A number of morphological indices have been proposed to characterize electrocardiographic patterns of ventricular repolarization mostly studying spatial and temporal patterns of T waves. Comparisons between different clinical populations exist but data on the suitability of the T-wave descriptors to characterize and quantify physiologic regulations of ventricular repolarization are lacking. To initiate such investigations, a study was conducted comparing the influence of provoked heart rate changes on the duration of QT interval, the roundness of T wave loop expressed by the relative T wave area, and on the 3-dimensional QRS-T angle. A population of 40 healthy subjects (18 women, mean age 30.4 ± 8.1 years) was studied. In each subject, provocative tests involving changes from strict supine to unsupported sitting and to unsupported standing positions were repeated twice during each of 3 separate monitoring days. Continuous 12-lead electrocardiograms were obtained during the provocative tests. The speed of the adaptation of the repolarization descriptors to heart rate changes was characterized by λ parameters of previously published exponential decay model of R-R interval related hysteresis. The comparisons showed that the adaptation of QT interval to heart rate changes was much slower than that of the investigated T-wave morphological descriptors: the mean (SD) values of λ parameters were 5.01 ± 1.13, 12.72 ± 8.66, and 12.90 ± 11.37 for QT interval, QRS-T angle, and relative T-wave area, respectively (P < .001 for the difference between QT interval and both morphological descriptors). The study suggests that the different numerical quantifiers of vertricular repolarization that may be derived from standard electrocardiographic tracings likely represent separate and distinct physiologic entities.
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