isolation transformer. If there is a conductive path
through the equipment as shown in Figure below, the meter in the LIM will read a current.
The meter on the LIM is calibrated to read what current would flow through a short-circuit
fault if it should occur from either secondary to ground. An alarm in the LIM is usually set
off when it is calculated that a short-circuit fault between a secondary lead and ground would
draw 2 to 5 mA of current. This alarm merely indicates that the backup system has failed and
the equipment is no longer isolated.
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Precautions to minimize Electric shock:
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The following are the precautions to be taken care to minimize the hazards due to
electric shock.
1.
In the hospitals, use only apparatus with 3 wire power cords.
2.
Provide isolated input circuits on monitoring equipment.
3.
Periodically check the ground wire continuity of all equipment.
4.
Staff should be trained to recognize potentially hazardous conditions.
5.
The functional controls of the equipment should be clearly marked and the
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operating instructions must be permanently displayed so that they can be easily
familiarized.
6.
The human assist devices such as pacemaker, ventilators, respirators, dialysers
must be properly grounded.
7.
The mechanical construction of the equipment must be such that the patient or
operator should not be injured by the equipment if properly handled.
8.
The connectors and the probes used in the lab must be standardized to avoid the
leakage current which may be picked up by the transducer.
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9.
High voltage and current operating equipment must not be placed where the
patient monitoring equipment is connected.
10. A potential difference of not more than 5mW should exist between the ground
points of one outlet to other outlet.
11. The patient equipment ground point must be individually connected to receptacle
ground point.
Biotelemetry:
Biotelemetry is “the measurement of biological parameters over longer distance”. The
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means of transmitting the data from the point of generation to the point of reception can take
any forms. Perhaps the simplest application of the principle of biotelemetry is the
stethoscope, whereby heart beat sounds are amplified acoustically and transmitted through a
hollow tube to be picked up by the ear of the physician for interpretation.
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Applications of Bio-Telemetry:
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In many situations, it becomes necessary to monitor physiological events from a
distance. To quote a few applications are,
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1. Radio frequency transmissions for monitoring the health of astronauts in space.
2. Patient monitoring in an ambulance and in other locations away from the hospital.
3. Collection of medical data from home or office.
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4. Patient monitoring, where freedom of movement is desired, such as in obtaining
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an exercise ECG. (In this instance, the requirement of trailing wires is
cumbersome and dangerous).
5. Research on unrestrained and unanesthetized animals in their natural habitat.
6. Use of telephone links for the transmission of ECGs or other medical data.
7. Special internal techniques, such as measuring pH or pressure in the
gastrointestinal tract.
8. Isolation of an electrically susceptible patient from power-line operated ECG
equipment, to protect him from accidental shock.
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Principles of Design of Bio-Telemetry System:
1. The telemetry system should be selected to transmit the bio-electric signal with
maximum fidelity and simplicity.
2. System should not affect living system by interference.
3. It should have more stability and reliability.
4. The power consumption at the transmitter and the receiver should be small to
extend the source lifetime in the case of implanted units.
5. The size and weight of the telemetry system should be compact.
6. For wire transmission, the shielding of cable is a must to reduce noise levels. At
the transmitter side, the amplifiers should be differential amplifiers to reject
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common mode interference.
7. Miniaturization of the radio telemetering system helps to reduce noise.
Physiological parameters adaptable to biotelemetry
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Based on the hardware systems, measurements can be applied to two categories:
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1. Bioelectrical Parameters, such as ECG, EEG and EMG.
2. Physiological variables that require transducers such as blood pressure,
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gastrointestinal pressure, blood flow and temperatures.
Bioelectric Parameters: (such as ECG, EMG and EEG)
The signal is obtained directly in electrical form.
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One example is ECG telemetry - the transmission of ECGs from an ambulance or site
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of emergency to a hospital. A cardiologist at the hospital can immediately interpret the ECG,
instruct the trained rescue team in their emergency resuscitation procedures and arrange for
any special treatment that may be necessary upon the patient‟s arrival at the hospital. In this
application, the telemetry to the hospital is supplemented by two-way voice communication.
Telemetry of EEG signals has also been used in studies of mentally disturbed
children. The child wears a specially designed “spaceman‟s helmet” with built-in electrodes,
so that the EEG can be monitored without traumatic difficulties during play. In some clinic,
the children are left to play with other children in a normal nursery school environment. They
are monitored continuously while data are recorded.
