Comparison of formulae for heart rate correction of QT interval in exercise electrocardiograms

The study investigated the differences in five different formulae for heart rate correction of the QT interval in serial electrocardiograms recorded i n healthy subjects subjected to graded exercise. Twenty-one healthy subject s (aged 37 +/- 10 years, 15 male) were subjected to graded physical exercis e on a braked bicycle ergometer until the heart rate reached 120 beats/min. Digital electrocardiograms (ECG) were recorded on baseline and every 30 se conds during the exercise. in each EGG, heart rate and QT interval were mea sured automatically (QT Guard package, Marquette Medical Systems, Milwaukee , WI, USA). Bazett, Fridericia, Hedges, Framingham, and nomogram formulae w ere used to obtain QT, interval values for each EGG. For each formula, the slope of the regression line between RR and QT, values was obtained in each subject. The mean values of the slopes were tested by a one-sample t-test and the comparison of the baseline and peak exercise QT, values was perform ed using paired t-test. Bazett, Hedges, and nomogram formulae led to signif icant prolongation of QT, intervals with exercise, while the Framingham for mula led to significant shortening of QT, intervals with exercise. The diff erences obtained with the Fridericia formula were not statistically signifi cant. The study shows that the practical meaning of QT, interval measuremen ts depends on the correction formula used. In studies investigating repolar ization changes (e.g., due to a new drug), the use of an ad-hoc selected he art rate correction formula is highly inappropriate because it may bias the results in either direction.