Electrical Principles I Coursework

Assignment 2: the defibrillator

Defibrillators are designed to restore the normal rhythm of heart activity when it is disturbed by

accident or disease, by passing a large but short-lived electric current through the chest. Although

the basic defibrillator circuit has a transient RC behaviour, an inductor may be introduced, giving

an RLC behaviour. The inductor allows the delivered pulse to be shaped in ways not possible

with the simple RC configuration.

Your assignment is as follows:

(a) Adapt the theory of RLC processes given in your lecture notes to model theoretically the

behaviour of the defibrillator. The basic differential equation for the current is unchanged, but the

initial conditions will be different, and this leads to different solutions: Start by stating the correct

initial conditions, then apply them to the general solutions to derive the solutions. The cases R2 ≥

4L/C and R² < 4L/C should both be covered. (Note: you do NOT need to rederive the general

solutions from the DE. Start at the point where the initial conditions for current and voltages are

used).

(b) Use SIMetrix to simulate the defibrillator circuit. Choose appropriate values for R, L and C.

Note: no switches are needed - by setting the initial state of the capacitor and inductor you can

reproduce the initial conditions of part (a). Compare your results for the current with the theory of

part (a) and the sketch graphs in the accompanying notes. (Hint: suitable values for R and C can

be derived from the notes - L can then be adjusted to give reasonable agreement).

(c)Finish off your report with a general discussion of defibrillators, informed by parts (a) and (b)

and by your reading and research (give adequate references to any sources used).

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. 1. Energy used for cardioversion & defibrillationa)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) x duration (seconds)

d) note: watt = current (amps) x voltage (volts)1 watt = 1 J s ̄1

e) Defibrillators are set according to the amount of energy stored; this depends on

both the charge & the potential

Power[W] CurrentfA]

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

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)

Potential[V] =

i) Cs'1

ii) J = CViii) 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. 2. Operationa) 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

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

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

v=- Js"1 J

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 sine wave 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 sine wave

a) Sine wave reverses slightly

b) May reduce defibrillation threshold

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

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

c) c) Patient impedance

i) The resistance to current that is offered by the chest is called chest

impedance: Average 75 Q ii) Variations in the patient's impedance

cause the delivered dose of current to

vary widely iii) Factors that influence impedance 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 impedance)

e) Time interval between discharges

f) Number of discharges (increase number -decrease impedance)

g) Phase of patient ventilation