Abstract
Beta-adrenergic blockade using intravenous propranolol was evaluated as a countermeasure for bed-rest-induced cardiovascular deconditioning. After propranolol administration, tolerance to a maximal lower body negative pressure (LBNP) test after bed rest improved to at least the −70 mm Hg level; following this, there was a sharp decrease in tolerance time. Propranolol decreased mean tolerance time by 36% (17.7 ± 2.4 to 11.5 ± 2.3 minutes) before bed rest, and by only half as much (16.6%) after bed rest (14.4 ± 2.2 to 12.0 ± 2.3 minutes). Systemic vascular resistance was maintained and even slightly increased after propranolol despite a decrease in cardiac output, indicating β 2-adrenergic blockade. Heart rates at all levels of LBNP were lower during β blockade, yet increases occurred with successive LBNP steps, both before and after bed rest, indicating withdrawal of parasympathetic nervous system influences. Results support the use of propranolol in small dosages as a countermeasure after bed rest, and the findings may also be extrapolated to space-flight deconditioning.
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