DYNAMICS OF THE QT INTERVAL DURING AND AFTER EXERCISE IN HEALTHY-CHILDREN

The dynamics and homogeneity of the QT interval have been used as indi cators of susceptibility to ventricular arrhythmias. We determined the relationship between QT intervals and heart rate during exercise test ing and subsequent recovery in 18 healthy children. The QT intervals w ere measured to the apex (early QT), to the end (total QT), and from t he apex to the end of the T wave (late QT) (inhomogeneity of repolariz ation) at heart rates from 60 by steps of 10 to 180 beats.min(-1). Gro up mean total QT and early QT exhibited better linear correlations wit h heart rate (r 0.998 and 0.999) than with cardiac cycle length (r 0.9 54 and 0.959). The slope relating total QT to heart rate was -1.30 dur ing exercise and -1.42 during recovery (P<0.05). The corresponding slo pes relating early QT to heart rate were -1.11 and -1.30 (P<0.05). Lat e 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 4 40 ms only. In conclusion, the relationship between QT and heart rate is linear and differs during exercise and recovery. Inhomogeneity of r epolarization increases al high heart rates. Linear correction of tota l QT and early QT intervals improves the evaluation of repolarization duration in exercise testing in children.