Choosing the Optimal Monophasic and Biphasic Waveforms for Ventricular Defibrillation

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Abstract
Optimal Monophasic and Biphasic Waveforms. Introduction: The truncated exponential waveform from an implantable cardioverter defibrillator can be described by three quantities: the leading edge voltage, the waveform duration, and the waveform time coastant (τs). The goal of this work was to develop and test a mathematical model of defibrillation that predicts the optimal durations for monophasic and the first phase of biphasic waveforms for different τs values. In 1932, Blair used a parallel resistor-capacitor network as a model of the cell membrane to develop an equation that describes stimulation using square waves. We extended Blair's model of stimulation, using a resistor-capacitor network time constant (τm), equal to 2.8 msec, to explicitly account for the waveform shape of a truncated exponential waveform. This extended model predicted that for monophasic waveforms with τs of 1.5 msec, leading edge voltage will be constant for waveforms 2 msec and longer; for τs of 3 msec, leading edge voltage will be constant for waveforms 3 msec and longer; for τs of 6 msec, leading edge voltage will be constant for waveforms 4 msec and longer. We hypothesized that the best phase 1 of a biphasic waveform is the best monophasic waveform. Therefore, the optimal first phase of a biphasic waveform for a given τs is the same as the optimal monophasic waveform. Methods and Results: We tested these hypotheses in two animal experiments. Part I: Defibrillation thresholds were determined for monophasic waveforms in eight dogs. For τs of 1.5 msec, waveforms were truncated at 1, 1.5, 2, 2.5, 3, 4, 5, and 6 msec. For τs of 3 msec, waveforms were truncated at 1, 2, 3, 4, 5, 6, and 8 msec. For τs of 6 msec, waveforms were truncated at 2, 3, 4, 5, 6, 8, and 10 msec. For waveforms with τs, of 1.5, leading edge voltage was not significantly different for the waveform durations of 1.5 msec and longer. For waveforms witb τs of 3 msec, leading edge voltage was not significantly different for waveform durations of 2 msec and longer. For waveforms with τs of 6 msec, there was no significant difference in leading edge voltage for the waveforms tested. Part II: Defibrillation thresholds were determined in another eight dogs for the same three τs values For each value of τs, six biphasic waveforms were tested: 1/1, 2/2, 3/3, 4/4, 5/5, and 6/6 msec. For waveforms with τs of 1.5 msec, leading edge voltage was a minimum for the 2/2 msec waveform. For waveforms with τs of 3 msec, leading edge voltage was a minimum for the 3/3 msec waveform. For waveforms with τs of 6 msec, leading edge voltage was a minimum and not significantly different for the 3/3, 4/4, 5/5, and 6/6 msec waveforms. Conclusions: The model predicts the optimal monophasic duration and the first phase of a biphasic waveform to within 1 msec as τs varies from 1.5 to 6 msec: for τs equal to 1.5 msec, the optimal monophasic waveform duration and the optimal first phase of a biphasic waveform is 2 msec, for τs equal to 3.0 msec, the optimal duration is 3 msec, and for τs equal to 6 msec, the optimal duration is 4 msec. For both monophasic and biphasic waveforms, optimal waveform duration shortens as the waveform time constant shortens.

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