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TRANSCRIPT
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EC1006 – MEDICAL ELECTRONICS / Panimalar Engg. College 1
UNIT III
ASSIST DEVICES AND
BIO-TELEMETRY
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UNIT III ASSIST DEVICES AND BIO-TELEMETRY
Cardiac pacemakers, DC Defibrillator, Telemetry principles, frequency
selection, Bio-telemetry, radio-pill and tele-stimulation.
Biotelemetry:
Biotelemetry is “the measurement of biological parameters over longer
distance”. The 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.
Applications of Bio-Telemetry:
In many situations, it becomes necessary to monitor physiological
events from a distance. To quote a few applications are,
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.
4. Patient monitoring, where freedom of movement is desired, such as
in obtaining 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 common mode interference.
7. Miniaturization of the radio telemetering system helps to reduce
noise.
Physiological parameters adaptable to biotelemetry
Based on the hardware systems, measurements can be applied to two
categories:
1. Bioelectrical Parameters, such as ECG, EEG and EMG.
2. Physiological variables that require transducers such as blood
pressure, gastrointestinal pressure, blood flow and temperatures.
Bioelectric Parameters: (such as ECG, EMG and EEG)
The signal is obtained directly in electrical form.
One example is ECG telemetry - the transmission of ECGs from an
ambulance or site 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
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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.
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 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.
Advantages of Biotelemetry:
1. Major advantage of using biotelemetry is removing the cables
from patient and providing a more comfortable medium to
patient. Patient needs to carry only a small transmitter.
2. Isolation of patient from high voltage completely. Transmitters
in the patient side work with batteries without any danger of
electrical shock.
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3. Battery operated amplifiers and transmitters will cause no
additional noise 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 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.
Classification of Telemetry Systems:
Telemetry Systems are classified as,
1. Based on data transmitted
a. Analog
b. Digital
2. Based on transmission distance
a. Short
b. Long
3. Based on whether user as control over transmission channel or
not.
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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 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. Where as
the final modulator is practically always an FM system.
Elements of Biotelemetry Systems:
� The essential blocks of a biotelemetry system are shown in Figure
below.
� The transducer converts the biological variable into an electrical sig-
na1.
� 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 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 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.
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
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.
At the receiver side, modulated subcarriers will be separated by the
proper band pass filters after the first discrimination (demodulation).
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:
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.
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' to avoid interference between channels.
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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.
The pill comprises an outer biocompatible capsule encasing micro
sensors, a control chip, radio transmitter and two silver-oxide cells.
INSIDE THE CAPSULE:
The schematic diagram of the microelectronic pill is as shown in figure
below. The outer casing of the pill is made by machining chemically
resistant polyetheterketone, which is 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
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4. batteries.
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.
Silicon diode:
The silicon diode is used to measure the body core temperature and
also identify local changes associated with tissue inflammation and ulcers.
ISFET:
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 excretion, reflux to
the oesophagus and the effect of GI-specific drugs on target organs.
Gold electrodes:
A pair of direct contact gold electrode is used to measure conductivity.
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.
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3- electrode electrochemical cell:
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.
The implementation of a generic oxygen sensor will also enable the
development of a first generation enzyme linked amperometric
biosensors, thus extending the range of future applications to include
(eg.) glucose and lactate sensing, as well as immunosensing protocols.
The microelectronic sensors are attached to the PCB chip carrier by a
10 pin, 0.5mm pitch polyimide ribbon connector. The PCB carrier is made
from 1.6mm thick fiberglass board. The transmitter and the ASIC are also
integrated on the board.
The integrated radio transmitter sends the signal to a local receiver
prior to data acquition on a computer.
The unit is powered by two standard 1.55V silver-oxide cells with a
capacity of 175mAh.The batteries are connected in series and provide an
operating time of 40 hours at the rated power consumption of 12.1mW.
The sensor chips are provided at the front end of the pill and are exposed
to the ambient environment through access ports. They are scaled by two
sets of stainless-steel clamps incorporating an o.8µm thick sheet of
fluoroelastomer seal. The 3mm diameter access channel in the center of
each steel clamp exposes the sensing region of the chips to the ambient
environment.
SENSORS:
The schematic diagram of sensor chips is as shown below.
The sensors are fabricated on two silicon chips located at the front
end of the capsule.
Chip1, measuring 4.75 x 5mm2, comprises the silicon diode
temperature sensor, the pH ISFET sensor and the two-electrode 5x 10-4mm2
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conductivity sensor. Predefined n-channels in the p-type bulk silicon form
the basis for the diode and the ISFET. The 15x600µm floating gate of the
ISFET is precovered with a 50nm thick proton-sensitive layer of Si3N4 for pH
detection. The pH sensor consists of the integrated 3x 10-2mm2 Ag/Agcl
reference electrode, a 500µm diameter and 10-nL electrolyte chamber and
15x600µm floating gate of the ISFET sensor.
