Background: Considerable controversy exists about the meaning of QT dispersion(QTD). The working hypothesis of the present paper was that the necessary although not sufficientcondition for the validity of QTD concept is the association of QTD with nondipolar voltage (NDPV)in T waves of the 12-lead ECG.
Methods and Results: ECGs of 4890 subjects, 966 with coronary heart disease (CHD) and3844 considered CHD-free were processed using computer programs for measurement of the ratio of the first twoeigenvalues (E2/E1), nondipolar voltage (NDPV), terminal T wave direction and ECGestimate of left ventricular mass (LVM). The mean NDPV in T wave was 11 μV (SD 3.9),with 6 μV (SD 1.3) in terminal 40 ms. NDPV alone explained only 6% and NDPV, E2/E1 andLVM combined 13% of QTD variance. There was a modest increase in the fraction of subjects with QTD>60 ms among subjects with NDPV in terminal T > 7 μV compared to those with NDPV ≥7 μV(15% vs. 10%). A more profound increase was associated with terminal T wave directiondeviating from normal (37% vs. 12% among those with normal direction), reflectingdipolar rather than nondipolar components.
Conclusions: The association between QTD and NDPV is weak, and QTD is unlikely to represent anymeaningful myocardial repolarization event in the interval domain. It seems more logical to use directmeasurement of NDPV as a potential marker of localized dispersion and heterogeneity of ventricularrepolarization for evaluation of the risk of adverse cardiac events.