Distinct unexpected phenomena with a shared mechanism.

Supernormal phenomena are uncommon findings in routine electrocardiographic evaluation, and both their electrophysiological substrate and their clinical significance remain controversial. We present two cases with electrocardiographic tracings consistent with supernormal excitability in a patient with a dysfunctional epicardial pacemaker, and supernormal conduction in another patient with complete atrioventricular block. In addition to their unexpected nature, both phenomena share a common electrophysiological substrate, which is thought to be mediated, at least in part, by the availability of functional Na + channels during the terminal portion of phase 3 of repolarization, known as the supernormal phase.