The QT interval and cycle length: the influence of atropine, hyoscine and exercise

Twenty-seven healthy male subjects of mean age 24.3 +/- 4.0 years and mean weight 74.9 +/- 9.1 kg took part in an investigation to assess the most suitable correction for the QT interval as a function of cardiac cycle length. 547 sets of data points were generated. Atropine 0.6, 1.2 and 1.8 mg, and hyoscine 0.4 and 0.8 mg, and exercise on a bicycle ergometer at power levels of 50-250 watts together with post-exercise values were employed to obtain a range of heart rates. Simultaneous measurement of cardiac output and total peripheral resistance were made. It was found that the traditional square root formula gave an unsatisfactory correction for the QT for supine subjects following atropine and hyoscine. The formula K = QT/RRN was linearized and fitted to the data by the least squares method and gave a best fit correction with N = 0.35, which is close to the cube root correction of Fridericia (1920). Neither stroke volume nor total peripheral resistance were found to provide a further enhancement of the correction. The relationship between QT and cycle length following the exercise protocol was found to be best represented by Bazett's correction but the complex changes in the QT produced by exercise were noted. These findings support the suggestion that either the cube root correction or the best fit correction with N = 0.35 provides a better correction factor than the traditional square root correction for the QT interval in clinical pharmacology experiments for data generated in resting patients.