Propranolol and the compensatory circulatory responses to orthostasis at high altitude

Tachycardia has been shown to be an important response involved in the maintenance of cardiac output during orthostasis at high altitude. This study was undertaken to determine if tachycardia, mediated by beta-adrenergic sympathetic stimulation, actually represents an essential response. Twelve young, healthy male subjects were administered either 80 propranolol (n = 6) or placebo (n = 6), t.i.d. at sea level and for 3 days (d) prior to and during the first 15 d of a 19-d altitude sojourn (On Treatment). Individuals were randomly assigned to each group. Upright tilt tests were performed at sea level and at high altitude during 2, 7, and 15 On Treatment. Subjects were also tilt-tested at sea level and on day 19 of the altitude exposure without placebo or propranolol administration (Off Treatment). Heart rate, stroke volume, calf blood flow, and blood pressure were obtained during supine rest and after 12 min of 60° tilt. We found no differences between groups in any of the circulatory. During the On Treatment phases at sea level and altitudes, propranolol caused reductions in heart rate and blood pressure values in each position (p < 0.05). Supine and upright cardiac output, however, were found not altered due to compensatory increased in stroke volume (p < 0.05). We concluded that tachycardia, both at rets and during upright tilt at high altitude is important, but not essential to maintain cardiac output.