Telemetry of EMG signals is useful for studies of muscle damage, partial paralysis
problems.
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Physiological variables:
The physiological parameters are measured as a variation of resistance, capacitance or
inductance. The differential signal obtained from these variations can be calibrated to
represent pressure flow, temperature and so on.
In the field of blood pressure and heart rate research in unanesthetized animals, the
transducers are surgically implanted with leads brought out through the animal‟s skin. A male
plug is attached post-operatively and later connected to the female socket contained in the
transmitter unit.
The use of thermistors to measure temperature is also easily adaptable to telemetry. In
addition to the continuous monitoring of skin temperature or systemic body temperature, the
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thermistor system has been found to be used in obstetrics and gynecology.
One more application is the use of “radio pill” to monitor stomach pressure of pH. In
this application, a pill that contains a sensor plus miniature transmitter is swallowed and the
data are picked up by a remote receiver and recorded.
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Advantages of Biotelemetry:
1.
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Major advantage of using biotelemetry is removing the cables from patient
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and providing a more comfortable medium to patient. Patient needs to carry
only a small transmitter.
2.
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Isolation of patient from high voltage completely. Transmitters in the
patient side work with batteries without any danger of electrical shock.
3.
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Battery operated amplifiers and transmitters will cause no additional noise
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as long as no connection with line voltage at patient side. (Electrical
Interference of 50Hz).
4.
Continuous monitoring of the patient can be obtained.
Radio Telemetry systems:
Many hospitals use radio telemetry systems to monitor certain patients. The most
common use of radio telemetry is to keep track of improving cardiac patients and at the same
time keep them ambulatory. These units are sometimes called as post coronary care units
(PCCU) or step down CCU.
The telemetry unit uses tiny VHF or UHF radio transmitter that is attached to the
patient either by a clip or a small sack is hung around the patient‟s neck. Most transmitters
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contain an analog ECG section that acquires the signal and uses it to modulate the frequency
of the radio transmitters.
The receiver station is equipped with a bank of radio receivers tuned to the same
frequency as the transmitters. The receiver demodulates the frequency modulation signal to
recover ECG waveform. The waveform is then displayed on an oscilloscope or strip chart
recorder as in other patient monitoring systems.
Modulation systems:
The modulation system used in wireless telemetry for transmitting biomedical signals
makes use of two modulators. This means that comparatively lower signal frequency carrier
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is employed in addition to the VHF which finally transmits the signal from the transmitter.
The principle of double modulation gives better interference free performance in
transmission and enables the reception of low frequency biological signals.
The sub-modulator can be a FM system or a PWM system, whereas the final
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modulator is practically always an FM system.
Elements of Biotelemetry Systems:
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
The essential blocks of a biotelemetry system are shown in Figure below.

The transducer converts the biological variable into an electrical signa1.

The signal conditioner amplifies and modifies this signal for effective transmission.

The transmission line connects the signal input blocks to the read-out device by wire
or wireless means.
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Single Channel Telemetry system:
A Single Channel Telemetry system is as shown in the figure below. The stages of a
typical biotelemetry system can be broken down into functional blocks as shown in figure
below for the transmitter and the receiver.
For a single channel system, a miniature battery operated radio transmitter is
connected to the electrodes of the patient. This transmitter broadcasts the biopotential to a
remotely located receiver. The receiver detects the radio signals and recovers the signals for
further processing.
Physiological signals are obtained from the subject by means of appropriate
transducers. The signal is then passed through a stage of conditioning circuit where
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amplification and processing is done. Later the processed signal is transmitted using radio
transmitter.
The radio frequency used in this system varies from few hundred KHZ to 300 MHZ.
Either amplitude modulation or frequency modulation can be used but due to reduced
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interference, FM transmission is often used for telemetry.
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Radio Telemetry with a subcarrier:
When the relative position of transmitter to the body or other conduction object
changes, the carrier frequency and amplitude will change. This is due to the loading change
of the carrier frequency resonant circuit. This effect is not distinguishable from the signal at
the receiver end.
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If the signal has a frequency different from the loading effect, they can be separated
by filters. Otherwise the real signal will be distorted by the loading effect. To avoid this
loading effect, the subcarrier system is needed.
