Comparison of formulae for heart rate correction of QT interval in exercise ECGs from healthy children

Objective To investigate the differences in four formulae for heart rate co rrection of the QT interval in serial ECG recordings in healthy children un dergoing a graded exercise test. Subjects-54 healthy children, median age 9.9 years (range 5.05-14.9 years), subjected to graded physical exercise (on a bicycle ergometer or treadmill ) until heart rate reached > 85% of expected maximum for age. Design-ECG was recorded at baseline, at maximum exercise, and at one, two, four, and six minutes after exercise. For each stage, a 12 lead digital ECG was obtained and printed. In each ECG, QT and RR interval were measured (l ead II), heart rate was calculated, and QTc values were obtained using the Bazett, Hodges, Fridericia, and Framingham formulae. A paired t test was us ed for comparison of QTc, QT, and RR interval at rest and peak exercise, an d analysis of variance for all parameters for different stages for each for mula. Results-From peak exercise to two minutes recovery there was a delay in QT lengthening compared with RR lengthening, accounting for differences observ ed with the formulae after peak exercise. At peak exercise, the Bazett and Hodges formulae led to prolongation of QTc intervals (p < 0.001), while the Fridericia and Framingham formulae led to shortening of QTc intervals (p < 0.001) until four minutes of recovery. The Bazett QTc shortened significan tly at one minute after peak exercise. Conclusions-The practical meaning of QT interval measurements depends on th e correction formula used. In studies investigating repolarisation changes (for example, in the long QT syndromes, congenital heart defects, or in the evaluation of new drugs), the use of an ad hoc selected heart rate correct ion formula may bias the results in either direction. The Fridericia and Fr amingham QTc values at one minute recovery from exercise may be useful in t he assessment of long QT syndromes.