Are Defibrillation Thresholds Ruled by a Hyperbolic Strength Duration Relationship?

Abstract

Our defibrillation theory claims that mean voltage is a hyperbolic function of pulse duration if voltages below the rheobase are avoided. To verify this theory, two animal experiments were carried out. The ‘rheobase condition’ demands that pulses are truncated when the trailing edge voltage reaches the rheobase. This creates a relationship between the time constant, pulse duration, and chronaxie. The integral over voltage pulse is a linear function of pulse duration from which the hyperbolic threshold function between mean voltage and pulse duration is derived. In this study, we determined defibrillation thresholds in swine. Parameters measured were: leading and trailing edge voltages and currents, pulse durations, and failure or success of shock. A step-up test was used; the lowest successful shock was defined as the ‘threshold’. Waveforms truncated according to theory yielded lower stored energy than either \(\raisebox{1mm}{\scriptsize 1}{\hspace*{-.5mm}\scriptsize /}\raisebox{-0.8mm}{\scriptsize 2}\) optimal duration or fixed 65% tilt pulses. Plots of voltage integral vs. pulse duration produced a strong linear correlation. Mean defibrillation voltage vs. pulse duration formed a hyperbola. Ranking of stored energy showed that lower capacitances reduce energy to the detriment of increased peak voltages. These are the first experiments in which defibrillation pulses were adjusted according to theory. Truncation above or below the rheobase increased stored energy. The experimental results are consistent with theory. The algorithm for optimal truncation should be incorporated into ICD. If the energy required is lower at smaller output capacitances, a compromise between clinical and technical aspects can be attained. The current concept of ‘constant tilt’ in ICD should be abandoned in favour of ‘optimal truncation’. Additional studies are needed to determine the applicability of this theory to humans.KeywordsDefibrillation thresholdhyperbolic strength duration relationshipchronaxierheobaserheobase condition