EC8073 Medical Electronics - Hints for Slow Learner Nadar Saraswathi College of Engineering and Technology, Theni, Tamilnadu, India.

1. N.Mathavan || AP – ECE – NSCET
DC Defibrillators
 Defibrillator is an electronic device that creates a sustained myocardial depolarization
of a patient's heart in order to stop ventricular fibrillation or atrial fibrillation.
 The instrument for administering the electric shock is called as defibrillator.
 Defibrillation is the application of electric shock to the area of the heart which makes all
the heart muscle fibers enter their refractory period together, after that normal heart
action may resume.
 If the heart does not recover spontaneously after delivering the shock to the heart using
defibrillator then a pacemaker may be employed to restart the rhythmic contraction of the
myocardium.
1. Types of Defibrillators
o Internal Defibrillators (Surgical Type)
o External Defibrillators (Therapeutic Type)
2. Types of defibrillator based on operation or Voltage delivered
o AC defibrillator
o DC defibrillator
o Synchronized DC defibrillator
o Square pulse DC defibrillator
o Double square pulse DC defibrillator
o Biphasic DC defibrillator
Types of Defibrillators
 Internal Defibrillators (Surgical Type)
 It is used when chest is opened.
 Here large spoon shaped electrodes with
insulated handle are used.
 Electrodes in the form of fine wires of
Teflon coated stainless steel are used.
 There are AC and DC defibrillator methods but DC defibrillator is used today.
 Since the electrode comes in direct contact with the heart, the contact impedance is
about 50 ohms.

2. N.Mathavan || AP – ECE – NSCET
 The Current passing through the heart is of the order of 1 to 20 amp.
 The Magnitude of the shock voltage is in the range from 50V to 1000V
 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.
 External Defibrillators (Therapeutic Type)
 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.
 The contact impedance on the chest is
about 100 ohms even after applying the gel.
 The Current passing through the heart is of the order of 10 to 60 amp.
 The Magnitude of the shock voltage is in the range from 1000V to 6000V
 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 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.
Types of defibrillator based on operation or Voltage delivered
There are six types of defibrillators based on the nature of the output voltage
delivered.
 AC defibrillator
 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 with various tapings on the secondary side.
 An electronic timer circuit is connected to the primary of the transformer.
 This timer connects the output to the electrodes for a pre-set time.
 The timing device may be a simple capacitor and resistor network or a monostable
multivibrator, which is triggered by a foot switch or a push button switch.

3. N.Mathavan || AP – ECE – NSCET
 The duration of the counter shock may vary from 0.1 second to 1 second depending
upon the voltage to be applied.
 For reasons of safety, the secondary coil of the transformer should be isolated from
earth so that there is no shock risk to anyone touching an earthed object.
 For external defibrillation, the voltages are in the range from 250V to 750V. For
internal defibrillation, the voltages are in the range from 60V to 250V.
Disadvantages:
1. Large currents are required in external defibrillation to produce uniform and
simultaneous contraction of the heart muscles fibers.
2. This current not only causes a violent contraction of the thoracic muscles but also
results in occasional burning of the skin under the electrodes.
2. It produces atrium fibrillation while arresting the ventricular fibrillation.
 DC defibrillator
This is also called Capacitive Discharge D.C. Defibrillators

4. N.Mathavan || AP – ECE – NSCET
 The DC defibrillator does not produce any undesirable side effect and at the same time
it produces normal heart beat effectively.
 The 230V AC main supply is connected to a variable autotransformer (T1 & T2) 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.
 The voltage to which C is charged is determined by the autotransformer in the
primary circuit.
 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 that
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 A to position B.
 The capacitor is discharged through the electrodes
and the patients are protecting by a resistive load
and the inductor L.
 The inductor L 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 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 to 10 millisecond.

5. N.Mathavan || AP – ECE – NSCET
 Once the discharge is completed, the switch automatically returns to position A 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
 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 60µF.
 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 mis) at a much lower voltage.
 In circuit diagram, the parallel combination of C1 and C2 stores the same energy as the
single capacitor.
 However, its discharge characteristic is more rectangular in shape (1onger duration of
approximately 10 mis) at a much lower voltage.
 With this type of waveform, effective defibrillation can be achieved in adults with lower
levels of delivered energy - between 50 and 200 watts.
E = ½ CV2

6. N.Mathavan || AP – ECE – NSCET
 Synchronized DC defibrillator
 DC defibrillator circuit consisting of defibrillator, electro cardio scope and pacemaker.
 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.
 Synchronizer 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:
ECG unit – To obtain the electrocardiogram of the patient who is going to receive
defibrillation pulse.
Switch - The medical attendant energizes the switch to deliver a defibrillation pulse.
The QRS detector - It pass a signal if R wave is absent in the electrocardiogram. If R Wave is
present it would not give any output.
The AND gate 'B' - Delivers on signal to the defibrillator only when the 'R' wave is absent, As
the two inputs of AND gate 'B' it receive the signal from the medical attendant & QRS
detector ,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. N.Mathavan || AP – ECE – NSCET
Fibrillation detector - It 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.
Thus when the AND gate B and C are simultaneously triggering the defibrillator, the
defibrillation pulse is delivered.
In the synchronization or Cardiovert 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.
 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.
 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.

8. N.Mathavan || AP – ECE – NSCET
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.
 Double Square Pulse Defibrillator
 Double square pulse defibrillator is normally used after the open-heart surgery.
 If the chest is opened, only lower energy electric shock should be given.
 Instead of 800 - 1500V, employed in capacitor discharge DC defibrillators, Here
8-60 V double pulse is applied with a mean energy of 2.4 watt-second.
 When the first pulse is delivered, some of the fibrillating cells will be excitable and will
be depolarized. 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. 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.
 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.