EFFECTS OF PROPRANOLOL AND PINDOLOL ON CARDIAC-OUTPUT DURING EXTENDEDPERIODS OF LOW-INTENSITY PHYSICAL-ACTIVITY

The effects of a nonselective beta-adrenergic blocking agent with (pin dolol) and without (propranolol) intrinsic sympathomimetic activity pr operties, compared with placebo-controlled conditions, on metabolic an d cardiorespiratory function during long-duration (2 hours) physical a ctivity were examined. After initial cardiorespiratory testing, subjec ts performed 2-hour walks at 25 and 45% of maximal oxygen consumption (VO2max) under each of the following 3 treatments: pindolol, propranol ol and placebo. Medication distribution was randomized and double-blin ded. A supine resting blood pressure and electrocardiogram were obtain ed before each exercise trial. Oxygen consumption, heart rate, stroke volume, cardiac output and blood pressure were determined after 5 minu tes of quiet sitting and every 30 minutes during each 2-hour exercise trial. Cardiac output was not significantly different at rest or durin g exercise, comparing pindolol and propranolol with placebo conditions . Cardiac output tended to decrease over time earlier during propranol ol treatment for the 25% VO2max trials in trained normotensive subject s than for the other treatments. Cardiac output decreased at approxima tely the same time across treatments during the 45% VO2max trials in t rained normotensive and untrained hypertensive groups. Finally, owing to the observation that a reduction in cardiac output was delayed or p revented in trained normotensive subjects when compared with that in u ntrained hypertensives while exercising at 2S% VO2max, developing a su bject's cardiovascular fitness level may be important in the maintenan ce of cardiac output during extended periods of low-to-moderate physic al activities while under the influence of beta-adrenergic blockade.