SATHYABAMA UNIVERSITY
(Established under Section 3 of UGC Act, 1956)
JEPPIAR NAGAR, RAJIV GANDHI SALAI
CHENNAI- 600119
THERAPEUTIC ISTRUMENTATION LAB MANUAL
DEPARTMENT OF BIOMEDICAL ENGINEERING
2010-2014 BATCH
VII SEMESTER
LIST OF EXPERIMENTS
1. SURGICAL DIATHERMY MACHINE
2. DEFIBRILLATOR
3. HEMODIALYSIS MACHINE- STUDY
4. VENTILLATORS
5. HEART LUNG MACHINE- STUDY
6. CARDIAC PACEMAKER
7. SHORTWAVE DIATHERMY
8. CEREBELLAR STIMULATORS/NERVE STIMULATORS
Experiment No 1
SURGICAL DIATHERMY MACHINE
AIM:
To study the working of a surgical diathermy machine.
THEORY:
Surgical diathermy equipment is used in operation theatres for cutting and coagulation in pure
and blend modes. This avoids the wastage of blood during surgery in operation theatres. highfrequency electric current is made to pass through the body between two contact electrodes. High
frequencies (0.5 to 3.0 MHz) are used because direct current, or low-frequency alternating
current (e.g. the 50 Hz or 60Hz of the mains supply) would cause muscle spasm. Surgical
diathermy apparatus comprising a power source, an active electrode for operation on a patient, at
least one circuit means for attachment to the patient, interconnection means operatively
interconnecting power source with active electrode and at least one circuit means, each circuit
means comprising a respective capacitive neutral plate for attachment to the patient, a
transformer interconnecting power source to capacitive neutral plate, and a potential
compensator comprising a amplifier transformer-coupled by transformer in series between
respective amplifier being operated to inject a voltage through the transformer which is
substantially equal to the potential drop across the capacitive reactance of the capacitive neutral
plate.
BLOCK DIAGRAM OF SURGICAL DIATHERMY
Figure: Block diagram
Figure: Principle of surgical diathermy machine
DEVICE DESCRIPTION
An electrosurgical unit is used for producing a modulated oscillating signal for coagulation
procedures and an unmodulated oscillating signal for cutting procedures. The unit includes a four
arm bridge rectifier interconnected directly to the line voltage. A first pair of opposing diagonal
terminals on the rectifier is connected directly to the line voltage. A cutting signal is obtained
from the other pair of diagonal terminals, and a coagulation signal is obtained across one arm of
the bridge rectifier. An oscillator tank coil provides the necessary RF isolation output. A switch
selectively interconnects either the coagulation signal output from the bridge rectifier, or the
cutting signal output from the bridge rectifier, to the oscillator. The output is taken across the
tank circuit from the RF oscillator and applied to a patient through a handpiece. Through the use
of high voltage transistors in the circuit, high efficiency is obtained by eliminating the input
transformer.
PROCEDURE:
Cutting and coagulation is achieved by applying electric current to the tissues via small handheld
probes (electrodes). The current flows out of the body (usually) through a very large electrode
laced on the skin at some remote site (e.g. on the thigh). At this electrode (the indifferent
electrode) the current density is very low, so little heating occurs. However, at the hand-held
electrode the density is very high due to its small contact area, and so great heat is developed.
Depending upon the speed of the current and the resulting heat, a diathermy unit can achieve the
following results:
1. Cutting with high speed or intensity. - Produced by continuous current, Sine wave-form.
2. Coagulation- Produced by interrupted pulses of current (50-100 per second) Square waveform
(a) If weak, epilation.
(b) If strong, coagulation producing destruction.
3. Desiccation ensues by passing medium current through a mono-terminal electrode at a slight
distance from the surface which produces sparking heat and results in drying up the tissues
4. Fulguration- The destruction of tissue, usually malignant tumors, by means of a highfrequency electric current applied with a needlelike electrode.
5. Electrotomy- The surgical use of high-frequency electric current for cutting or destroying
tissue.