Chip2, measuring 5 x 5mm2, comprises the electrochemical oxygen
sensor and a NiCr resistance thermometer. The oxygen sensor is embedded
in the electrolyte chamber. The 3-electrode electrochemical cell of the oxygen
sensor comprises the 1x10-1 mm2 counter electrode made of gold, a
microelectrode array of 57x10µm diameter working gold electrodes and an
integrated 1.5x 10-2mm2 Ag/Agcl reference electrode.
The microelectrode array has an inter-electrode spacing of 25µm and a
combined area of 4.5x 10-3mm2. It promotes electrode polarization and
reduces response time by enhancing transport to the electrode surface.
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The NiCr resistance thermometer is made from a 100nm thick layer of
NiCr and is 5µm wide and 11mm long.
The 500nm thick layer of thermally evaporated silver is used to
fabricate the reference electrode. It is then oxidized to Ag/Agcl by
chronopotentiometry.
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.
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.
The conductivity circuit operates at direct current, measuring the
resistance across the electrode pair as an inverse function of solution
conductivity. An incorporated potentiostat circuit operates the amperometric
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oxygen sensor with a 10-bit DAC controlling the working electrode potential
with respect to the reference.
The analogue signals have a full-scale dynamic range of 2.8V with the
resolution 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.
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,
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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Defibrillators
Introduction:
Defibrillator is an electronic device that creates a sustained
myocardial depolarization of a patient’s heart in order to stop ventricular
fibrillation or arterial 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 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)
Internal defibrillator:
It is used when chest is opened.
Here large spoon shaped electrodes with insulated handle are used.
Sometimes 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.
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 pediatric patient and 8 to 10 cm diameter for adult
patients with a highly insulated handle.
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Mechanism:
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.
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
1. AC 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 fibres 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 transformer.
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To achieve defibrillation with internal electrodes placed on the surface
of the heart (in open heart surgery), voltage ranging from 80 to 300V rms is
required.
When external electrodes are used on the chest, voltages of twice the
value are required.
The transformer must be capable of supplying 4 to 6 amperes current
during the stimulus period.
Disadvantages:
1. There are many disadvantages in using AC defibrillators.
2. Successive attempts to correct ventricular fibrillation are often
required.
3. AC defibrillator cannot be successfully used to correct atrial
fibrillation.
2. Capacitive Discharge DC Defibrillators
The Capacitive Discharge type DC Defibrillator is as shown in the
figure below.
The 220V 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.
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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.
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.
The capacitor is discharged through the electrodes and the patient's
torso represented by a resistive load, and the inductor L.
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.
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.
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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 = 2
1CV2
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 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.
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 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.
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.
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'.
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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. Suppose a patient is suffered by atrial fibrillation. First the
doctor diagnoses it correctly and then the treatment is initiated using
this circuit.
10. 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.
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
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
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
EP = TE
D
RR
V
+2
2
.TD = ID2 (2RE + RT)TD
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
= TE
T
RR
R
+2EP
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 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 =
DTE
T
RRR
R
++2EC
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.
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Advantages:
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.
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.
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
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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 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.
Defibrillator Electrodes
The two defibrillator electrodes applied to the thoracic walls are called
either Anterior-Anterior or Anterior-Posterior paddles.
With anterior-anterior paddles, both paddles are applied to the chest.
Anterior-posterior paddles are applied to both the patient's chest wall
and back, so that the energy is delivered through the heart. This method of
paddle application offers better control over arrhythmias that occur as a
result of atrial activity.
These two methods are shown in Figure below.
To maintain good contact, the electrodes must be firmly placed
against the patient. The posterior paddle is flat and has a larger disc (with a
radial handle) than the anterior paddle (axial handle). The electrodes must
be sufficiently well insulated, so that the operator holding the electrodes is
safe.
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(a) (b)
(c) (d)
(a) Anterior-Anterior Electrode Placement on the Chest
(b) Anterior-Posterior Electrode Placement on Chest & Back
(c) Paddle-Type External Electrode which is Applied on Chest Wall
(d) A Spoon-shaped Internal Electrode which is Applied Directly to the Heart
Muscle
Two types of electrodes for defibrillation are shown in the above figure,
and Figure (c) shows the type of electrode used for external defibrillation.
This electrode consists of a large metal disc, approximately 100 mm in
diameter, in an insulated housing.
A control switch is located on the handle so that, once the electrodes
are in place, the operator can push the switch to initiate the pulse. While
being used, the electrodes surface is coated with a conducting gel of the type
used with an ECG recording.
Figure (d) shows an internal type of electrode which is spoon shaped,
for applying directly on the myocardium (during open-chest surgery), or it
may be applied to the chest of an infant.
