The effect of action potential propagation on a numerical simulation of a cardiac fiber subjected to secondary external stimulus.

Computer simulations have been performed on a particular model of propagating cardiac action potentials (AP), over a large enough strand (15 mm) and a long enough time (500 msec) that the complete AP can develop free of end effects. External stimuli of either polarity of transmembrane current are applied in the plateau phase of the AP to a position midway in the strand. In our case, stimuli of either polarity can produce increases in action potential duration (APD), whereas repolarizing stimuli produce decreases in APD for space-clamped calculations. An analysis of the ion fluxes allows us to characterize these events within the parameters of the model. For repolarizing stimuli in our long fiber case, we obtain an APD increase, whereas a decrease might be expected. The APD increase is due to a local deep drop in potential, which resets the fast sodium channels so that subsequent diffusional return of charge from regions of positive potential fires JNa at the stimulus site.