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The Strength‐Duration Relationship of Monophasic Waveforms with Varying Capacitance Sizes in External Defibrillation

The shape of the shock waveform influences defibrillation efficacy. However, the optimal combination between capacitance size and truncation/tilt which can determine monophasic waveform's shape, has not been determined for external defibrillation. The purpose of this study was to assess the effects of varying capacitance and tilt on external defibrillation using exponential monophasic waveforms. In a pig model of external defibrillation (n = 10, 30 ± 6 kg), nine exponential monophasic waveforms combining three capacitance values (30 μF, 60 μF, and 120 μF) and three tilt values (55%, 75%, and 95%) were tested randomly. The energy and leading edge voltage at 50% defibrillation success (E50 and V50) were used to evaluate defibrillation efficacy. E50 and V50 were determined by the Bayesian technique. The lowest stored E50 for the 30μF, 60 μF, and 120 μF waveforms were 90 ± 12 J (95% tilt), 106 ± 45 J (55% tilt), and 107 ± 52 J (75% tilt), respectively. The lowest V50 for the 30 μF, 60 μF, and 120 μF waveforms were 2,439 ± 166 V (95% tilt), 1,849 ± 375 V (55% tilt), and 1,301 ± 322 V (75% tilt), respectively. The average current at external defibrillation threshold demonstrated a strength versus pulse duration relationship similar to that seen with pacing. Reducing capacitance has the same effect as truncating the waveform. The E50 is more sensitive to tilt values changes in larger capacitance waveforms. This study suggests that the optimal combination between capacitance and tilt may be 120 μF and 55%–75% for external defibrillation. (PACE 2003; 26:2213–2218)