Principle of operation of DC defibrillator with reference to: nature & waveform of the defibrillating pulse; necessity for the use of ‘joules’ to describe the level required for defibrillation

1. Energy used for cardioversion & defibrillation

a) Electrical output of defibrillators is expressed in terms of energy

b) Joules (J) or watt-seconds (Ws) describe the power (watts) and the length of time it is applied (sec)

c) Thus Energy (Joules) = power (watts) ´ duration (seconds)

d) note: Watt = current (amps) ´ voltage (volts)

watt = 1 J sec^-1

^e)

Capacitance is the measure of the ability of an object to hold an electrical charge.

SI unit is Coulomb (C)

Coulombs = Amperes (A) x seconds (s)

Defibrillators are set according to the amount of energy stored; this depends on both the charge & the potential

f)

g) Power (W) = Energy (J) per second

h) Current(A) = Charge(C) per second

i)

ii) J = CV

iii) Stored energy (J) = 1/2 X stored energy (C) x Potential (V)

iv) EG: potential of 5000 V is applied across two plates of a capacitor, produces a store of electrons of 160mC of charge

v) Energy = 1/2 x 160 mC x 5000 V = 400 J

2. Operation

a) Defibrillation energy is temporarily stored in a capacitor

b) The large capacitor is charged to the selected energy and then discharged through the paddles applied to the chest

i) The energy stored in the capacitor is released as a current pulse (eg, 35 A for 3 ms) resulting in a synchronous contraction of the heart after which a refractory period and normal beats may follow

c) Inductor

i) Included in circuit to ensure that the electric pulse has an optimal shape & duration

ii) During discharge, the inductor absorbs some of the energy so that not all is discharged to the patient

a) Defibrillators are calibrated in terms of delivered energy not stored energy

3. DC Defibrillator pulse shapes (waveforms)

a) The defibrillation waveform is a major factor in determining efficacy of defibrillation

b) Damped sinewave defibrillator

i) Consists of a capacitor, inductor and electrodes

ii) Placement of an inductance coil in series with the capacitor, the resultant waveform is half sinusoidal in configuration

iii) A slight variation is the Underdamped sinewave

a) Sine wave reverses slightly

b) May reduce defibrillation threshold

iv) The duration of the current for adult is 5 — 10 msec

v) Current intensity is dependent on set stored energy on defibrillator

c) Patient impedance

i) The resistance to current that is offered by the chest is called chest impedence: Average 75 W

ii) Variations in the patient’s impedance cause the delivered dose of current to vary widely

iii) Factors that influence impedence between the defibrillator paddles (resistance):

a) Delivered energy

b) Paddle (electrode) size & composition

c) Interface between paddle & skin (gel used to reduce)

d) Paddle pressure (increase press — decrease imped)

e) Time interval between discharges

f) Number of discharges (incr number -dec imped)

g) Phase of patient ventilation

h) Distance between paddles