A Conceptual Basis for Defibrillation Waveforms

A model is developed for defibrillation that treats the heart as a first order time constant. Such a model allows ready evaluation of different monophasic waveforms. For implantabls devices where the voltage is provided by the discharge of a capacitor, it can be seen that the effective voltage within the heart rises rapidly to a peak and then decays to zero. The time interval at which this peak occurs is defined as the optimal duration, and there is no advantage in extending the pulse beyond this point. Characteristics are presented that show how the time course of this voltage within the heart changes with different device capacitors and load impedances. The effect of different heart time constant are also examined. Forbiphasic waveforms, a contour plot of threshold voltage is presented with phase 1 and phase 2 durations on the two axes. It is seen that there is a region of reliable low threshold defibrillation from 3.5/1.5 ms to 9/6 ms.