A two-dimensional simulation of the effects of late stimulation on a cardiac action potential

The response of cardiac tissue to a stimulus delivered late in the action potential (AP) is important in studies of defibrillation and certain models of ischemia in which reentrant trajectories produce such stimulation. Such stimuli are often strong and delivered in complex spatial arrangements. In this work a two-dimensional computer simulation of a thin, flat layer of cardiac tissue subjected to rather small sizes of late stimulation delivered at a simple point has been performed. The calculation covered a plane of 95 x 140 space units of 72 u each and the stimulus was near the center. The Drouhard and Roberge version of the physiological routine due to Beeler and Reuter was used in an anisotropic bidomain model of the tissue. Late stimuli of either polarity were applied from 150 to 250 msec after initiation of the AP. Depolarizing (positive) voltage stimuli produced small positive voltage peaks, narrowly localized in space, which decayed with a time constant of 8 to 10 msec. Repolarizing (negative) stimuli, if not applied too late in the AP, produced, after a variable delay, a depolarizing fast sodium peak, which was also positive and spatially quite localized. It decayed quickly so that little effect on the AP duration was found. Since stimuli were of limited size and spatial extent in this study the small effects found here are expected but they indicate that major effects from late stimulation require strong pulses and/or large electrode arrays.