MODULATION OF THE QT INTERVAL - EFFECTS OF GRADED-EXERCISE AND REFLEXCARDIOVASCULAR STIMULATION

During exercise, as heart rate (HR) increases, the QT interval of the electrocardiogram shortens. The mechanism(s) involved in this QT short ening has not been clearly defined. To distinguish the influence of in creased circulating catecholamines from myocardial efferent stimulatio n, the relationship between HR and QT interval was investigated during exercise and cardiovascular reflex stimulation in cardiac transplant patients and normal control subjects. Because of cardiac denervation, increases in HR in these patients are solely due to circulating catech olamines and thus allow isolation of their effect on the QT interval. Twenty-one cardiac transplant patients were studied and compared with 16 normal control subjects. The QT-HR relationship was determined acco rding to an exponential model during treadmill exercise in both groups [QT = 0.12 +/- 0.492e(-0.008.HR) and QT = 0.12 + 0.459e(-0.007.HR) in normal subjects and transplant patients, respectively] and was statis tically similar between groups, suggesting similar QT interval shorten ing in both groups. During cold pressor and Valsalva maneuvers, HR inc reased significantly in normal subjects only, whereas QT interval chan ged minimally in both groups. These results suggest that during exerci se the QT interval is influenced predominantly by increases in circula ting catecholamines rather than by neurally mediated reflex autonomic changes.