Figure 4.

Spatially concordant and discordant alternans in simulated cardiac tissue. Shown are the results of rapidly pacing a 2-dimensional homogeneous sheet of simulated cardiac tissue (from the top left corner), including the spatial distribution of action potential durations (bottom) and the action potentials occurring at two sites in the tissue (top traces). (A) During pacing at a cycle length of 220 ms, spatially concordant alternans is seen, in which the APD of all myocytes alternate in-phase. (B) Pacing at a cycle length of 180 ms, however, induces spatially discordant alternans, in which the action potentials of adjacent regions of tissue alternate out-of-phase. This is seen in the top traces, where the action potentials of site a alternate short-long at the same time as those of site b alternate long-short. As seen in the bottom panel, this spatially discordant alternans causes a large gradient in APD across the tissue, with large regions of tissue with counter-phase APD-alternans, separated by a “nodal line” where alternans is not seen (and where the APD gradient is steepest). Adapted from Weiss et al. (2006).