C.M.Lathers, CVM, FDA, Rockville, MD
Head-out water immersion (HOWI) simulates weightlessness and is used to study renal patients, inducing primary central hypervolemia due to cephalad fluid redistribution, diuresis and natriuresis, and decreased plasma volume (PV). Decreased colloidal osmotic pressure due to leg capillary bed fluid shifts accounts for 25% increased sodium excretion. Healthy subjects but not acute renal failure exhibit a correlation between HOWI-induced changes in volume regulatory hormones. Orthostatic tolerance post space flight exhibits decreased PV and total body water. Frey et al (J Clin Pharmacol 1991, 1994) compared oral rehydration solutions of varying osmolality and sugar content. Urine flow was greater after water and glucose containing 1% solutions and lowest after saline with no glucose. PV increased more at 2, 3, and 4-hrs after 1.07% saline than after other solutions, including the 0.9% saline countermeasure (3740 vs. 3520 mL). Systolic blood pressure increased upon standing after saline but decreased when hypohydrated (20 mg iv Lasix) or euhydrated. Skylab 24-hr urine revealed elevated sodium in-flight and post flight. Acute renal disease decreases sodium excretion; hypotonic 0.45% solutions correct it. 1.07% saline may be better than 0.9% saline as a post space flight countermeasure for orthostatic intolerance. Opinions are the author’s and not FDA’s.