Determinants of precordial QT dispersion in normal subjects

Dispersion of precordial QT intervals has been attributed to delay in the recovery process in the myocardium under the exploring electrode, a local effect. However, the phenomenon also could be explained by different projections of the heart vector, in which case the la-lead electrocardiogram (ECG) derived from the heart vector would show similar dispersion that could not be local in nature because the electrical activity of the heart is represented by a single dipole. Using an analog device that switched between the two, conventional and derived ECGs were obtained from 129 normal subjects. Measured as the difference between the longest and shortest precordial QT intervals, QT dispersion from the derived ECGs (mean +/- SD, 40 +/- 20 ms) was nearly identical in magnitude to that from the standard ECGs (41 +/- 18 ms, P = NS). Further analysis of the derived ECGs revealed nonuniform distributions of both the maximal and minimal QT intervals across the precordial leads. In addition, a weak correlation was found between the QT interval and the T wave amplitude in the two precordial leads with the lowest T-wave amplitudes (r = -0.303 in V1, P = .001, and r = 0.253 in V6, P = .005). While findings in patients with disease or with abnormal ECGs may differ and require separate examination, these data suggest that the observed magnitude of precordial QT dispersion in normal subjects can be explained by differences in precordial projection of the end of the T wave rather than by local effect.