PATHOPHYSIOLOGY AND NATURAL HISTORY ELECTROPHYSIOLOGY The QT interval during reflex cardiovascular adaptation

We examined the relationship between changes in heart rate and the measured QT interval of the electrocardiogram in healthy subjects after exercise and during breath holding, hyperventilation, the dive reflex, the Valsalva maneuver, and the cold-pressor test. The tachycardia of exercise was accompanied by the familiar shortening of the QT interval, but substantial heart rate changes encountered in other more "sedentary" maneuvers were accompanied by very small changes in QT. Calculating the corrected QT in the latter instances, therefore, yielded spurious results. The data suggest very little, if any, direct effect of heart rate on the QT interval. The length of the interval in healthy subjects appears to be determined largely by reflexly elicited discrete autonomic influences. Those associated with exercise result in QT shortening but, during neurally mediated cardiovascular adjustments that do not involve exercise, QT is maintained within narrow limits. Circulation 69, No. 1, 22-25, 1984. PROLONGATION of the QT interval of the electrocardiogram has been correlated with susceptibility to sudden death in patients who have recovered from myocardial infarction,1 in those who have a QT interval prolonged by drugs,2 3 in liquid-protein-modified diet users,' in association with idiopathic mitral valve prolapse,5 in drug-free depressed patients,6 and among patients afflicted with the congenital long-QT syndrome.' An inverse relationship between the length of the QT interval and heart rate ascertained from single resting and postexercise heart rate recordings among large groups of healthy subjects led to the development of formulas for the determination of QTc, the QT interval corrected for heart rate. The formula for correction most often applied, that of Bazett,5 uses an exponential factor, although Akhras and Rickards' found a consistent linear relationship between the uncorrected QT interval and heart rate during exercise and recovery. Changes in the heart rate during artificial cardiac pacing in human subjects"'2 are accompanied by only small changes in QT interval. Thus, calculating the QTc by the most widely used modification of Bazett's formula was shown in all three studies to overcorrect, actually yielding lengthened QT values. These and other studies have led to the suggestion that whatever From the Milton S. Hershey Medical Center, Pennsylvania State University School of Medicine, Hershey, and Temple University School of Medicine, Philadelphia, PA. Address for correspondence: Stewart Wolf, M.D. R.D. Number 1, Box 1262, Bangor, PA 18013. Received May 2, 1983; revision accepted Sept. 22, 1983. 22 direct effect heart rate may have on the QT interval must be relatively small. On the other hand there is ample evidence of autonomic nervous system influence on the QT interval, chiefly from the sympathetic innervation of the heart and from circulating catecholamines, but also to some extent from vagal activity. 'Whether or not the familiar shortening of the QT interval that accompanies acceleration of heart rate during exercise is attributable to associated increases in circulating catecholamines or to the local effects of the release of adrenergic neurotransmitters from activation of specific stellate ganglion neurons has not been determined. It would be anticipated, however, that the generalized influence of circulating catecholamines would differ from the effects of discrete localized activation of cardiac nerves. The present study was designed to explore the relationship ofQT and QTc to changes in heart rate during various autonomic nervous system-mediated cardiac reflexes unassociated with vigorous exercise. The aim was to test a widely held supposition that there is a uniform reciprocal relationship between heart rate and QT interval and thereby to determine the applicability of QTc.