BIPOLAR SURGICAL DIATHERMY
As an alternative to the conventional (monopolar) surgical diathermy, in which the electrical
current flows from a small active probe through the body to a large indifferent electrode, the path
of the current can be constrained to pass only through the tissue being treated. This is achieved
using a special forceps in which the two halves of the instrument are insulated from one another
and in effect one half becomes the source of the current and the other the destination, thus
replacing the active and indifferent electrodes mentioned above. Bipolar diathermy has the
advantage that electric currents do not pass through parts of the body which are not being treated
and also it is possible to be much more precise with the quantity of tissue being coagulated.
RESULT:
The study of surgical diathermy is thus performed.
Experiment No 3:
HEMODIALYSIS MACHINE – STUDY
AIM:
To study the working of hemodialysis machine.
THEORY:
Dialysis is a process for removing waste and excess water from the blood, and is used primarily
to provide an artificial replacement for lost kidney function in people with renal failure. The
principle of hemodialysis is that it involves diffusion of solutes across a semipermeable
membrane. Hemodialysis utilizes counter current flow, where the dialysate is flowing in the
opposite direction to blood flow in the extracorporeal circuit. Counter-current flow maintains the
concentration gradient across the membrane at a maximum and increases the efficiency of the
dialysis.Fluid removal (ultrafiltration) is achieved by altering the hydrostatic pressure of the
dialysate compartment, causing free water and some dissolved solutes to move across the
membrane along a created pressure gradient.
The dialysis solution that is used may be a sterilized solution of mineral ions. Urea and other
waste products, potassium, and phosphate diffuse into the dialysis solution. However,
concentrations of sodium and chloride are similar to those of normal plasma to prevent loss.
Sodium bicarbonate is added in a higher concentration than plasma to correct blood acidity. A
small amount of glucose is also commonly use.
COMPONENTS:
Water system
An extensive water purification system is absolutely critical for hemodialysis. Since dialysis
patients are exposed to vast quantities of water, which is mixed with dialysate concentrate to
form the dialysate, even trace mineral contaminants or bacterial endotoxins can filter into the
patient's blood. Water used in hemodialysis is carefully purified before use. Once purified water
is mixed with dialysate concentrate, its conductivity increases. During dialysis, the conductivity
of dialysis solution is continuously monitored to ensure that the water and dialysate concentrate
are being mixed in the proper proportions.
Dialyzer
The dialyzer is the piece of equipment that actually filters the blood. Almost all dialyzers in use
today are of the hollow-fiber variety. A cylindrical bundle of hollow fibers, whose walls are
composed of semi-permeable membrane, is anchored at each end into potting compound. This
assembly is then put into a clear plastic cylindrical shell with four openings. One opening or
blood port at each end of the cylinder communicates with each end of the bundle of hollow
fibers. This forms the "blood compartment" of the dialyzer. Two other ports are cut into the side
of the cylinder. These communicate with the space around the hollow fibers, the "dialysate
compartment." Blood is pumped via the blood ports through this bundle of very thin capillarylike tubes, and the dialysate is pumped through the space surrounding the fibers. Pressure
gradients are applied when necessary to move fluid from the blood to the dialysate compartment.
Membrane
Dialyzer membranes used to be made primarily of cellulose (derived from cotton linter).
Nowadays cellulose acetate and modified cellulose dialyzers are used. Cellulosic membranes can
be made in either low-flux or high-flux configuration, depending on their pore size.
Another group of membranes is made from synthetic materials, using polymers such as
polyarylethersulfone, polyamide, polyvinylpyrrolidone, polycarbonate, and polyacrylonitrile..
Synthetic membranes can be made in either low- or high-flux configuration, but most are highflux.
Blood pump
The blood pump takes and returns the blood from the patient via the arterial and venous needles.
