Time dependence of anodal and cathodal refractory periods in cardiac tissue

Mehra et al. (PACE 2980; 3:526) observed that immediately after implantatio n of a pacing electrode in a dog heart, the anodal refractory period (RP) i s shorter than the cathodal RP, but after several weeks the anodal RP becom es longer than the cathodal RP. We examine this experiment using numerical simulations based on the bidomain model of cardiac tissue and a Beeler-Reut er membrane. Our hypothesis is that accumulation of inexcitable tissue arou nd the electrode following implantation causes the effective size of the el ectrode to increase and that this increase is the mechanism underlying the change in RP. We calculate that the anodal RP is shorter than the cathodal RP for both large and small electrodes. However, for large electrodes the t hreshold for anode "break" stimulation is greater than 8 mA. Mehra et al. d efined RP experimentally as the interval at which the threshold stimulus st rength becomes greater than 8 mA. If we restrict the stimulus current in ou r calculations to less than 8 mA, we exclude anode break stimulation from o ur calculation of the RP. In that case, our results are consistent with Meh ra et al. and suggest that their observation resulted from their definition of RP.