The relationship between heart rate and QT interval during atrial stimulation

The relationship between heart rate and QT interval was investigated during atrial stimulation (intrinsic effect of heart rate) in ten healthy male volunteers prior to and after administration of sotalol. The QT interval in the ECG (paper speed 200 mm/s) was determined at rates of 70, 85, 100, 115, 130, 145, and 160 beats/min and at pacing periods of 180 s each at 30, 60, 120, and 180 s. After a 15-minute period, 2.0 mg sotalol/kg body weight were administered iv and the stimulation protocol was repeated. The analysis of QT interval behavior reveals contradictions to the mathematical implications of Bazett's equation QT = QTc square root of 60/HR, so that the relationship between heart rate and QT interval is not adequately described under the given conditions. After examination of approaches reported in the literature and our own approaches, the expression QT = a e-b (HR-60) is used as a possibility differentially to describe the data by nonlinear regression. The parameters a and b may be interpreted as QT reference value and shortening parameter. The QT reference value a, a parameter in reference to heart rate of 60 beats/min, has a comparable significance to the expression QTc in the Bazett equation. A reduction in the shortening parameter b indicates whether substances influencing the QT interval additionally produce overproportional shortening of the QT interval with increasing heart rate. After administration of sotalol, an increase can be observed in both the QT reference value and also in the shortening parameter. The suggested approach is an attempt to provide a more precise assessment of the QT interval under different conditions.