The blood is confined to the disposable plastic tubing and doesn't come in contact with any part
of the machine. The blood pump is the distinct feature on all dialysis machines. The pumping is
done by squeezing the plastic tube inside the pump using a pair of spring loaded rollers. The
suction is done by the elasticity of the tube, which expands after released from the rollers and
sucks the blood from the arterial needle.The blood coming from the pump flows to the dialyzer
and the blood that leaves the dialyzer returns to the patient through the venous needle.
Dialysis Process
Concerning the blood, dialysis performs 2 different functions that are normally done by healthy
kidneys:
1. Removing excess fluid.
2. Removing waste like urea, and excess electrolytes (chemicals) like potassium, magnesium,
sodium, etc.
The dialysis is performed inside the dialyzer, which is a plastic cylinder, in which the blood
enters from the top (the red header), flows through thousands of extremely thin hollow fibers and
leaves from the bottom. At the same time the dialysate enters from the bottom (the blue header),
flows around and in between the fibers, and leaves from the top.
The fibers are semi permeable membranes, that is, smaller molecules in the blood stream can
pass through them into the dialysate and bigger molecules as well as blood cells cannot. The
dialysate is a water-based solution and its purpose is to absorb from the blood all that should be
removed and nothing else. Wastes and electrolytes move from the blood into the dialysate
because their concentration in the blood is higher. This process is called diffusion.
The dialysate flow ensures that fresh dialysate is present at all times so that the dialysate doesn't
become saturated and the process never ends.Fluid is removed from the blood in the same way
the kidneys do it - by blood pressure. This process is called Ultra Filtration (UF) and is similar to
Reverse Osmosis (RO). In UF, the membrane pore size is larger, allowing some bigger
molecules to pass through the pores with the water. The rate at which fluid is removed from the
blood is called UFR (UF Rate).
There is higher pressure in the blood passing through the dialyzer and lower pressure in the
dialysate. This pressure difference is called TMP (Trans Membrane Pressure). The higher the
TMP, the higher the UFR.
RESULT:
The working of the hemodialysis machine is studied.
Experiment No: 4
VENTILATORS
AIM:
To study the working of a ventilator.
THEORY:
A medical ventilator (or simply ventilator in context) is a machine designed to mechanically
move breatheable air into and out of the lungs, to provide the mechanism of breathing for a
patient who is physically unable to breathe, or breathing insufficiently.
While modern ventilators are computerized machines, patients can be ventilated with a bag valve
mask, a simple hand-operated machine.
Figure: Mask, breathing valve and self filling bag for artificial ventilation
WORKING/FUNCTION
In its simplest form, a modern positive pressure ventilator consists of a compressible air reservoir
or turbine, air and oxygen supplies, a set of valves and tubes, and a disposable or reusable
"patient circuit". The air reservoir is pneumatically compressed several times a minute to deliver
room-air, or in most cases, an air/oxygen mixture to the patient. If a turbine is used, the turbine
pushes air through the ventilator, with a flow valve adjusting pressure to meet patient-specific
parameters. When overpressure is released, the patient will exhale passively due to the lungs'
elasticity, the exhaled air being released usually through a one-way valve within the patient
circuit called the patient manifold. The oxygen content of the inspired gas can be set from 21
percent (ambient air) to 100 percent (pure oxygen). Pressure and flow characteristics can be set
mechanically or electronically.
Ventilators may also be equipped with monitoring and alarm systems for patient-related
parameters (e.g. pressure, volume, and flow) and ventilator function (e.g. air leakage, power
failure, and mechanical failure), backup batteries, oxygen tanks, and remote control. The
pneumatic system is nowadays often replaced by a computer-controlled turbo pump.
Modern ventilators are electronically controlled by a small embedded system to allow exact
adaptation of pressure and flow characteristics to an individual patient's needs. Fine-tuned
ventilator settings also serve to make ventilation more tolerable and comfortable for the patient.
The patient circuit usually consists of a set of three durable, yet lightweight plastic tubes,
separated by function (e.g. inhaled air, patient pressure, exhaled air). Determined by the type of
ventilation needed, the patient-end of the circuit may be either noninvasive or invasive.
