CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 413

stimulus waveform over the pulse duration. It describes how steeply the capacitor dis-
charges into the patient's body and gives an indication of how much of the stored energy
is delivered to the tissues. Tilt is dependent on the RC time constant (device capacitance
times the impedance of the transthoracic discharge pathway). Duration refers to the extent
of the de fi brillation waveform. Commonly, duration is in the range 8 to 40 ms.

Bare-Bones De fi brillator

Figure 8.31 shows a damped sine waveform de brillator. A transformer T1 steps up the

power line voltage to a high voltage (a few kilovolts). Capacitor C1 is charged through
recti

fi

er D1, current-limiting resistor R1, and charge switch S1 to some voltage V (meas-
ured through voltmeter M1) in order to store energy E

fi

2 (C1) V 2 . When the de fi brillation

switch S2 closes, the de flows through the inductor L1 and the patient,

who has a transthoracic impedance of R patient . The discharge waveform depends on the
values of C1, L1, and total impedance (R inductor

fi brillation current I fl

R patient ). Note that the critical damping

resistance of the circuit is R critical 2 L 1 /C 1 . Since de fi brillators are commonly designed

assuming that a patient impedance of 50 Ω , and if we assume an inductor impedance

R inductor of 10 Ω , a suitable R critical could be 67 Ω . The actual energy delivered to a patient

depends on patient impedance and is given by

R

pati
R

en t

nductor

E delivered E C1

R patien

t i

For this example, the discharge is underdamped (biphasic, also referred to as a Gurvich
waveform ) when the patient resistance is less than about 56 Ω because

R patient R inductor 56 Ω 10 Ω 66 Ω R critical 67 Ω

In this case, the waveform is underdamped and produces a biphasic (oscillating) wave-
form.
If the patient impedance is higher than 67

, the waveform is overdamped (monopha-
sic, also referred to as an Edmark waveform ). In this case the inductor slows the rate of rise

Ω

Defibrillate

S1

S2

D1 L1

R1

R. Patient

RL1

T1

1 5

Line

V

AC Power

C1

4 8

Neutral

Figure 8.31 Simplified block diagram of a damped sine waveform defibrillator. Transformer T1 steps up the power line voltage to charge
C1 through rectifier D1, R1, and charge switch S1 to some voltage V (measured through voltmeter M1) in order to store energy E

1
2 (C1) V 2 .

When the defibrillation switch S2 closes, the defibrillation current flows through L1 and the patient.