UNIT – II
Bio potential Recorders
Introduction
The bio-potential recorder is playing an important role in the biomedical instrumentation.
Normally each doctor is performing his diagnosis based on the output from the recorder. If the
recording is not faithful or the fidelity is very poor, then there is improper diagnosis. Thus the
recorders are very useful to interpret the nature of the biosignal source during its working
condition.
During 1887, Waller recorded the first electrocardiogram using capillary electrometer,
immersion type electrodes and moving light sensitive paper. Then the string galvanometer was
introduced. From 1920 onwards, electronic amplifiers were introduced. About 1946, the direct
writing recorders, which use ink to record the picked up by biosignal trace on a moving paper
strip are introduced. These are further improved by introducing servomotors, inkjet recorders to
extend the frequency response and signal recovery circuits.
Characteristics of recording system
The following are the characteristics of the recording system
Sensitivity : The sensitivity of the recording system is the magnitude of input voltage required to
produce a standard deflection in the recorded trace.. In the case of EEG recording, it is usually
measured in microvolt per millimeter. The sensitivity is composed of gain of the amplification
system and sensitivity of the writer in the recorder.
Linearity: A recorder is said to be linear if the pen deflection is proportional to the amplitude of
the input signal. The range over which the system is linear is called the dynamic range. Beyond
th dynamic range, the non-linear distortion occurs.
Frequency Response: Frequency Response of the recorder is the frequency characteristics of the
system such that variation of the sensitivity for different frequency components of the biosignal.
A recorder is said to have good frequency response when the sensitivity of the system is constant
for all the frequencies present in the signal with different amplitudes and the amplification
should be done without any distortion.
Phase Response: Phase response of a recorder is measured by means of time delay between the
input and output. A complex waveform will be distorted if the time displacements introduced are
different for different frequencies. A low noise level is maintained by special circuits and use of
quality electronic components.
Writer and pen damping effects
The writer is usually a pen motor and is an electromagnetic system of moving coil or
moving iron type. The moving coil type has the pen attached to a coil, carrying current and so
behaves like a magnet whose strength is proportional to the current. The coil rotates in the field
of a permanent magnet and the angle of rotation is proportional to the current in the coil which is
supplied by the driver amplifier and modulated by the input signal. The sensitivity of the writer
is dependent on frequency because of inertia, mechanical resonance and friction.
Electrocardiography (ECG)
ECG is an interpretation of the electrical activity of the heart over a period of time, as
detected by electrodes attached to the outer surface of the skin and recorded by a device external
to the body. The recording produced by this noninvasive procedure is termed as
electrocardiogram. An electrocardiogram (ECG) is a test that records the electrical activity of the
heart. ECG is used to measure the rate and regularity of heartbeats as well as the size and
position of the chambers, the presence of any damage to the heart, and the effects of drugs or
devices used to
The ECG device detects and amplifies the tiny electrical changes on the skin that are
caused when the heart muscle depolarizes during each heartbeat. At rest, each heart muscle cell
has a negative charge (membrane potential) across its outer wall (or cell membrane). Increasing
this negative charge towards zero (via the influx of the positive ions, Na+ and Ca++) is called
depolarization, which activates the mechanisms in the cell that cause it to contract. During each
heartbeat a healthy heart will have an orderly progression of a wave of depolarization that is
triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through
"intrinsic conduction pathways" and then spreads all over the ventricles. This is detected as tiny
rises and falls in the voltage between two electrodes placed either side of the heart which is
displayed as a wavy line either on a screen or on paper.
Origin of cardiac action potential
Electrocardiogram
Heart is a muscle with many functions; it also has a special electrical conduction system
that is made of two nodes and several conduction fibers. The electrical conduction capability of
the heart allows electrical propagation to be transmitted from the Sinoatrial node passing through
the atria and moving forward to the antriventricular node.
The normal functioning of the heart starts with an electrical impulse from the sinoatrial
mode, which is located within the right atrium. The sinoartrial node is essentially the pacemaker
of the heart as it is responsible for setting the heart rate and rhythm. This impulse spreads
through the atria, which causes the walls to contract. The neurons that supply the nerves to the
cardiac muscle hold several similarities to the neurons that innervate the skeletal muscle. These
neurons can be greatly influenced by the sympathetic part of the autonomic nervous system.