Noninvasive methods, which are adequate for patients who require a ventilator only while
sleeping and resting, mainly employ a nasal mask. Invasive methods require intubation, which
for long-term ventilator dependence will normally be a tracheotomy cannula, as this is much
more comfortable and practical for long-term care than is larynx or nasal intubation.
Types of ventilators
Ventilators come in many different styles and method of giving a breath to sustain life. There are
manual ventilators such as Bag valve masks and anesthesia bags require the user to hold the
ventilator to the face or to an artificial airway and maintain breaths with their hands. Mechanical
ventilators are ventilators not requiring operator effort and are typically computer controlled or
pneumatic controlled.
Mechanical ventilators
Mechanical ventilators typically require power by a battery or a wall outlet (DC or AC) though
some ventilators work on a pneumatic system not requiring power.
• Transport ventilators - These ventilators are small, more rugged, and can be powered
pneumatically or via AC or DC power sources.
• Intensive-care ventilators - These ventilators are larger and usually run on AC power. This style
of ventilator often provides greater control of a wide variety of ventilation parameters (such as
inspiratory rise time). Many ICU ventilators also incorporate graphics to provide visual feedback
of each breath.
• Neonatal ventilators - These are a specialized subset of ICU ventilators which are designed to
deliver the smaller, more precise volumes and pressures required to ventilate these patients.
• Positive airway pressure ventilators (PAP) - These ventilators are specifically designed for noninvasive ventilation. This includes ventilators for use at home for treatment of chronic conditions
such as sleep apnea or COPD.
Modes of mechanical ventilation
Volume controlled
Volume controlled systems of ventilation are based on a measured volume variable which is set
by the clinician. When the ventilator detects the set volume having been applied, the ventilator
cycles to exhalation.
Pressure controlled
Pressure controlled cycling is based on an applied positive pressure that is set by the clinician. In
pressure controlled modes the total volume is variable as the ventilator is using only the pressure
as a measurement for cycling.
Spontaneously controlled
Spontaneously controlled cycling is a flow sensed mode dependant on a spontaneously breathing
patient to cycle. Spontaneously controlled ventilation is typically only in reference to continuous
spontaneous ventilation, also called continuous positive airway pressure (CPAP).
RESULT:
The working mechanism of ventilators are studied.
Experiment No: 5
HEART LUNG MACHINE/CARDIOPULMONARY BYPASS
AIM:
To study the working of a heart lung machine
THEORY:
A device used in open heart surgery to support the body during the surgical procedure while the
heart is stopped. The heart-lung machine is often referred to as the "pump", and does the work of
the heart and lungs during the operation. The heart-lung machine consists of a chamber that
receives the blood from the body, which is normally the responsibility of the heart’s right atrium.
This blood is then pumped by the machine through an oxygenator, a function normally the
responsibility of the right ventricle. The oxygenator removes the CO2 and adds oxygen, which is
normally the work of the lungs. The pump then pumps this newly oxygenated blood back to the
body, which is normally the work of the left heart. The heart-lung machine is connected to the
patient by a series of tubes that the surgical team places. At the end of the operation, the surgeon
gradually allows the patient’s heart to resume its normal function, and the heart-lung machine is
"weaned off".
It is used in cases of cardiac valve repair and/or replacement, Repair of heart defects,
Transplantation (heart transplantation, lung transplantation, heart–lung transplantation).
Figure: Block Diagram
COMPONENTS
A heart lung machine consists of two main functional units, the pump and the oxygenator which
removes oxygen-deprived blood from a patient’s body and replaces it with oxygen-rich blood
through a series of tubes (hoses).
Tubing
The components of the CPB circuit are interconnected by a series of tubes made of silicone
rubber or PVC.
Pumps
Roller pump
The pump console usually comprises several rotating motor-driven pumps that peristaltically
"massage" tubing. This action gently propels the blood through the tubing. This is commonly
referred to as a roller pump, or peristaltic pump.
