The metabolic and hemodynamic effects of prolonged bed rest in normal subjects.

The metabolic and hemodynamic effects of prolonged bed rest were studied in 6 normal subjects. Bed rest of 2-3 weeks duration produced exaggerated responses of heart rate, cardiac output, stroke volume, and peripheral vascular resistance to 700 tilt. Sympathetic nerve function and catecholamine metabolism were not impaired by bed rest. Pressor responses to infusions of norepinephrine and angiotensin and reflex vasoconstriction to a cool and warm environment were essentially unchanged during bed rest. Plasma catecholamines and urinary vanillylmandelic acid excretion were somewhat lower during bed rest than during ambulation, but the response of plasma catecholamines to 700 tilt was not diminished. The apparent turnover of norepinephrine in plasma was also similar in bed rest and control periods. Negative sodium and potassium balances and reductions in plasma volume were observed in all subjects, but plasma renin activity and aldosterone secretory rate showed no significant change. The major decreases in sodium balance and plasma volume occurred in the early bed rest period and did not correlate closely with the degree of orthostatic intolerance. The reductions in potassium balance appeared to be progressive throughout the study.