Effects of beta-adrenergic antagonists on the QT measurements from exercise stress tests in pediatric patients with long QT syndrome.

It has been proposed that beta-adrenergic antagonist protection against cardiac events in patients with long QT syndrome (LQTS) may be related to a decrease in baseline QTc dispersion. To determine the effects of beta-blocker therapy on QT measurements, we evaluated the exercise tests of 25 pediatric patients with LQTS. Measurements were made of the maximum QTc interval and QTc dispersion during the various segments of the exercise test. There was no statistically significant difference between the pre-beta-blocker and post-beta-blocker maximum QTc interval during the supine (0.473 +/- 0.039 vs 0.470 +/- 0.038 sec), exercise (0.488 +/- 0.044 vs 0.500 +/- 0.026 sec), or recovery (0.490 +/- 0.031 vs 0.493 +/- 0.029 sec) phases of the exercise stress test. There was also no statistically significant difference between the pre-beta-blocker and post-beta-blocker QTc dispersion during the supine (0.047 +/- 0.021 vs 0.058 +/- 0.033 exercise vs 0.063 +/- 0.028 sec), or recovery (0.045 +/- 0.023 vs 0.052 +/- 0.026 sec) phases of the exercise stress test. Therefore, the protection that beta-blockers offer appears not to be related to a reduction of the baseline QTc interval or a decrease of QTc dispersion.