Electrocardiographic measures of repolarization revisited: why? what? how?

Ventricular repolarization continues to be an enigma to clinical cardiologists and cardiac electrophysiologists. On the one hand, a century of experience has documented an association between abnormal T-wave morphology, QT prolongation and dispersion, T-wave alternans, and nonspecific ST-T waves with arrhythmia risk or negative prognostic outcome. On the other hand, recent advances in molecular electrophysiology have definitively implicated abnormal function and structure of cardiac ion channels associated with repolarization as primary arrhythmogenic mechanisms in long QT syndrome, Brugada's Syndrome, and idiopathic ventricular fibrillation and ventricular tachycardia. In spite of this extensive clinical experience and newly established mechanistic knowledge, robust measurements of repolarization and sensitive algorithms for reliable assessment of risk and prediction of arrhythmia occurrence have remained elusive. New insights into electrocardiographic waveform that reflect and capture the underlying spatial and dynamic characteristics of repolarization offer opportunity to devise clinical indices of repolarization that might be more predictive of risk or outcome than those currently used. Experimental and model data show evidence that the location and size of repolarization lesions may be deduced from T waveform. The changes of repolarization induced by altered activation sequence, and cycle length mediated alterations to repolarization offer additional means to assess the magnitude and significance of such lesions that are linked to increased arrhythmogenic risk. This article explores indices of repolarization that are sensitive to repolarization and its change and that provide opportunity to better characterize and assess repolarization for risk stratification.