Dysfunction and safety factor strength-duration curves for biphasic defibrillator waveforms.

Newly developed biphasic waveforms significantly lower defibrillation threshold in animal and clinical models. However, underlying mechanisms and optimum waveform shape are unknown. Defibrillation shocks produce dysfunction; safety factor, the ratio of shock intensity inducing dysfunction to that producing stimulation of partially refractory cells, is an important parameter for defibrillator waveforms. We determined dysfunction and safety factor strength-duration curves for symmetric and asymmetric (50% undershoot) monophasic and biphasic rectangular (0%-tilt) waveforms. Dysfunction threshold, defined as the voltage producing a 4-s postshock contractile arrest, was determined for waveforms with total durations from 1 to 40 ms. For all waveforms, dysfunction threshold decreased with waveform duration. At all durations, dysfunction threshold was similar for symmetric monophasic and biphasic waveforms with the same total duration. In contrast, asymmetric biphasic waveforms increased dysfunction threshold 14 +/- 3% (P < 0.005) compared with monophasic control waveforms. Because long-duration, low-tilt, biphasic waveforms improve excitation threshold for refractory cells, they should improve defibrillation threshold. Asymmetric waveforms have the additional advantage of improving safety factor by reducing postshock dysfunction.