The dependence on heart rate of the human ventricular action potential duration

For the human electrocardiogram, the relationship between QT interval and heart rate was examined in conditions where the heart rate was changed at different rates. The QT interval provides a measure of the ventricular action potential duration. For slow changes of heart rate, elicited by mild exercise, the dependence of QT interval on heart rate obtained when the heart rate was rising was the same as that obtained when the heart rate was falling. The QT interval was approximately proportional to 1/(heart rate)1/2. For the rapid transient changes of heart rate associated with respiration, the QT interval was approximately constant, independent of the instantaneous heart rate. For rapid sustained changes of heart rate, elicited by heavy exercise or immersion of the face in cold water, the changes in QT interval lagged behind the heart rate changes. Consequently the dependence of QT interval on heart rate obtained when the heart rate was increasing differed from the dependence obtained during a fall in heart rate. This time dependence of the relationship between QT interval and heart rate is similar to that seen for the relationship between action potential duration and stimulation rate in isolated pieces of mammalian ventricle.