The components of the conduction system of the heart include:

The Sinoatrial Node. This is the first part of the conduction system. It can be found in the
posterior wall of the right atrium, right at the entrance of the superior vena cava. The
Atrioventricular Node. This is the second part of the conduction system and is situated in
the floor of the right atrium, near the interatrial septum. The cells contained in this node
conduct impulses slightly slower than other parts; as a result, there is a brief delay to the
impulses that travel in the AV node.

The Bundle of His or otherwise known as the atrioventicular bundle is where the
impulses pass through from the AV node. These impulses go straight to the right part of
the left bundle branches and extend further to the right and left sides of the apex and the
interventricular septum of the heart.

The heart’s cardiac cycle refers to the contraction and relaxation of the myocardium in
the walls of the heart chambers. This process is done by the heart’s conduction system
during the time it takes to make one heartbeat. The two atria of the heart contracts at the
same time; and when they start to relax, the ventricles start contracting. This contraction
phase is known as the systole, and the relaxation phase is called the diastole.
The cardiac action potential differs significantly in different portions of the heart. This
differentiation of the action potentials allows the different electrical characteristics of the
different portions of the heart. For instance, the specialized conduction tissue of the heart has the
special property of depolarizing without any external influence. This is known as cardiac muscle
automaticity. The electrical activity of the specialized conduction tissues are not apparent on the
surface electrocardiogram (ECG or EKG - From German word). This is due to the relatively
small mass of these tissues compared to the myocardium.
A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS
complex, a T wave, and a U wave which is normally visible in 50 to 75% of ECGs. [23] The
baseline voltage of the electrocardiogram is known as the isoelectric line. Typically the
isoelectric line is measured as the portion of the tracing following the T wave and preceding the
next P wave.
ECG lead configuration
The term "lead" in electrocardiography causes much confusion because it is used to refer
to two different things. In accordance with common parlance the word lead may be used to refer
to the electrical cable attaching the electrodes to the ECG recorder. As such it may be acceptable
to refer to the "left arm lead" as the electrode (and its cable) that should be attached at or near the
left arm. There are usually ten of these electrodes in a standard "12-lead" ECG.The word lead
may refer to the tracing of the voltage difference between two of the electrodes and is what is
actually produced by the ECG recorder. Each will have a specific name. For example "Lead I"
(lead one) is the voltage between the right arm electrode and the left arm electrode, whereas
"Lead II" (lead two) is the voltage between the right limb and the feet. (This rapidly becomes
more complex as one of the "electrodes" may in fact be a composite of the electrical signal from
a combination of the other electrodes
Bipolar limb leads
The Einthoven triangle
Placement of electrodes
Usually surface electrodes are used with jelly as electrolyte between skin and electrodes.
The potentials generated in the heart are conducted to the body Surface. The potential
distribution changes in a regular and complex manner during each cardiac cycle. Therefore to
record electrocardiograms, standardized electrode positions are to be used.
The classical 12-lead ECG can be extended in a number of ways in an attempt to improve
its sensitivity in detecting myocardial infarction involving territories l. This includes an rV4 lead
which uses the equivalent landmarks to the V4 but on the right side of the chest wall and
extending the chest leads onto the back with a V7, V8 and V9.
Different types of electrode systems are
i)Limb lead : In both the 5- and 12-lead configuration, leads I, II and III are called limb leads.
The electrodes that form these signals are located on the limbs—one on each arm and one on the
left leg.[ The limb leads form the points of what is known as Einthoven's triangle.
Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode
I = LA − RA.
Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA)
electrode:
II = LL − RA.
Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA)
electrode:
III = LL − LA.
Unipolar vs. bipolar leads
There are two types of leads: unipolar and bipolar. Bipolar leads have one positive
and one negative pole. In a 12-lead ECG, the limb leads (I, II and III) are bipolar leads.
Unipolar leads also have two poles, as a voltage is measured; however, the negative pole is
a composite pole
ECG recording Setup
The important parts of ECG recorder are
1)Patient cable and Defibrillator Protection circuit
The patient cable connects the different leads from the limbs and chest to the defibrillator
protection circuit. It consists of buffer amplifiers and over voltage protection circuit. The leads
are connected with the buffer amplifiers such that one buffer amplifier for each patient lead. The
input impedance is increased and the effects arising from the variations in the electrode
impedance are reduced. Further the over voltage protection circuit is necessary to avoid any
damage to the bioamplifiers in the recorder. The over voltage protection circuit is necessary to
avoid any damage to the bioamplifiers in the circuit.This over voltage protection circuits consists
of a network of resistors and neon lamps which fire when a pulse from a defibrillator is present.
