Duration of QT Interval at Slow Ventricular Rates

Abstract

The QT interval of the ecg. at slow ventricular rates (50 or less/ minute) has an inverse relationship to the heart rate, but the scatter is very marked. The correlation is clearer for patients less than 50 years of age with clinically normal hearts than for patients 50 years of age or older. The mean QT indices (Bazett formula) for groups of individuals with clinically normal hearts and for those with clinically abnormal hearts (patients with bundle-branch block excluded) were not strikingly different, and the ranges in QT indices were also similar. Approximately 10% of the patients had QT indices that were greater than 0.42. In patients with incomplete heart block with varying atrioventricular conduction (for example, 2:1, 3:1 block), the QT interval was slightly longer following a long cycle than after a short cycle. At slow heart rates, indeterminate physiologic variables other than heart rate per se are apparently more active in determining the QT interval. It is re-emphasized that QT interval may not be of the same duration as mechanical systole.