Certain biphasic waveforms require less energy to defibrillate than do monophasic pulses of equal duration, although the mechanisms of this increased effectiveness remain unclear. This study used strength-duration and percent success curves for defibrillation with monophasic and biphasic truncated exponential waveforms to explore these mechanisms. In part 1, defibrillation thresholds were determined for both high-and low-tilt waveforms. The monophasic pulses tested ranged in duration from 1.0 to 20.0 msec, and the biphasic waveforms had first phases of either 3.5 or 7.0 msec and second phases ranging from 1.0 to 20.0 msec. In part 2, defibrillation percent success curves were constructed for 6.0 msec/6.0 msec biphasic waveforms with a constant phase-one amplitude and with phase-two amplitudes of approximately 21%, 62%, 94%, and 141% of phase one. This study shows that if the first phase of a biphasic waveform is held constant and the second phase is increased in either duration or amplitude, defibrillation efficacy first improves, then declines, and then again improves. For pulse durations of at least 14 msec, the second-phase defibrillation threshold voltage of a high-tilt biphasic waveform is higher than that of a monophasic pulse equal in duration to the biphasic second phase (p<0.05), indicating that the previously proposed hypothesis of stimulation by the second phase is not the sole mechanism of biphasic defibrillation. These facts indicate the importance of the degree of tilt for the defibrillation efficacy of biphasic waveforms and suggest at least two mechanisms exist for defibrillation with these waveforms, one that is more effective for smaller second phases and another that becomes more effective as the second phase is increased. (Circulation 1990;82:2128-2141 C ertain biphasic waveforms require less energy to defibrillate or produce less cardiac dysfunction than do monophasic pulses of equal duration.1-4 The lower defibrillation threshold (DFT) for certain biphasic waveforms has important implications for the effectiveness and lifespan of automated, implantable cardioverter/ defibrillators with power supplies that are constrained by space considerations.5-7Research seeks to answer the following two closely related questions. First, what is the mechanism that

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