help needed with defibrillator simulation

Thread Starter


Joined Jan 8, 2007

im having problems with my assignment for simulating a defibrillator
it is based on an RLC circuit behavior
the problem is that i cant understand wot initial conditions i should take for R,L and C.

please help me out in this!

im attaching a doc file of the assignment

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).

------------------------- EXTRA INFO -----------------------------------

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) 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

f) Potential[V] = Power[W] / Current[A]
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)

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

V= Js-1 / Cs-1 = J/C

i) Cs'1
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. 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
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

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) Patient impedance
i) The resistance to current that is offered by the chest is called chest
impedance: Average 75 ohms
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
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



Joined Apr 20, 2004

The patient impedance is given to you in the above text (the R). The defib equipment contains the capacitor (the C). And it's up to you to add the L to tailor the pulse discharge characteristics.


Joined Feb 5, 2009
Well now two years latter (02-2009) so probablly the author will not see this. As a biomedical engineer I was curious about the outcome. Was it measurable? I was retired and came back into the workplace - always interested to see the new ones coming up.