The signal is modulated on a subcarrier to convert the signal frequency to the
neighbourhood of the subcarrier frequency. Then the R.F carrier is modulated by this sub
carrier carrying the signal.
At the end, the receiver detects the R.F and recovers the subcarrier carrying the signal.
Since the sub carrier frequency is quite different form all noise interference and loading
effect, it can be separated by filters. So one additional stage of demodulation is needed to
convert the signal from the modulated subcarrier back to its real frequency and amplitude.
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Multiple Channel telemetry Systems:
For most bio-medical experiments, it is desirable to have simultaneous recordings of
several signals for correlation study. Each signal requires a telemetry channel. When the
number of channels is more than two or three, the simultaneous operation of the several
single channel units is difficult. At that time, multiple channel (multiplex) telemetry system
adopted.
There are two types:
1. Frequency division multiplexing
2. Time division multiplexing
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Frequency division multiplex System:
Each bio-signal is frequency modulated on a subcarrier frequency. Then these
modulated subcarrier frequencies are combined to modulate the main R.F. carrier.
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At the receiver side, modulated subcarriers will be separated by the proper band pass
filters after the first discrimination (demodulation).
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Later the individual signals are recovered from these modulated subcarriers by the
second set of discriminators (Demodulators).
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The frequency of the subcarriers has to be carefully selected to avoid interference.
The low pass filters are used to extract the signals without any noise.
Time division multiplex System:
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Most of the biomedical signals have low frequency bandwidth requirements;
so, time division multiplex system can be used by the time sharing basis.
The transmission channel is connected to each signal-channel input for a short time to
sample and transmit that signal. Then the transmitter is switched to the next signal channel in
a definite sequence.
When all the channels have been scanned, the operation is repeated from the first
channel.
At the receiver end, the process is reversed. The sequentially arranged, signal pulses
are distributed to the individual channels by a synchronized switching circuit.
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If the number of scanning cycles per second is large and if the transmitter and the
receiver are synchronized, the signal in each channel at the receiver side can be recovered
without noticeable distortion.
Conditions:
1. The scanning frequency fn should be at least greater than twice the maximum signal
frequency fs.
(i.e) fn > 2fsmax
2. If Tn = 1/ fn = scanning period, and tn is the sampling time of each channel, then the
maximum number of channels that can be obtained is n = Tn/tn.
Practically the number of channels allowed is smaller than the calculated value of „n'
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to avoid interference between channels.
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UNIT - V
RECENT TRENDS IN MEDICAL INSTRUMENTATION
---------------------------------------------------------------------------------------------------------------Thermograph, Endoscopy unit, Laser in medicine, cryogenic application, Introduction to telemedicine
THERMOGRAPHY
Thermography is the process of recording temperature distribution over the surface of
the body or skin.
Our body absorbs infrared radiation without any reflection and at the same time our
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body emits a part of its own thermal energy in the form of infrared radiation. so, at certain
distance from the surface of the body, the surface temperature can be measured.
In normal healthy person, the body temperature may change from time to time but the
skin temperature pattern remains the same. so, by analysing the skin pattern, the pathological
changes can be studied.
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The thermogram contains qualitative and quantitative information relevant to the
image and its temperature. Thermography is an important tool in many diseases particularly
in

Breast Cancer

Joint Diseases
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Based on the detection of thermal radiation from skin areas, we can classify
thermography into three methods.
1. Infrared Thermography
2. Liquid crystal Thermography
3. Microwave Thermography
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Advantages of Thermography
1. Non-invasive
2. No contact between the patient and system
3. No radiation hazard
4. It is a real time system
Applications:
1. Examination of female breast as a reliable aid for diagnosing breast cancer is the
best known example of Thermography.
2. In assessment and monitoring joint diseases.
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Infrared Thermographic Equipment:
Simplified block diagram of thermographic equipment
The equipment used in thermography consists of two units
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1) Special infrared Camera
- Scans object.
2) Display unit
- Displays thermal picture on the screen.
Camera Unit:
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The camera unit consists of,
1. An optical system
2. Detector
3. Amplifier
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The beam sweeps across the tube face in a pattern corresponding to the scanning
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pattern of the camera. The picture on the screen can be adjusted for contrast and brightness
by means of controls on the display unit.
The optical system forms an important component in the scanning system. The lenses
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used are made of silicon and are coated with anti-reflective materials which match the
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spectral response of scanner detector. 12 and 20 the working distance is greater than 1.5m
then 12 lens is used.
