Effects of oral propranolol on left ventricular size and performance during exercise and acute pressure loading.

Oral propranolol (160 mg/day) was administered to 19 normal subjects for 2 weeks. Echocardiograms were performed at rest, during graded supine bicycle exercise in 10 subjects and during acute pressure loading with intravenous phenylephrine in the remaining nine subjects. Resting heart rate on propranolol decreased compared with control (52 +/- 8 vs 63 +/- 10 beats/min; p less than 0.001), as did systolic blood pressure (99 +/- 9 vs 107 +/- 9 mm Hg; p less than 0.01). Left ventricular end-diastolic dimension was slightly enlarged (48.3 +/- 4.2 vs 47.1 +/- 3.6 mm, p = 0.05), but percent dimensional shortening was unchanged (37 +/- 4 vs 38 +/- 5%). At each stage during supine bicycle exercise, heart rate was slower, blood pressure lower, left ventricular dimensions were larger and percent dimensional shortening was reduced on propranolol by an analysis of variance. In contrast, during acute pressure loading, while the heart rate response to increased blood pressure was blocked by atropine, there was no significant difference in left ventricle size and performance compared with control. We conclude that prolonged oral propranolol therapy has little, if any, intrinsic effect on myocardial performance in normal subjects. Its major action on the heart is competitive inhibition of beta-adrenergic tone, which is most manifest during conditions associated with increased sympathetic tone, such as exercise.