Physiologic Relation Between Cardiac Cycle and QT Duration in Healthy Volunteers

To determine the relation between QT duration and RR interval on the electrocardiogram, 2 studies were undertaken in 23 young healthy volunteers (mean age 24 years). In study 1, the electrocardiogram (paper speed at 50 mm/s, 800 measurements/subject) was recorded in 11 subjects (5 men, 6 women) at rest, during dynamic exercise and after rapid intravenous injections of isoproterenol (0.2 to 12.8 micrograms), before and after intravenous atropine (2 mg). In study 2, the QT-RR relation was studied at rest and during exercise in 12 subjects (6 men, 6 women) before and after oral propranolol (80 mg). The results confirmed a monoexponential individual relation of the QT and RR intervals during rest and exercise: QT = A-B.Exp (-k.RR). By pooling the RR-QT plots from the 11 subjects in study 1, we found that the measured QT interval = 425-676.Exp (-3.7.10(-3) RR. During isoproterenol-induced tachycardia, QT either did not change or increased and this may indicate an increase in temporal dispersion of ventricular repolarization. Atropine-induced tachycardia produced changes similar to those resulting from exercise testing. Propranolol did not change the QT-RR relation despite a lengthening in RR intervals. These results suggest that physiologic QT-RR adaptation is mainly under parasympathetic control.