This figure illustrates the concept of reentrant tachycardia due to an infarction scar (red). Left panel: Activation (arrows) comes from the left, enters a narrow surviving pathway, and blocks at a point where the pathway suddenly expands. This can happen when the cells in the narrow bundle cannot generate enough current to activate the cells in the wider area, a condition known as current-to-load mismatch. Right panel: Activation arriving from the other side, delayed due to a long and tortuous route, can re-activate the first pathway and the bulk myocardium, and initiate a reentry.

Guidance for catheter ablation of ventricular arrhythmia.
Mark Potse and Vidal Essebag
Med. Biol. Eng. Comput. 47(3):241-243 2009 (editorial)