Strength-duration relationship for human transvenous defibrillation.
BACKGROUND
One of the basic characteristics of electrical defibrillation is the strength-duration relationship, or the effect of pulse width on defibrillation efficacy. This relationship is important for understanding the mechanism of defibrillation and for the design of optimal waveforms. However, a detailed evaluation of the strength-duration relationship for human transvenous defibrillation has not been performed previously.


RESULTS
This was a prospective study of 29 patients undergoing initial defibrillator implantation with a uniform dual coil, transvenous lead. In each patient defibrillation thresholds were measured for either short (2, 3, 4, 6 ms) or long (6, 12, 18 ms) pulse durations, with the order of testing randomized. The shock waveform was a truncated monophasic pulse from a capacitor of 150 microF. The leading edge voltage at defibrillation threshold was 566+/-100 V for 2-ms pulses. Voltages declined exponentially with increasing pulse width reaching an asymptote by 6 ms (451+/-68 V, P<.05). Defibrillation threshold voltage was insensitive to longer pulse widths. Stored energy at defibrillation threshold showed a similar relationship with pulse width. In contrast, mean current decreased monotonically over the full range of pulse durations evaluated, and there was no evidence of a rheobase.


CONCLUSIONS
The shape of the strength-duration curve and the lack of rheobase current indicate a fundamental difference between cardiac stimulation and defibrillation. The relationship between pulse duration and defibrillation threshold voltage or stored energy is well modeled by a parallel capacitor resistor circuit with a time constant of 5.3 ms.
Gold
M R
MR
Department of Medicine, University of Maryland School of Medicine, Baltimore 21201, USA. MGold@medicine.ab.umd.edu
Shorofsky
S R
SR
D000368:Aged; D004056:Differential Threshold; D004554:Electric Countershock / Q000379:methods*; D005069:Evaluation Studies as Topic; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008955:Models, Cardiovascular; D011446:Prospective Studies; D013997:Time Factors; D016896:Treatment Outcome