[Heterogeneities of ventricular repolarization and vulnerability to arrhythmia. How to detect them with noninvasive...

Vulnerability to arrhythmias can be influenced by two conditions: a dynamic (beat-to-beat) variation of repolarization sequence, and a state of heterogeneity of repolarization, i.e. a greater than normal dispersion of recovery time. The first condition is well reflected by T-wave alternans, a phenomenon characterized by alternation on every other beat basis of amplitude and morphology of T waves. Experimental studies provided evidences of close temporal correlations between ischemia-induced alternans, dispersion of repolarization and susceptibility to ventricular fibrillation. Gross T-wave alternans can be occasionally observed in patients with long QT syndrome or during acute ischemia before the onset of arrhythmias. Recent studies have demonstrated that measurement of microvolt level T-wave alternans at rest and during exercise is a promising technique for the identification of patients at risk of ventricular arrhythmias and sudden death. A state of repolarization inhomogeneity can be revealed by methods which analyze a single cardiac beat. The QT dispersion, defined as the difference between maximum and minimum QT interval measured at 12 lead ECG, is the most simple and widely used index of repolarization inhomogeneity. The major limitation is that this measure cannot be related to the actual spatial heterogeneity of repolarization, since each surface lead reflects, in different degree, the electrical activity of the whole heart. The majority of studies reported that, in various pathological conditions, the QT dispersion is higher in patients with than without ventricular arrhythmias. On the other hand, a recent large prospective study in post-myocardial infarction patients failed to demonstrate the predictive value of QT dispersion, even when measured with the best available methodology. Body surface potential mapping has proven to be a useful method for detecting repolarization inhomogeneities not revealed by the analysis of conventional ECG leads. Different methods of analysis of the potential maps have been used. By applying principal component analysis of the ST-T waves, we computed the similarity index, defined as the ratio of the first principal component to the sum of all remaining components. A low value of similarity index suggests a high degree of repolarization inhomogeneity. The similarity index was found significantly lower in patients with idiopathic long QT syndrome and in patients with arrhythmogenic right ventricular dysplasia with episodes of ventricular tachycardia than in normal subjects. Future researches should aim at identifying novel reliable indices of repolarization inhomogeneity, first deduced from extensive body surface mapping, then possibly computed from digital recording of the 12 conventional leads.