2) Lead selector switch
After the defibrillator protection circuit, there is lead selector switch which is used to feed
the input voltage from the appropriate electrode to the preamplifier
3) Calibrator
A push button allows the insertion of a standardization voltage of 1mV to the
preamplifier. This enables the technician to observe the output on the display unit and adjust the
scale so that a known deflection corresponds to a 1mV input signal. From the lead selector
switch the ECG signal goes to bio-amplifier.
4) Bioamplifier
It consists of a preamplifier and power amplifier. The sensitivity or the gain of the
amplifier can be varied. Followed by preamplifier, there is a power amplifier
ECG Recording set up
Practical Considerations for ECG recording
Several practical aspects must be observed in order to obtain diagnostically useful
electrocardiogram
i).Artifacts
Since the ECG unit is a sensitive device, it can pick up unwanted electrical signal which
may modify the actual ECG signals. Even though AC interference is reduced by increasing the
CMRR of the bioamplifier, the operator before recording EEG should check the following things
1. Be sure that the patient dose not touch or make contact with any metal object such as bed
rail, bed stand or furniture
2. Remove or unplug any other electrical appliances such as clocks, radios, lamps etc in the
vicinity of patient
3. If adder ECG machines are used, make sure that the polarity test has been performed
before connecting the cable to the patient
4. Be sure that all electrodes have been applied with right amount of paste or jelly and that
all electrode straps are tight enough
ii) Wandering of Base line
The wandering of base line results from the gross movements of patients or from
mechanical strain on the electrode wires. If there is no proper application of jelly between the
electrode and the skin, during that time also the wandering of base line occurs
iii) Solid Base Line
An indistinct trace or solid base line appears due to improper adjustment of stylus
temperature or by buildup of waxy residue on heated stylus.
iv) Frequency Response
Generally the upper frequency limit of the bioamplifier is about 100Hz. But the pen
inertia limits the ECG unit response tom about 50Hz.
Analysis of recorded ECG signals
Typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, The baseline
voltage of the electrocardiogram is known as the isoelectric line. Typically the isoelectric line is
measured as the portion of the tracing following the T wave and preceding ECG curve
1) Normal ECG curve
2) Here PQ segment has prolonged conduction time i.e greater than 2.22 second
Result: First degree AV block
3) Here QRS complex is widened i.e. QRS interval is greater than 0.1 second
Result: Bundle block
ECG Signals
4)Here ST segment is elevated
Result: Myocardial infraction
5)Here ST segment is depressed and negative T wave is present
Result : Coronary insufficiency
6)Here there is a train of pulses instead of PQRST waves.
Result : Ventricular fibrillation which may lead to death if it is not properly corrected by
defibrillatortion
If the normal conduction system is disturbed, then the beat rate will be slower than the
normal rate. This state is called heart block. There are different types of heart block
1st degree AV block : Due to prolonged conduction time
2nd degree AV block : Due to conduction of few pulses instead of all from atrium
3rd degree AV block : Due to asynchronous action of atrium and ventricle.
Adams – Stoke attack : Due to sudden attack of total block
ECG Signals
Vectorcardiography
Vectorcardiography was developed by E. Frank in the early 30's. Since the human body
is a three dimensional structure, the basic idea is to construct 3 orthogonal leads containing all
the electric information. Vectorcardiography is a method of recording the magnitude and
direction of the electrical forces that are generated by the heart by means of a continuous series
of vectors that form curving lines around a central point. The vector cardiogram appears as loops
in each plane. There are three loops corresponding to P,QRS and T waves. Among this the QRS
complex loop is dominating one. A Polaroid camera photographs the oscilloscope screen to
provide a permanent record. With the modern computer facilities, the vectorcardiography may be
clinically used in an extensive manner, even though it is not widely used clinically now. It is the
recording of the moment-to-moment electromotive forces of the heart on a plane of the body
surface delineated as a vector function of time.