The double scanning movement of the plane mirror causes each spot on the patient’s
body to be focussed in turn on the cooled detector of indium antimonide.
The detector is mounted in a dewar flask which is cooled with nitrogen in order to
obtain optimum sensitivity and eliminate thermal noise. An optical collector system is
employed in an infrared detecting system. The horizontal and vertical movements of the
scanning mirror are controlled by individual motors.
The scanning mechanism has an arrangement for rapidly oscillating the flat mirror to
give a horizontal line scan and slowly tilting it to give frame scan. Simultaneous horizontal
synchronous pulse and vertical sweep signals are generated for display unit.
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The chopper disc interrupts the infrared beam. so that an AC signal is produced from
the detector. Chopper disc also interrupts light from standard heat source.
The AC signal developed at the detector is amplfied by the pre-amplifier. It is then
rectified and fed into the bandpass filter whose central frequency is determined by the
chopping frequency.
The total voltage gain in the amplifier is around 120dB.
A 13cm CRT is used for display of scanned image.
The object is scanned like television raster where in each line consists of temperature
information divided into picture elements. The thermal picture on screen is virtually flicker
free as a result of high frame rate of 16 frames/ second.
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In spite of high frame rate, temperature differences of less than 0.2C can be detected.
Medical Applications of Thermography:
1. Healthy Case:
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The distribution of a healthy person's skin temperature is symmetrical, particularly
true with regard to head, limb and face.
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The location of the disease can be easily diagnosed if there is any abnormality in this
symmetry.
2. Tumours:
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In the case of tumours, there is difference in temperature with the surrounding tissues.
for example,
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Benign tumour - difference in temperature with surrounding tissue is 1 lower.
Benign angioma - 2 higher
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Lipom & Cystic Struma - low temperature
Superficial malignant tumour - 2-3 higher
3. Burns and Perniones:
In the treatment of burns and perniones, the decision is made based on degree of
severity. The first degree burns register high temperature and third degree burns register low
temperature than normal value.
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4. Skin Grafts and Organ Transplantation:
The condition of skin rolls after transplantation can be detected by means of local
blood flow. In kidney transplantation, the rejection causes a high temperature.
5. Orthopedic Diseases:
Fractures, arthrities, sprains can be easily diagnosed because of local skin temperature
rises.
6. Harmone Disease:
Thyroid glands normally register high temperature due to their active metabolism. If a
person affected with hyper-thyrodism, the site of the struma shows a high temperature but if a
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patient suffering from cyst, the corresponding area presents a low temperature.
7. Examination of Placenta:
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Detection of location of placenta
is possible by means of thermogram because
increase in local flow leads to a high temperature. There is no possibility of damage to the
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fetus, since the infrared radiation detected is naturally radiated from skin.
Advantages:
1. Non-destructive
2. Non- invasive
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3. Comfortable for patient
4. Possible to achieve faithful reproducible results
8. Inflammation:
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The area of inflammation shows high temperature because of active metabolism and
increase in local blood flow. So, this makes difficult to diagnosis malignant tumour.
Malignant tumour is diagnoised by using thermal recovery method. In this method the
tumour and the inflammation are cooled. The tumour takes time to recover the temperature,
whereas the inflammation quickly recovers the temperature. This temperature difference
allows us to diagnose tumour.
9. Diseases of peripheral vessels:
In cases where the arteries are occluded, the blood flow of peripheral vessels either
decreases or disappears resulting in a low temperature in that part. In reverse, the arteries
venous fistula increases the blood flow showing high temperature.
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ENDOSCOPY UNIT
Introduction:
Due to the recent advanced technology, the major surgical intervention are made
faster and safer. The surgical interventions are possible through a small incision. They help
the surgeon to achieve operative interventions without the need for major surgical incisions.
Then also help surgeons to accurately diagnose visceral diseases.
"Endoscope" means viewing the inside. Initially people used the long rigid tube with
light attached to its tip, pushing it into the oesophagus and the stomach was difficult as the
old Indian "Sword Swallowing" magic. The discovery of the fiber optics helped images to be
transmitted even through bent fibers. The light can also be sent from a lamp situated outside
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the body. Hence it is called cold light source.