Centrifugal pump
Many CPB circuits now employ a centrifugal pump for the maintenance and control of blood
flow during CPB. By altering the speed of revolution (RPM) of the pump head, blood flow is
produced by centrifugal force. This type of pumping action is considered to be superior to the
action of the roller pump by many because it is thought to produce less blood damage
(Hemolysis, etc.).
Oxygenator
The oxygenator is designed to transfer oxygen to infused blood and remove carbon dioxide from
the venous blood.
Bubble Oxygenator
The venous blood is passed up a vertical column through which bubbles of oxygen are rising.
Oxygen enters the blood and carbon dioxide is released
Membrane Oxygenator
It uses a thin semi-permeable membrane to separate the blood and gas phases.
Heparin-coated Membrane Oxygenator
It is a new type which is believed to produce less inflammation and decrease the proper density
for blood to clot in the CPB circuit.
Cannulae
Multiple cannulae are sewn into the patient's body in a variety of locations, depending on the
type of surgery. A venous cannula removes oxygen deprived blood from a patient's body. An
arterial cannula infuses oxygen-rich blood into the arterial system. A cardioplegia cannula
delivers a cardioplegia solution to cause the heart to stop beating.
WORKING
The first step in using a heart-lung machine during open-heart surgery is to give the patient a
drug called heparin, which is a powerful anticoagulant . Heparin reduces the blood's ability to
clot, reducing the risk of clots forming in the heart-lung machine and within the tubes placed in
the heart. Once the medication has taken effect, a tube (called a cannula ) from the heart-lung
machine is placed in the upper-right chamber of the heart (the right atrium ), which contains
oxygen-poor blood from the body. Another cannula is placed in the aorta , a large artery that
carries oxygen-rich blood from the heart to the rest of the body. By setting up the machine in this
way, oxygen-poor blood drains into the machine, receives fresh oxygen and is returned to the
aorta to be carried to the rest of the body.
Once the machine is functioning, the surgeon can carefully stop the heart in order to perform the
necessary surgery. When the surgery is complete, the surgeon will restart the heart. Once the
surgical team is satisfied that the heart is beating strongly again, the tubes are removed from the
right atrium and the aorta. To reverse the effects of the heparin given at the beginning of the
process, the patient will be administered another medication called protamine .
Throughout this process, the heart-lung machine is operated by a perfusionist . While the heart is
stopped and the machine is working, the perfusionist continually monitors blood pressure , blood
oxygen levels, carbon dioxide levels, blood temperature and breathing.
During the procedure, hypothermia is maintained; body temperature is usually kept at 28ºC to
32ºC (82.4-89.6ºF). The blood is cooled during CPB and returned to the body. The cooled blood
slows the body's basal metabolic rate, decreasing its demand for oxygen. Cooled blood usually
has a higher viscosity, but the crystalloid solution used to prime the bypass tubing dilutes the
blood.
RESULT:
The working of heart lung machine is studied.
Experiment No: 6
CARDIAC PACEMAKERS
AIM:
To understand the working of an external pacemaker and the various modules included in it.
EQUIPMENTS REQUIRED:
a) Oscillator
b) Refra Generator –2 Nos.
c) Pulse width control
d) Amplitude Control
e) Paced output
f) Synch Generator
g) QRS Detector
h) QRS Filter
i) ECG Amp Pacemaker
j) Patient Simulator
k) Electrodes
l) Charger
m) Battery
THEORY:
A pacemaker is an electronic device equipped with a battery, electronic circuits and memory that
generates electronic signals (pacing pulses), which are carried along insulated wires (leads) to the
heart to make the muscle beat at a normal rhythm. Bradycardia, a heartbeat that slows to an
unhealthy rate, is the most frequent reason for a pacemaker. There are three basic types of
temporary or permanent pacemakers, and each may work on demand, constantly or according to
the heart's activity. Pacemakers are also of internal and external type. Internal pacemakers are
mostly used for permanent heart damages and are surgically implanted beneath the skin near the
chest or abdomen with its output leads connected directly to the heart muscle. The external
pacemakers are mostly used for temporary heart irregularities and are placed outside the body in
the form of a wrist watch or in the pocket, from which one wire will go into the heart through the
vein. Pacemakers may also be of single-, dual-, or triple chambered.
