Simulations of electrical field stimulation of cardiac myocytes during the relative refractory period

Global stimulation of the heart during defibrillation may be the result of local field stimulation at the cellular level, in which one end of the cell is depolarized while the other end is hyperpolarized. We have formulated a model which describes the spatial and temporal variation of transmembrane potential along the surface membrane of a single cell subjected to a spatially uniform, pulsed electric field. The ability of the applied field to prolong the action potential may be a relevant factor during defibrillation, and we have used the model to determine the extent of prolongation when the waveform is applied during repolarization. Using an S1-S2 protocol, we find that despite the complicated membrane response, 5 msec, rectangular monophasic S2 pulses (with intensity 2.5 times the SI threshold) prolong the action potential as a monotonie function of the Sl-S2 interval. This finding is similar to cellular responses to stimulation by intracellular current injection.