Strength-duration and probability of success curves for defibrillation with biphasic waveforms.

Abstract

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

Influence of Phase Duration of Biphasic Waveforms on Defibrillation Energy Requirements With a 70-μF Capacitance

“…Similar findings have been described by Feeser et al 26 Using a constant phase-1 duration of 3.5 ms, they found two voltage minima at the DFT for phase-2 durations of 4 ms and 7 ms with significantly higher voltages at the DFT for phase-2 durations between these two values. They suggested two possible mechanisms accounting for these observations: (1) Depolarization after hyperpolarization: The role of the second phase is to depolarize that half of the myocardial cell membrane that has been hyperpolarized during phase 1.…”

“…Halting of myocardial excitation by a hyperpolarizing impulse after the excitation threshold has been reached was described by Weidmann. 29 It was proposed 26 that with increasing phase-2 durations first, the mechanism of depolarization after hyperpolarization might be predominating, thus resulting in lower voltages at the DFT. Then, with longer phase-2 durations the effect of hyperpolarization after depolarization may outweigh the beneficial effect of depolarization after hyperpolarization on the other side of the myocardial cells, thus leading to an increase of the voltage at the DFT.…”

“…However, the finding of more than one optimal phase-2 duration may be extrapolated to other lead configurations and impedances because similar results have been found despite different models. 25,26 Although the relative heart weight of the applied pigs, which is 0.3% of the body weight of the animal, is comparable to humans, 31 differences of this model due to the lack of organic heart disease and the mode of induction of ventricular fibrillation may have caused differences to spontaneous episodes of ventricular fibrillation in patients.…”

Optimization of Bipolar Pulses of External Defibrillators Based on the Repolarization Hypothesis

“…It was shown experimentally in [12] that decreasing phase II to phase I amplitude ratio from 0.5 0.6 to 0.37 0.42 would result in a decrease in the Gurvich-Venin defibrillation pulse efficacy. Similar results were reported in [13] for a bipolar trapezoidal defibrillation pulse with amplitude ratio of phase II to phase I equal to 0.21 ± 0.03, 0.62 ± 0.08, and 0.94 ± 0.08. In experiments with laboratory animals reported in [14], the optimal range of amplitude ratio of phase II to phase I was determined to be 0.5 0.75 for a bipolar quasi sinu soidal defibrillation pulse.…”

Ventricular Defibrillation With Triphasic Waveforms

“…2,27,28 For monophasic shocks, when the RV electrode is an anode, the DFT is significantly lower than when it is a cathode. 23 In some cases, polarity appears to affect the efficacy of biphasic waveforms, whereas in other cases it does not.…”