Simulations of the transmembrane potential distribution around an extracellular electrode in an active 2-D sheet of cardiac tissue

The authors have created a 2-dimensional model of cardiac tissue in order to study the distribution of the transmembrane potential after stimulation with a circular extracellular electrode. The tissue is modeled as a collection of uncoupled fibers, and the fibers are discretized into elements with active Luo-Rudy kinetics. The authors assume that gradients in the extracellular potential drive the membrane, and they use an S1-S2 protocol to determine the tissue response during the absolute refractory period. The authors see that: (1) the V_m isopotential lines form a “dog-bone” shape, as other models predict; (2) in the longitudinal direction the response reverses polarity (virtual electrode effect), while in the transverse direction the polarity does not change; (3) the responses to anodal and cathodal electrodes are not symmetric; and (4) the location of the point of reversal depends on the polarity of the pulse. These last two results have been reported in recent experiments