QT INTERVALS AT HEART-RATES FROM 50 TO 120 BEATS PER MINUTE DURING 24-HOUR ELECTROCARDIOGRAPHIC RECORDINGS IN 100 HEALTHY-MEN - EFFECTS OF ATENOLOL

Background. Conflicting reports about changes in QT intervals suggest that QT values should be compared at similar heart rates. Methods and Results. Relations between QT and RR intervals were determined after measurement of QT values by Holter recording at heart rates of 50, 60, 70, 80, 90, 100, 110, and 120 beats per minute in 100 healthy young men. Fifteen men underwent a second recording during acute treatment with the beta-blocking agent atenolol. At heart rates between 80 and 120 beats per minute, the QT interval was significantly longer (from 9 to 16 msec), and at a heart rate of 50 beats per minute significantly shorter (26 msec), than values calculated from Bazett's formula. Sleep prolonged QT values by 18 msec at a heart rate of 60 beats per minute and by 21 msec at a heart rate of 50 beats per minute compared with the waking state. Atenolol lengthened QT intervals significantly (by 11-14 msec) at heart rates between 90 and 110 beats per minute and shortened them (by 12 msec) at a heart rate of 60 beats per minute. During sleep, QT intervals were the same before and after atenolol. Conclusions. Method of plotting QT against RR intervals after measurement of QT values at similar stable spontaneous heart rates before and after intervention allows changes in autonomic state and heart rate to be taken into account. By this method, QT values can be compared without distortion effects caused by correction formulas.