Endoscope paved the way for many interventions that are of diagnostic and
therapeutic importance. The gastroduodenoscope is used primarily to visualize the interior of
the stomach and the duodenum. Various organs can be reached by their respective
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endoscopes. The organ and concerned type of endoscope required to reach that organ are
enumerated as below.
S.No Organ
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Type of Endoscope
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1
Food Pipe
Oesophagoscope
2
Stomach and duodenum
Gastroduodenoscope
3
Lungs
Bronchoscope
4
Abdomen
Laparoscope
5
Urinary Bladder
Cystoscope
6
Kidney
Nephroscope
7
Large Intestine
Colonoscope
8
Blood Vessels
Angioscope
9
Joints
Arthoscope
10
Tympanic membrane
Otoscope
11
Rectum
Proctoscope
12
Rectum and Distal part of colon
Sigmoidoscope
13
Stomach
Gastroscope
14
Heart Cavities
Cardioscope
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Endoscopic laser Coagulator:
The figure shows the schematic diagram of a gastric optical coagulator using argon
ion laser as high energy optical source and endoscope as the delivery unit. It is known that
argon ion lasers are very useful in the coagulation of blood vessels since its green light is
highly absorbed by the red blood cells and haemoglobin. The absorption of green light results
in the photocoagulation of the blood protein and micro hemostasis. Further it is more
advantageous to use argon ion laser for the photocoagulation of retina because of its smaller
beam diameter and ability to do coagulation without affecting the surrounding tissue.
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Endoscopic laser Coagulator
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To control the gastric haemorrhage photocoagulation technique is adopted. With the
help of fibre optic endoscope, the output from the argon ion laser is delivered to the required
spot to arrest the gastric bleeding. We can move the laser beam in any direction with the
flexible endoscope. The argon ion laser gives an output of 13 watts in the form of continuous
waves. Using an endoscope, the beam can be delivered to the required site as well as it can be
viewed for proper alignment. The diameter of the quartz fibre endoscope is about 2mm.
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An array of optical imaging system is available now to aid the physician in the
diagnosis of disease. The endoscope permits direct visualization of the diseased area in the
body.
There are two types of endoscopes
1. Rigid Endoscope
2. Flexible Endoscope
1. Rigid Endoscope:
Rigid Endoscope consists of a series of lenses to convey the image of a remote organ
to the physicians. This instrument is used for many applications in the management of
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diseases of the ear, throat, urinary tract, bone joints and abdominal cavity.
In applications where natural opening is not available, a small incision is made to
enable passage of the endoscope. For example, 1 cm incision is made in the abdominal wall
to facilitate passage of the laparoscope’s to provide visualization of the fallopian tubes during
tubal ligation.
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Rigid Endoscope
These scope incisions are generally atraumatic and heal in a few days. A Special
optical fiber or cylinder with a graded index of refraction from the center to the periphery
produces in effect. a continuum of closely packed low power lenses.
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2. Flexible Endoscope:
The Flexible endoscope is as shown in the diagram below. This endoscope is used to
observe remote anatomy.
In this endoscope, the image is first focused onto the distal array of optical fibers by a
remote lens and the image is broken up into an array of picture elements or pixels.
Each component of the picture is transported back to the proximal eyepiece by an
individual optical fiber.
If the picture is to have a field resolution of 200 x 200 elements, then 40,000 fibers
are required. The endoscope is provided with longitudinal metal cables for distal guiding of
the scope. A proximal control handle allows 4-way vectoring of the distal tip.
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Flexible Endoscope
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These endoscopes have been specifically developed for visualization of the entire
human colon, the stomach and duodenum and bronchus.
The outer diameter of this endoscope ranges from 6-15mm, depending upon the
application. The smaller scopes sacrifice some level of resolution but permit bronchoscopic
or pediatric applications.
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Laser in Medical Application:
Within a very short time and amazing period of research and development of optical
fibers changed the area of communications completely.
Applications of fibers in medicine depend mainly on the interaction of the light
transmitted through the fiber and the part of the human body to be treated. With respect to the
interaction between light and tissues, a lot of investigations have been made. Besides the light
wavelength, the light power and the type of tissue to be treated is also important.
