Effect of Beta-blocker Therapy on QTc Response in Exercise and Recovery in Normal Subjects
Background. In congenital long QT syndrome type 1 (LQT1), episodes of ventricular tachycardia are usually triggered by exercise and can be prevented in most patients by beta-blocker therapy. In addition, LQT1 associated with a normal resting QT interval can be unmasked by the abnormal QT response to exercise testing (failure of the QT interval to shorten normally). Preliminary data from our laboratory show that the exercise QT intervals of patients with LQT1 are partially normalized by beta-blocker therapy. It is still currently not known if beta-blockers modify the QT/heart rate relationship (a primary effect on repolarization) or if the "normalizing" effect is due to the inability of subjects on beta-blockers to attain sufficiently high workloads (due to reduced heart rate) for prolongation to occur. Moreover, the physiologic response of the exercise QT interval to beta-blockers in healthy control subjects is not known.
Objective. The objective of this study is to define the impact of beta-blocker therapy on the QT response to exercise and recovery in normal subjects.
Methods. Approximately 36 healthy adult subjects age-matched to previously studied LQT1 subjects will undergo 1) screening history, 2) two weeks of beta-blocker therapy ending in an exercise test, and 3) two weeks of placebo therapy ending in an exercise test. Beta blocker and placebo will be given in random order in a double-blind fashion. The QT response to exercise and recovery will be compared between drug-free and beta-blocker-treated states. These data will be compared to those previously collected for LQT1 subjects.
Implications. These results will provide new information about the effect of beta-blocker therapy on repolarization parameters in normal subjects, and will provide a context in which to interpret the previous findings that beta-blocker administration modifies the QT response to exercise in LQT1 subjects.
|Study Design:||Allocation: Randomized
Intervention Model: Crossover Assignment
Masking: Double Blind (Subject, Investigator)
Primary Purpose: Basic Science
|Official Title:||Effect of Beta-blocker Therapy on QTc Response in Exercise and Recovery in Normal Subjects|
- QTc Response to Exercise on Versus Off Beta-blocker. [ Time Frame: 2 weeks on each treatment then exercise test ] [ Designated as safety issue: No ]To minimize the effect of heart rate on QT, QT was measured at heart rates between 100 and 110 beats per minute during exercise (on and off beta-blocker) and during recovery (on and off beta-blocker).
- Tpeak-end Interval (Tpe) [ Time Frame: Measured after 2 weeks on each intervention ] [ Designated as safety issue: No ]Tpeak-end interval was measured at rest, exercise, and recovery on placebo and on propranolol.
|Study Start Date:||August 2007|
|Study Completion Date:||December 2008|
|Primary Completion Date:||December 2008 (Final data collection date for primary outcome measure)|
Placebo Comparator: 1
Subjects are assigned to placebo.
Placebo will be given 1 pill daily for a week, then 2 pills daily, followed by the exercise test.
Active Comparator: 2
Subjects will take propranolol LA 80 mg daily for one week then 160 mg for one week followed by the exercise test.
Drug: Propranolol LA
Subjects will receive propranolol LA 80 mg one pill daily for 1 week then 2 pills daily for 1 week followed by exercise test.