Figure 5.

Spatially discordant alternans predisposes premature excitations to induce conduction block and reentry. One mechanism by which SDA predisposes otherwise benign premature beats to conduction block and reentry is diagrammed. An ectopic beat (asterisk) initiated during SDA in the region with a short APD encounters more refractory tissue as it propagates toward the region with long APD. Given a sufficiently steep gradient of refractoriness, conduction block can occur (dashed line). Meanwhile, this beat can successfully propagate laterally, allowing time for recovery of the long APD region and subsequent reentry. Adapted from Weiss et al. (2006).