Electrocardiographic Quantitation of Heterogeneity of Ventricular Repolarization

Background: QT interval dispersion (QT d ) measured from the surface ECG has emerged as the most common noninvasive method for assessing heterogeneity of ventricular repolarization. Although QT d correlates with dispersion of monophasic action potential duration at 90% repolarization and with dispersion of recovery time recorded from the epicardium, total T‐wave area, representing a summation of vectors during this time interval, has been shown to have the highest correlation with these invasive measures of dispersion of repolarization. However, recent clinical studies suggest that the ratio of the second to first eigenvalues of the spatial T‐wave vector using principal component analysis (PCA ratio) may more accurately reflect heterogeneity of ventricular repolarization. Methods: To better characterize the ECG correlates of surface ECG measures of heterogeneity of ventricular repolarization and to establish normal values of these criteria using an automated measurement method, the relations of QRS onset to T‐wave offset (QT o d) and to T‐wave peak (QT p d) dispersion and the PCA ratio to T‐wave area and amplitude, heart rate, QRS axis and duration, and the QT o interval were examined in 163 asymptomatic subjects with normal resting ECGs and normal left ventricular mass and function. QT o d and QT p d were measured by computer from digitized ECGs as the difference between the maximum and minimum QT o and QT p intervals, respectively. Results: In univariate analyses, a significant correlation was found between the sum of the T‐wave area and the PCA ratio (R =−0.46, P < 0.001), but there was no significant correlation of the sum of T‐wave area with QT o d (R = 0.11, P = NS) or QT p d (R=0.09, P = NS). There were only modest correlations between QT o d and QT p d (R = 0.45) and between the PCA ratio and QT o d (R = 0.29) and QT p d (R = 0.49) (each P < 0.001). In stepwise multivariate linear regression analyses, the PCA ratio was significantly related to the sum of T‐wave area, T‐wave amplitude in aVL, and to female gender (overall R = 0.54, P < 0.001), QT o d correlated only with the maximum QT o 0 interval (R = 0.39, P < 0.001), and QT p d was related to heart rate and QRS axis (overall R = 0.36, P <0.001). In addition, the normal interlead dispersion of repolarization as measured by QT o d was significantly greater than dispersion measured by QT o d (23.5 ± 11.5 ms vs 18.3 ± 11.2 ms, P < 0.001). Conclusions: These findings provide new information on ECG measures of heterogeneity of repolarization in normal subjects, with a significantly higher intrinsic variability of Q to T‐peak than Q to T‐offset dispersion and only modest correlation between these wo measures. The independent relation of the PCA ratio to the sum of T‐wave area suggests that the PCA ratio may be a more accurate surface ECG reflection of the heterogeneity of ventricular repolarizat on. A.N.E. 2000;5(1):79–87