In these applications, the energy levels required for defibrillation may
range from 10 to 50 watts. Special pediatric paddles are available with
diameters ranging from 2 to 6 cm.
The energy of a defibrillator is usually given in terms of watts/sec,
referenced across a 50 ohm resistor. Most defibrillators today have a
charging capacity of 400 watts.
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Typical defibrillating values used (in watt are as follows:
S.No Patient Defibrillating Value
1 Adult (external) 200-400
2 Adult (internal) 35-75
3 Pediatric (external) 100-200
4 Pediatric (internal) 25-50
Due to energy dissipation as heat in components inside the unit and
to some extent at the electrode skin interface, there is usually a 20% loss of
energy. Most defibrillators include watt meters to indicate the amount of
energy stored in the capacitor prior to discharge.
PACEMAKER
INTRODUCTION:
Pacemaker is an electrical pulse generator for starting and/or
maintaining the normal heart beat.
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.
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.
The pulse voltage is made variable to allow adjustments in the energy
delivered by the pacemaker to the heart during each pulse.
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.
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The typical ranges of parameters of the pacemakers available today
are,
S.No Parameters Ranges
1 Pulse rate - 25 - 155 pulses per minute
2 Pulse width - 0.1 - 2.3 milliseconds
3 Pulse amplitude - 2.5 - 10 volts
4 Battery capacity - 0.44 - 3.2 amp-hours
5 Longevity - 3.5 - 18 years
6 End-of-life indicator - 2 - 10% dropin pulse rate
7 Weight - 33 - 98 grams
8 Size - 22 - 80 cm3
9 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 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 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.
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Classification of Pacemakers based on placement:
Based on the placement of the pacemaker, there are two types
1. External pacemaker and
2. Internal (Implanted) pacemaker
S. No
External Pacemaker Internal Pacemaker
1
The pacemaker is placed outside
the body. It may be in the form of
wrist watch or in the pocket, from
that one wire will go into the
heart through the vein.
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.
2
The electrodes are called
endocardiac electrodes and are
applied to the heart by means of
an electrode catheter with
electrode's tip situated in the
apex of the right ventricle. These
are in contact with the inner
surface of the heart chamber.
The electrodes are called
myocardiac electrodes and are in
contact with the outer wall of the
myocardium.
3 It does not need the open chest
surgery
It requires a minor surgery to place
the circuit.
4
The battery can be easily
replaced and any defect or
adjustment in the circuit can be
easily attended without getting
any help from a medical doctor.
The battery can be replaced only by
minor surgery. Further any defect
or adjustment in the circuit cannot
be easily attended. Doctor's' help is
necessary to rectify the defect in the
circuit.
5
During placement, swelling and
pain do not arise due to
minimum foreign body reaction.
During placement swelling and pain
arise due to foreign body
reaction.
6 Here there is no safety for the Here there is a cent percent safety
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pacemaker particularly in the
case of children carrying the
pacemaker.
for the circuit from the external
disturbances.
7 Mostly these are used for
temporary heart irregularities
Mostly these are used for
permanent heart 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
3. Ventricular inhibited pacemaker (demand pacemaker)
4. Atrial synchronous pacemaker
5. Atrial sequential ventricular inhibited pacemaker
1. Ventricular asynchronous pacemaker (fixed rate pacemaker)
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. This produces a stimulus at a fixed rate irrespective of
the behaviour 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 αα
+−
1
1
Where ‘α’ is the feedback voltage fraction such that
21
2
RR
R
+=α
The period of the oscillator can be changed by changing ‘α’ 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.
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The square wave generator is an astable multivibrator which
periodically switches between the output voltages |V sat| and -|Vsat|.
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
TD = 5Cc
+ 43
43
RR
RR = -R5 Cm ln
+ 43
3
RR
R
Advantages:
1. It has the simplest mechanism and the longest battery life.
2. It is cheap.
3. It is least sensitive to outside interference.
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Disadvantages:
1. There may be competition between the natural heart beats
and pacemaker beats
2. Using the fixed rate pacemaker, the heart rate cannot be
increased to match greater physical effort.
3. 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:
Using the sensing electrode, the heart rate is detected and is given to
the timing circuit 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.
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.
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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.
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. They could not undergo diathermy treatment and could
not be exposed to airport security metal detector. Further they
could not ride motor or scooters. But in the newer pacemakers,
this is eliminated by connecting a low pass filter in the input
circuit of the pacemaker
3. Ventricular Inhibited Pacemaker (Demand Pacemaker)
The R wave inhibited pacemaker allows the heart to pace at its normal
rhythm when it 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.
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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.
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.
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.
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.
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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 block diagram for the atrial synchronous pacemaker is as shown
below. The atrial activity is picked up by a sensing electrode placed in a
tissue close to the dorsal wall of the atrium.
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.
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.
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 patients 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.