Dynamics of the QT interval during and after exercise in healthy children.

The dynamics and homogeneity of the QT interval have been used as indicators of susceptibility to ventricular arrhythmias. We determined the relationship between QT intervals and heart rate during exercise testing and subsequent recovery in 18 healthy children. The QT intervals were measured to the apex (early QT), to the end (total QT), and from the apex to the end of the T wave (late QT) (inhomogeneity of repolarization) at heart rates from 60 by steps of 10 to 180 beats.min-1. Group mean total QT and early QT exhibited better linear correlations with heart rate (r 0.998 and 0.999) than with cardiac cycle length (r 0.954 and 0.959). The slope relating total QT to heart rate was -1.30 during exercise and -1.42 during recovery (P < 0.05). The corresponding slopes relating early QT to heart rate were -1.11 and -1.30 (P < 0.05). Late QT, as a proportion of total QT, increased at high heart rates. Rate correction using Bazett's method gave abnormal total QT values (> 440 ms) in 12 children (67%) whereas linear correction gave values below 440 ms only. In conclusion, the relationship between QT and heart rate is linear and differs during exercise and recovery. Inhomogeneity of repolarization increases at high heart rates. Linear correction of total QT and early QT intervals improves the evaluation of repolarization duration in exercise testing in children.