TYPES OF PACEMAKERS
Demand Pacemakers
When the heart's rate is too slow or it misses a beat, demand pacemakers, which monitor the
heart's activity, will send an electrical pulse to set the heart back to a more normal rhythm.
Fixed-rate Pacemakers
Fixed-rate pacemakers discharge steadily, regardless of the heart's natural electrical activity.
Rate-responsive Pacemakers
Rate-responsive pacemakers have sensors that adjust automatically to changes in your physical
activity. They are designed to raise or lower the heart rate to meet the body's needs.
BLOCK DIAGRAM:
BLOCK DESCRIPTION:
Oscillator
An oscillator produces a repetitive electronic signal, often a sine wave or a square wave. The
oscillator synchronises with the synch generator. If an R wave is detected the oscillator circuit is
reset. In the absence of R wave, the oscillator circuit starts and delivers pulses at a paced rate till
the heart rate climbs above the paced rate. The output voltage is 5V and a 10msec pulse. The
oscillator has rate control knob on the top panel.
Refractory Generator
The refractory generator is a non-retrigerrable monostable multivibrator which generates a
250ms delay following an output pulse or a sensed R-wave during which the amplifier in the
sensing circuit will not respond to outside signals. The input is a pulse of 0-5V (Min 1msec) and
output is pulse of 0-5V (250msec).
Pulse Width Control
The pulse width circuit is a basic RC network which determines the duration of the pulse
delivered to the heart. The pulse width control has a range of 0.1- 2.1 msec and output of 5V.
Amplitude Control
The output of the pulse width control is given to the amplitude control which controls the
amplitude of the delivered pulses.
Paced output
The paced output delivers the pulse to the heart, the duration and amplitude being controlled by
the pulse width control and amplitude control. The output also goes to the ECG amplifier as a
feedback signal.
ECG Amp. Pacemaker
The input to the ECG amplifier is from the Paced output which gives ECG signal as the feedback
and it is amplified here.
QRS Filter
The Demand type pacemaker works on the presence or absence of R wave hence a QRS filter is
used to selectively filter the QRS wave. The QRS Filter used here is a Bandpass Filter having
limits from 22Hz to 45Hz with a gain of 10.
QRS Detector
With the presence of an R wave, the QRS detector will generate a pulse. The input voltage is 1V
Pk –Pk and output voltage is 5V. It has a frequency range of 15-30Hz.
Synch Generator
The Synch Generator synchronises with the QRS Detector and in presence of an R wave it resets
the oscillator circuit. In the absence of R wave it allows the oscillator to deliver pulses at its
preset rate.
Patient Simulator
The Patient Simulator simulates the abnormal heart condition- Bradycardia, Tachycardia and AV
block.
PROCEDURE:
1. Connect the modules as per the block diagram.
2. Select one of the abnormal conditions in the Pacemaker Simulator and keep holding the button
for simulating the condition.
3. The Paced output leads detect this abnormality and the output is given as a feedback to the
ECG Amplifier.
4. As a normal QRS wave is not detected, the Oscillator is now triggered and the Pacemaker
takes over and Green LEDs flash.
5. Once the abnormal condition is removed the Paced output leads detect this change, the
oscillator is reset and the Pacemaker stops functioning.
RESULT
Hence the working of a Pacemaker along with its various modules is studied.
Experiment No: 7
SHORTWAVE DIATHERMY
AIM:
To study the working of Shortwave Diathermy.
