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(Circulation. 1997;96:3517-3520.)
© 1997 American Heart Association, Inc.

Articles

Strength-Duration Relationship for Human Transvenous Defibrillation

Michael R. Gold, MD, PhD; ; Stephen R. Shorofsky, MD, PhD

From the Department of Medicine, Division of Cardiology, University of Maryland School of Medicine, Baltimore Md.

Correspondence to Michael R. Gold, MD, PhD, Division of Cardiology, N3W77, University of Maryland Medical System, 22 S Greene Street, Baltimore, MD 21201. E-mail MGold{at}medicine.ab.umd.edu

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.

Methods and 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 µF. 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.

Key Words: defibrillation • electrophysiology • pulse width • electrical stimulation • tachyarrhythmias

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