QT dispersion, T-wave projection, and heterogeneity of repolarization in patients with coronary artery disease

The clinically useful prognostic value of precordial QT dispersion in patie nts with heart disease is generally attributed to its measurement of region al heterogeneity of ventricular repolarization. However, when repolarizatio n is abnormal, differences in measured QT intervals might result simply fro m variation in projection of the T-wave loop. To provide insight into the m echanism of QT dispersion, we used an analog device to transform convention al 12-lead electrocardiograms (ECGs) of 78 patients to derived 12-lead ECGs based on the heart vector. Because the electrical activity of the heart is represented by a single dipole, all QT dispersion in the transformed ECGs results from variation in projection of the T-wave loop and cannot be due t o local heterogeneity of repolarization. Measured as the difference between the longest and shortest precordial QT intervals, QT dispersion in the der ived ECGs, with no local heterogeneity of repolarization, was 53 +/- 49 ms (mean +/- SD). QT dispersion in these derived ECGs was similar in magnitude to that measured from the original standard 12-lead ECGs in these patients (49 +/- 23 ms, p = NS). Therefore, the precordial QT dispersion measured f rom standard ECGs of patients with coronary artery disease can be explained by interlead variation in precordial projection of the T-wave loop. Althou gh regional heterogeneity might still contribute to precordial repolarizati on findings and to prognosis, this is nor required to explain the QT disper sion observed in patients with coronary artery disease, Therefore, QT inter val dispersion is not equivalent to heterogeneity of repolarization. (C) 20 01 by Excerpta Medica, Inc.