Regulation of QT interval during postural transitory changes in heart rate in normal subjects

A lack of homogeneity, and the presence of intrinsic abnormalities in ventricular myocardial repolarization are thought to be important mechanisms underlying the development of malignant ventricular arrhythmias.^1-4 These abnormalities are believed to be reflected, at least in part, in the finding of a relative elongation in the QT interval to the preceding RR interval.^5,6 Although the QT-RR relation has been examined clinically as a static relation (QTc),⁷ it appeared important to evaluate it in the framework of the dynamic QT response to transitory changes in heart rate, because it is possible in some cases that the abnormalities in the ventricular repolarization process that could precipitate malignant ventricular arrhythmias emerge only in its dynamic response, and not in a static relation. Although some studies have reported the response of the QT interval to the sudden and sustained changes in heart rate produced by strenuous exercise,^5,6,8 this method appears impractical, particularly for evaluating patients with malignant ventricular arrhythmias. In this study, we developed a microcomputer-based, automated technique for assessing the beat-to-beat QT-RR relation to examine the dynamic response of the QT interval to transitory changes in the heart rate during the head-up tilt test.