CHOOSING THE OPTIMAL MONOPHASIC AND BIPHASIC WAVE-FORMS FOR VENTRICULAR DEFIBRILLATION

Optimal Monophasic and Biphasic Waveforms. Introduction: The truncated exponential waveform from an implantable cardioverter defibrillator c an be described by three quantities: the leading edge voltage, the wav eform duration, and the waveform time constant (tau(s)). The goal of t his work was to develop and test a mathematical model of defibrillatio n that predicts the optimal durations for monophasic and the first pha se of biphasic waveforms for different tau(s) values. In 1932, Blair u sed a parallel resistor-capacitor network as a model of the cell membr ane to develop an equation that describes stimulation using square wav es. We extended Blair's model of stimulation, using a resistor-capacit or network time constant (tau(m)), equal to 2.8 msec, to explicitly ac count for the waveform shape of a truncated exponential waveform. This extended model predicted that for monophasic waveforms with tau(s) of 1.5 msec, leading edge voltage will be constant for waveforms 2 msec and longer; for tau(s) of 3 msec, leading edge voltage will be constan t for waveforms 3 msec and longer; for tau(s) of 6 msec, leading edge voltage will be constant for waveforms 4 msec and longer. We hypothesi zed that the best phase 1 of a biphasic waveform is the best monophasi c waveform. Therefore, the optimal first phase of a biphasic waveform for a given tau(s) is the same as the optimal monophasic waveform. Met hods and Results: We tested these hypotheses in two animal experiments . Part I: Defibrillation thresholds were determined for monophasic wav eforms in eight dogs. For tau(s) of 1.5 msec, waveforms were truncated at 1, 1.5, 2, 2.5, 3, 4, 5, and 5 msec. For tau(s) of 3 msec, wavefor ms were truncated at 1, 2, 3, 4, 5, 6, and 8 msec. For tau(s) of 6 mse c, waveforms were truncated at 2, 3, 4, 5, 6, 8, and 10 msec. For wave forms with tau(s) of 1.5, leading edge voltage was not significantly d ifferent for the waveform durations of 1.5 msec and longer. For wavefo rms with tau(s) of 3 msec, leading edge voltage was not significantly different for waveform durations of 2 msec and longer. For waveforms w ith tau(s) of 6 msec, there was no significant difference in leading e dge voltage for the waveforms tested. Part II: Defibrillation threshol ds were determined in another eight dogs for the same three tau(s) val ues. For each value of tau(s), six biphasic waveforms were tested: 1/1 , 2/2, 3/3, 4/4, 5/5, and 6/6 msec. For waveforms with tau(s) of 1.5 m sec, leading edge voltage was a minimum for the 2/2 msec waveform. For waveforms with tau(s) of 3 msec, leading edge voltage was a minimum f or the 3/3 msec waveform. For waveforms with tau(s) 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 t he optimal monophasic duration and the first phase of a biphasic wavef orm to within 1 msec as tau(s) varies from 1.5 to 6 msec: for tau(s) e qual to 1.5 msec, the optimal monophasic waveform duration and the opt imal first phase of a biphasic waveform is 2 msec, for tau(s) equal to 3.0 msec, the optimal duration is 3 msec, and for tau(s) equal to 6 m sec, the optimal duration is 4 msec. For both monophasic and biphasic waveforms, optimal waveform duration shortens as the waveform time con stant shortens.