THEORY:
Short Wave diathermy current is a high frequency alternating current. The heat energy obtained
from the wave is used for giving relief to the patient. Its frequency is 27.33 MHz and the
wavelength is 11 metres. Short wave diathermy machines utilize two condenser plates that are
placed on either side of the body part to be treated. Another mode of application is by induction
coils that are pliable and can be molded to fit the part of the body under treatment. As the highfrequency waves travel through the body tissues between the condensers or the coils, they are
converted into heat. The degree of heat and depth of penetration depend in part on the absorptive
and resistance properties of the tissues that the waves encounter. Short wave diathermy usually is
prescribed for treatment of deep muscles and joints that are covered with a heavy soft-tissue
mass, for example, the hip. In some instances short wave diathermy may be applied to localize
deep inflammatory processes, as in pelvic inflammatory disease.
The units can be operated in either Continuous mode or Pulsed mode.
Two basic types of electrodes (applicators) are in use: Capacitor-type and Inductor type. In the
first case tissue heating is basically due to the radiofrequency electric field, while for the
inductive electrodes (coils), heating occurs by a combination of electric field effects and currents
induced in the tissue by the magnetic field.
WORKING OF SHORT WAVE DIATHERMY:
Shortwave diathermy heats the tissue by causing oscillations of electromagnetic energy of high
frequencies. The physiologic effects of temperature occur at the site of the application and in
distant tissue. The local effects occur due to the elevated local temperature which is associated
with increased local blood flow, capillary dilatation and capillary permeability. It results in
higher level tissue metabolism and more rapid transfer of nutritional ingredients to the end
organs and tissues. It promotes faster healing. Short wave heat increases connective tissue
elasticity, reduces muscle spasm, and sedates the nerve endings to change the pain threshold.
Distant changes from the heated target location include reflex vasodilatation and reduction of
muscle spasm, increase in body temperature, respiratory and pulse rates and decreased blood
pressure. Diathermy increases white blood cell concentration in the area of chronic
inflammation.
RESULT:
The working principle of shortwave diathermy is studied.
Experiment No: 8
CEREBELLAR STIMULATORS/NERVE STIMULATORS
AIM:
To stimulate the nerves using modules.
THEORY:
Electrical nerve stimulators, also known as neuromodulators, send mild electrical pulses to
nerves in the lower back and help manage urinary function or offer relief of chronic pain.
Neuromodulators have been helpful for many IC patients who don’t get enough relief from other
therapies. In order to propagate a nerve impulse, a certain threshold stimulus must be applied to
the nerve. Of note, below this threshold, no impulse is propagated, and increase in the intensity
of the stimulus above this threshold propagation or triggered impulse is further increased.
Assuming a square pulse of the current is used to stimulate the nerve, the total energy (charge)
applied to the nerve is a product of the intensity of the current and the duration of the of the
pulse. when the nerve is stimulated by an electrode, significantly less current is needed to
obtained a response to nerve stimulation when the cathode (negative) is adjacent to the nerve,
rather than the anode (positive is adjacent to the nerve). The reason for this phenomenon is
because when the stimulating electrode is negative, the current flow alters the resting membrane
potential adjacent to the needle, producing an area of depolarization which then spreads across
the nerve. When the electrode adjacent to the nerve is an anode, the current causes an arc
hyperpolarization adjacent to the needle and a ring of depolarization distal to the needle tip.
FEATURES OF NERVE STIMULATORS
Constant current output
The impedance of tissues, needles, connecting wires and grounding electrodes may vary (e.g., 1
kOhm-20 kOhm). The constant current design of the stimulator allows for an automatic
compensation for changes in tissue or connection impedance during nerve stimulation, assuring
accurate delivery of the specified current.
Current meter
The ability to read the current being delivered is of utmost importance, since the current
intensity at which the nerve is stimulated gives the operator an approximation of the needlenerve distance; 0.5 mA or less indicating intimate needle-nerve relationship.
Current intensity control
This can be either via digital means or better an analog dial.
Short pulse width
A short pulse width, e.g. 50 µsec -100 µsec corresponds to chronaxies of a Alpha fibers,
triggering muscle twitches rather than causing pain on stimulation.