Automatic adjustment of chopping-modulated defibrillation pulses to patient transthoracic resistance

Abstract

Defibrillation of the heart requires a high amplitude short duration current pulse to be passed through large electrodes placed on the patient's chest. The current meets a virtually active resistance, which can vary in the approximate range of 25 to 180 &#122. As the delivered current or energy depends on the resistance, several methods have been developed to reduce or compensate its influence. For example, pre-shock resistance has been measured by a high-frequency current and the current or energy set accordingly; measurements have been made from the initial tilt and the pulse durations adjusted; and pre-shock measurements have been made by a sub-shock pulse to generate an appropriately selected constant current. A method is proposed using high-frequency chopped biphasic pulses, with pulse-width and period modulation of the elementary pulses. Patient resistance is measured with the first elementary pulse and depending on its value a modulated waveform is generated, selected by a micro-controller from a preprogrammed set. Thus the selected energy is accurately delivered to the patient. In addition, this method allows the shaping of a desired mean patient current waveform, maintaining adequate charge balance between the two phases and securing an appropriate time course of the model-derived transmembrane potential.