Lasers in medical have wide applications under different specialties as listed below
1) Laser in Ophthalmology:
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The laser heating of tissues is used for 2 distinct surgical functions, cutting and
photocoagulation. First medical application of laser was in ophthalmology. Ophthalmologist
used to treat variety of eye problems including retinal bleeding, the excessive growth of
blood vessels in the eye caused by diabetes and also for spot welding –reattaching retinas that
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have become partly detached from the back surface of the eye, the choroid.
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The argon ion lasers, which emit blue green light, that is readily absorbed by the
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blood are preferred for photocoagulation of small blood vessels in the eye. Nd-YAG lasers
are also used for blood vessels and tissue coagulation and have found wide applications in
ophthalmology.
Lasers are used for tissue interaction like,
i) Photocoagulation theraphy
ii) Photo distruption theraphy
iii) Photo fragmentation theraphy
i) Photocoagulation theraphy:
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In the case to stop bleeding or to burn retinal layer, this type of theraphy is used. The
patients with sugar complaint are treated. The laser can burn retina layer 4 to 5 micron for
each shot.
ii) Photo distruption theraphy
In this type of therapy, treatment is given to posterior capsule, just behind intraocular
lens.
iii) Photo fragmentation theraphy
In this technique using a small inclusion on the eye, the laser is guided through a fiber
optic cable. The laser energy is delivered to vaporize the nucleus of the cataractous lens.
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2) Laser in Dermatology:
The uses of laser in Dermatology are,
1. To use laser as investigative tools for the study of skin exposures.
2. To use laser as tools for studies on the optical properties of the skin.
3. To evaluate the effect of acute and chronic laser exposures in the development of
laser safety program
4. To evaluate lasers for dermatological surgery.
5. To determine the role of the laser as a research tool in the combined fields of
biomedical engineering and dermatology.
Lasers used:
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All types of lasers have been used for investigative studies in dermatology. These
include pulsed ruby laser, Neodymium, Argon, Krypton, Helium-neon and CO2.
Port-wine lesions
– Pulsed ruby laser
Tattoos
- Q-Switched ruby laser, Argon laser, YAG Laser.
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3) Laser in Cancer research:
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A Laser is a powerful source of monochromatic light, Because of its coherent
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properties; it can be used with precision in many phases of cancer research. Cancer research
program have shown that laser energy may be of value because of the following properties.
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1. Due to its enormous power, laser energy can cause coagulation necrosis of any
tissue.
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2. Because the beam is highly collimated and may be finely focused, it may be used
as a cutting instrument.
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3. Due to the monochromatic nature of the beam, darkly pigmented tissues will more
readily absorb the beam from most visible transition lasers in an selective manner.
4. Due to the enormous power density of the focused beam, the laser produces tissue
cuts with a rapid cauterizing effect. i.e., the cuts are almost bloodless.
Laser Used:
Pulsed ruby laser and CO2 laser is used.
4) Laser in Neurosurgery:
Laser can also be used where other forms of surgery such as cryosurgery cannot be
used. Some examples are,
1. Laser thalamotomy
2. Relief of intractable pain by delivering laser doses to the spinal cord.
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3. Treatment of cerebral neoplasms which cannot be completely or effectively
removed by other modalities.
Laser Used:
Argon, YAG and CO2 laser.
5) Laser in Dentistry:
Another significant field of laser research is the use of laser in dentistry.
1. Initially laser dentistry is used for selective destruction of calculus and dental
caries.
2. Used to prevent caries.
3. Used in study of enamel and dentine.
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4. Used in examining the composition and change of the structures of the teeth.
Laser Used:
The most popular and result oriented lasers used in dental treatment are,
1. Ruby laser
2. CO2 Laser
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3. Neodymium Laser.
6) Laser transillumination:
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Transillumination with intense incoherent light source is a recognized procedure in
clinical medicine.
1. It is used primarily for visualization of foreign bodies, paranasal sinuses and
transillumination of the infant skull.
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2. Used in diagnosis of breast tumours and location of foreign bodies in soft tissues.
Laser Used:
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Helium-Neon and Krypton lasers are used.
7) Laser exposures done in the pituitary gland:
1. Ruby laser exposures cause diffuse necrosis, sometimes in depth in a
nonhomogenous manner.
2. Argon laser exposure will give highly specific reactions to red coloured vessels.
Deep penetrations may be seen in vascular areas.
3. CO2 lasers will have a highly specific absorption at the outer surface of the
pituitary.
Laser Used:
Pulsed Ruby laser, Argon laser and CO2 Laser.
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