Why did QT dispersion die?

Abstract

Considerable controversy exists about the meaning of QT dispersion (QTD). The working hypothesis of the present paper was that the necessary although not sufficient condition for the validity of QTD concept is the association of QTD with nondipolar voltage (NDPV) in T waves of the 12-lead ECG. ECGs of 4,890 subjects, 966 with coronary heart disease (CHD) and 3,844 considered CHD-free were processed using computer programs for measurement of the ratio of the first two eigenvalues (E2/E1), nondipolar voltage (NDPV), terminal T wave direction and ECG estimate of left ventricular mass (LVM). The mean NDPV in T wave was 11 muV (SD 3.9), with 6 muV (SD 1.3) in terminal 40 ms. NDPV alone explained only 6% and NDPV, E2/E1 and LVM 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 muV compared to those with NDPV >/=7 muV (15% vs. 10%). A more profound increase was associated with terminal T wave direction deviating from normal (37% vs. 12% among those with normal direction), reflecting dipolar rather than nondipolar components. The association between QTD and NDPV is weak, and QTD is unlikely to represent any meaningful myocardial repolarization event in the interval domain. It seems more logical to use direct measurement of NDPV as a potential marker of localized dispersion and heterogeneity of ventricular repolarization for evaluation of the risk of adverse cardiac events.