Propranolol and inspiratory muscle performance in normal subjects

We studied the effects of propranolol on respiratory muscle performance during inspiratory threshold-loaded endurance breathing in eight normal subjects. Propranolol (mean daily dose = 160 + 17 mg, SEM) reduced loaded 15-second MVV (92 versus 81 L/min;P = .01) and maximal sustainable ventilatory capacity (52.3 versus 44.5 L/min, P = .02) but did not affect the fraction of MVV, which was sustainable. Maximal static inspiratory pressures were reduced at two of three lung volumes, whereas maximal static expiratory pressures were unaffected by propranolol. The reduction in inspiratory muscle performance in the whole population could be accounted for almost entirely by four subjects who developed symptoms of "tiredness" and easy fatigability while receiving propranolol. There was no significant difference in propranolol dose, in degree of beta-receptor blockade, or in physical fitness in symptomatic and asymptomatic groups, and in neither group did propranolol alter pulmonary function test results. Propranolol directly depresses inspiratory muscle strength in subjects who develop drug-induced symptoms of fatigue by a mechanism probably unrelated to beta-adrenergic-receptor blockade.