Short‐and Long‐Term Reproducibility of QT, QTc, and QT Dispersion Measurement in Healthy Subjects

The study investigated interobserver and intrasubject reproducibility of QT interval duration and dispersion measured in standard 12‐lead ECGs recorded at 25 mm/sec. Twenty‐eight healthy volunteers were studied. Each undenvent four ECG recordings, which were performed 1, 7, and 30 days apart. Two independent observers analyzed each ECG record. In each lead with a distinguishable T wave pattern, the RR interval, Q‐peak of T interval, and Q‐end of T interval were measured using a digitizing board with a 0.1‐mm resolution. From each recording the following measures were derived: the maximum, minimum, and mean QT interval; maximum, minimum, and mean heart rate corrected QT interval (QTc); QT and QTc dispersion (the difference between the maximum and minimum QT interval among the 12 leads); and adjusted QT and QTc dispersion (dispersion divided by the square root of the number of leads measured). The interobserver and short‐term (1 day) and long‐term (1 week and 1 month) reproducibility of individual indices was assessed by computing the relative errors and comparing them by a standard sign test. In addition, the distributions of maximum and minimum QTc values among electrocardiographicleads, and the differences between QT‐end and QT‐peak based measurements were investigated. The results showed that: (1) the measurement of the QT interval from standard ECG recordings is feasible and not operator dependent (interobserver relative error <4%); (2) the duration of the QT interval in healthy volunteers is stable and its short‐ and long‐term reproducibility is high (intrasubject relative error < 6%); (3) parameters that characterize dispersion of the QT interval in the 12‐lead ECG are highly nonreproducible, both between subsequent recording (relative error of 25%–35%) and between observers (relative errar 28%–33%), the reproducibility of QT dispersion is significantly lower than that of QT duration (P < 0.01); and (4) the duration of the entire QT interval correlates only weakly with the duration of the Q‐peak of T interval.