Radio Pill

UNIT IIIUNIT IIIUNIT IIIUNIT III ASSIST DEVICES AND BIO ASSIST DEVICES AND BIO ASSIST DEVICES AND BIO ASSIST DEVICES AND BIO ---- TELEMETRY TELEMETRY TELEMETRY TELEMETRY

EC1006 – MEDICAL ELECTRONICS / Panimalar Engg. College 1

UNIT III

ASSIST DEVICES AND

BIO-TELEMETRY



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.



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



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.



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.



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.

�



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.



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.



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).



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.



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



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.



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



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.



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



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).



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.



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.



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.



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.



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.



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.



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.



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'.



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.



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.



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



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.



(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.



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.



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.



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.



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



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.



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.



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.



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.



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.



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.



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.



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.

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