QTd

QT dispersion (QTd) is the difference between the longest and shortest QT interval observed across the twelve leads of a standard electrocardiogram (ECG) in a single recording. In some studies, dispersion is reported for heart-rate–corrected QT intervals (QTc dispersion) as the difference between the maximum and minimum QTc across leads.

Physiological interpretation of QTd has centered on it as a marker of nonuniform ventricular repolarization, reflecting regional differences in myocardial recovery. It has been studied as a potential predictor of malignant ventricular arrhythmias and sudden cardiac death, particularly in contexts such as acute coronary syndromes, heart failure, and congenital long QT syndrome. However, the associations identified across studies are inconsistent, and QTd has not proven reliable enough for routine risk stratification in clinical practice.

Measurement and limitations of QTd are significant. Accurately identifying the end of the T wave can be difficult, and U waves, noise, and baseline wander can affect QT measurements. Differences between leads may stem from measurement error as well as true physiologic heterogeneity. Interobserver and intraobserver variability can be substantial. Although some automated ECG systems provide QTd values, there is no universal standard, and QT intervals remain sensitive to heart rate, electrolyte status, and medications.

Clinical use of QTd remains limited. It is largely viewed as a research tool rather than a standard component of clinical risk assessment, with emphasis typically placed on QTc intervals and other markers of repolarization heterogeneity.