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Early fluid retention and severe acute mountain sickness

¹Lovelace Respiratory Research Institute; ²Hypo-hyperbaric Facility, University of New Mexico; ³Cardiology Section, Veterans Affairs Medical Center, Albuquerque, New Mexico; ⁴Institute of Adaptive and Spaceflight Physiology, A-8010 Graz, Austria; and ⁵Colorado Center for Altitude Medicine and Physiology, University of Colorado Health Sciences Center, Aurora, Colorado

Submitted 17 May 2004 ; accepted in final form 14 October 2004

Field studies of acute mountain sickness (AMS) usually include variations in exercise, diet, and environmental conditions over days and development of clinically apparent edemas. The purpose of this study was to clarify fluid status in persons developing AMS vs. those remaining without symptoms during simulated altitude with controlled fluid intake, diet, temperature, and without exercise. Ninety-nine exposures of 51 men and women to reduced barometric pressure (426 mmHg = 16,000 ft. = 4,880 m) were carried out for 8–12 h. AMS was evaluated by Lake Louise (LL) and AMS-C scores near the end of exposure. Serial measurements included fluid balance, electrolyte excretions, and plasma concentrations, regulating hormones, and free water clearance. Comparison between 16 subjects with the lowest AMS scores near the end of exposure ("non-AMS": mean LL = 1.0, range = 0–2.5) and 16 others with the highest AMS scores ("AMS": mean LL = 7.4, range = 5–11) demonstrated significant fluid retention in AMS beginning within the first 3 h, resulting from reduced urine flow. Plasma Na⁺ decreased significantly after 6 h, indicating dilution throughout the total body water. Excretion of Na⁺ and K⁺ trended downward with time in both groups, being lower in AMS after 6 h, and the urine Na⁺-to-K⁺ ratio was significantly higher for AMS after 6 h. Renal compensation for respiratory alkalosis, plasma renin activity, aldosterone, and atrial natriuretic peptide were not different between groups, with the latter tending to rise and aldosterone falling with time of exposure. Antidiuretic hormone fell in non-AMS and rose in AMS within 90 min of exposure and continued to rise in AMS, closely associated with severity of symptoms and fluid retention.

antidiuretic hormone; extracellular water; water retention; free water clearance

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