Plasma volume restoration with salt tablets and water after bed rest prevents orthostatic hypotension and changes in supine hemodynamic and endocrine variables

Abstract

Head-down bed rest changes the values of many cardiovascular and endocrine variables and also elicits significant hypovolemia. Because previous studies had not controlled for hypovolemia, it is unknown whether the reported changes were primary effects of bed rest or secondary effects of bed rest-induced hypovolemia. We hypothesized that restoring plasma volume with salt tablets and water after 12 days of head-down bed rest would result in an absence of hemodynamic and endocrine changes and a reduced incidence of orthostatic hypotension. In 10 men, we measured changes from pre-bed-rest to post-bed-rest in venous and arterial pressures; heart rate; stroke volume; cardiac output; vascular resistance; plasma norepinephrine, epinephrine, vasopressin, renin activity (PRA), and aldosterone responses to different tilt levels (0 degrees, -10 degrees, 20 degrees, 30 degrees, and 70 degrees); and plasma volume and platelet alpha2- and lymphocyte beta2-adrenoreceptor densities and affinities (0 degrees tilt only). Fluid loading at the end of bed rest restored plasma volume and resulted in the absence of post-bed-rest orthostatic hypotension and changes in supine hemodynamic and endocrine variables. Fluid loading did not prevent post-bed-rest increases in beta2-adrenoreceptor density or decreases in the aldosterone-to-PRA ratio (P = 0.05 for each). Heart rate, epinephrine, and PRA responses to upright tilt after bed rest were increased (P < 0.05), despite the fluid load. These results suggest that incidents of orthostatic hypotension and many of the changes in supine hemodynamic and endocrine variables in volume-depleted bed-rested subjects occur secondarily to the hypovolemia. Despite normovolemia after bed rest, beta2-adrenoreceptors were upregulated, and heart rate, epinephrine, and PRA responses to tilt were augmented, indicating that these changes are independent of volume depletion.

Hindlimb unloading and female gender attenuate baroreflex-mediated sympathoexcitation

“…Increased incidence of orthostatic intolerance after real or simulated microgravity has been associated with hypovolemia (17,47,48), myocardial dysfunction (20,39), changes in peripheral vascular responsiveness (17,48), and alterations in neurohumoral regulation of cardiovascular function (11,18,48). Decreases in plasma volume and cardiac atrophy after exposure to microgravity or prolonged bed rest may contribute to orthostatic intolerance by reducing the ability to maintain stroke volume during an orthostatic challenge (20,46,47). Similarly, impaired peripheral vascular function could contribute to reduced orthostatic tolerance after real or simulated microgravity by limiting the ability to vasoconstrict and thus decreasing vascular resistance or venous return during an orthostatic challenge (46,49).…”

“…Upon return to a 1G environment or to normal activity, individuals display resting tachycardia, decreased exercise capacity, and increased incidence of orthostatic intolerance (6,17,19). The mechanisms that account for these changes are still unresolved, although cardiac atrophy (39), altered vascular reactivity (17,32), hypovolemia (47), and altered baroreflex control of sympathetic nerve activity (11,18,48) have been suggested, and all potentially contribute. In addition, females appear to be less tolerant of the negative effects of returning from spaceflight or a period of bed rest, because females exhibit a greater incidence of orthostatic intolerance after spaceflight compared with men (23,48).…”

Body Fluid Changes, Cardiovascular Deconditioning and Metabolic Impairment Are Reversed 24 Hours after a 5-Day Dry Immersion

“…It is interesting to compare our study, in which plasma volume was normalized spontaneously, with studies in which plasma volume was restored after a prescription. Plasma volume normalization with salt tablets and water after a 12-day head down bed rest (HDBR) prevented orthostatic hypotension and changes in supine hemodynamics and endocrine variables, but was not effective to prevent tachycardia and epinephrine hyperresponsiveness to upright tilt [31]. Plasma volume restoration with fludrocortisone (a synthetic mineralocorticoid) after a 7-day HDBR significantly limited orthostatic intolerance (only one out of 7 subjects remained intolerant), and prevented changes in the baroreflex sensitivity [32].…”

Simulated Stand Tests and Centrifuge Training to Prevent Orthostatic Intolerance on Earth, Moon, and Mars

“…Fluid loading, 19 exercise and lower body negative pressure, 17 and midodrine 14 have been shown to be inconsistently effective as countermeasures for orthostatic intolerance. Anti-G suits, which constitute the only reliable countermeasure currently available 12 are heavy and uncomfortable for long-term wear by all crew members.…”