New methods of analysis of the ventricular repolarization by using surface potential mapping

Abstract

The identification of patients at risk for sudden death and ventricular arrhythmias in non-invasive way still represents an important clinical challenge. In recent years many investigations have focused on electrocardiographic T-wave abnormalities reflecting heterogeneities of ventricular repolarization, which have been shown to favor the occurrence of ventricular tachycardias and ventricular fibrillation. Several methods have been proposed, based on the analysis of the 12-lead ECG or of multiple thoracic leads. Body surface potential maps (BSPM) have the advantage over the conventional 12 leads to explore the entire chest surface and to be more sensitive in detecting local electrical events, such as regional heterogeneities of ventricular recovery. Different methods of analysis of BSPM can be used to identify signs of repolarization heterogeneities. QRST integral maps. Areas of QRST deflections mainly reflect the intrinsic repolarization properties and are largely independent of ventricular excitation sequence. A complex, multipolar pattern has been related, on the basis of experimental observations, to local heterogeneities of the ventricular recovery process and thus to cardiac states of vulnerability to arrhythmias. Actually, we found a clear multipolar pattern only in a small percentage of patients affected by ventricular arrhythmias. A multipolar distribution most likely reflects only gross regional inequalities of repolarization, and may not represent a marker sufficiently sensitive for minor disparities. Principal Component Analysis. By applying principal component analysis of the ST-T waves, we computed the similarity index (ratio of first eigenvalue by the sum of all eigenvalues). The value of similarity index is inversely proportional to the variability of T-wave morphologies and a low value is considered a marker of repolarization heterogeneity. In our experience, similarity index was significantly lower than normal in patients affected by idiopathic LQTS, in patients with arrhythmogenic right ventricular dysplasia and ventricular tachycardias and in patients with myocardial infarction. Other repolarization variables In order to analyzing the instantaneous variations of repolarization potentials we considered two new indices: early repolarization deviation index (ERDI) and late repolarization deviation index (LRDI). Visually, the pattern of potential maps is generally constant during normal repolarization, apart from changes in amplitude. The ERDI and LRDI are numerical indices which describe deviations from this behaviour during repolarization, from the J point to the T peak and from the peak to the end of T wave, respectively. We computed these indices in small series of patients with different cardiac diseases, and in some groups significant differences from normals were found. On the T wave of the root mean square (RMS) signal of 62 thoracic leads the following measurements were performed: T width, T area, T highest amplitude, T asymmetry. Then linear discriminant analysis was applied to all indices considered (including ERDI, LRDI). A good separation was obtained between groups of normals and patients with idiopathic VT, between ARVD patients with and without VT, between myocardial infarction patients with and without VT. Although the development of a clinically useful index will necessitate further investigations, these preliminary results show that the repolarization potentials do provide sufficient information to identify patients prone to VT.