Repolarization alternans: toward a unifying theory of reentrant arrhythmia induction.

Abstract

The appearance of electrocardiographic T-wave alternans with elevated heart rate or metabolic insult has been observed for nearly a century.1 Macroscopic T-wave alternans is often noted as a harbinger of sudden arrhythmic death.2 Efforts to quantify the magnitude of more subtle repolarization alternans and relate these measurements to arrhythmia susceptibility have been pursued since the 1980s.3 4 Only in the last few years, however, have the mechanisms underlying repolarization alternans and their role in the genesis of arrhythmias been addressed. Several investigators have described alternations in action potential duration and morphology coinciding with T-wave alternans in the surface ECG in a variety of proarrhythmic settings.5 6 7 The ionic basis for these beat-to-beat changes in action potential has only recently been explored. Shimizu and Antzelevitch8 found that under conditions mimicking congenital long-QT syndrome physiology in a ventricular wedge preparation, alternans of T-wave and action potential duration were elicited during rapid pacing and abolished by ryanodine and low extracellular calcium, implicating intracellular calcium cycling in the maintenance of T-wave alternans. Pastore et al9 showed that alternations in action potential duration induced by rapid pacing are not uniform across the myocardium. Much of the ventricle exhibits sequential lengthening and shortening of the action potential, but fluctuations in some regions are 180 degrees out of phase with those in other regions, constituting a phenomenon known as discordant alternans. The resulting spatial gradients in transmembrane potential during the repolarization phase alternate in magnitude …