Active Ascent to 3600m Induces A Plasma Volume Expansion That Likely Exacerbates Acute Mountain Sickness

PURPOSE: Plasma volume typically decreases following passive ascent to high altitude (HA) but strenuous exercise alters this response likely due to a reduction in central mixed venous tension and associated antidiuresis. The purpose of this study was to examine the change in plasma volume (ΔPV) and acute mountain sickness (AMS) following active and passive ascent to 3600 m. METHODS: Forty-one healthy unacclimatized Soldiers (n = 41, mean ± SD; age = 26 ± 5 yr; body weight = 83.5 ± 11.6 kg; VO2max = 47.9 ± 5.5 ml · kg−1 · min−1) were tested at sea level (SL) in Ft. Leonard Wood, MO, flown to Taos Ski Valley, NM (2845 m) where they arrived around 1400, and then either hiked (n = 21; 5.0 km, 15.5% mean grade) or were driven (n = 20; 30-min drive) to a HA ski patrol facility located at 3600 m. All volunteers had ad-libitum access to water during the ascent. Energy expenditure during ascent was assessed using an ankle accelerometer (Actical, Respironics, Inc.). The venous hemoglobin concentration [Hb] and hematocrit (Hct) were measured (Nova Biomedical, Inc) and used to estimate the ΔPV from SL to HA1 (~3 to 4 h following the completion of ascent) using the Dill equation. AMS was assessed ~4 h following ascent using the Environmental Symptoms Questionnaire and if AMS-C was ≥0.7, individuals were considered sick. RESULTS: The active group expended a greater (P < 0.05) amount of energy (745 ± 435 kcal) than the passive group (55 ± 10 kcal) during ascent. The ΔPV was greater (P < 0.05) in the active (+5.29 ± 6.90%) versus passive ascent group (-0.02 ± 6.8%) with a divergence in direction on HA1. The AMS prevalence was also higher (P < 0.05) in the active versus passive ascent group (53 vs. 35%) on HA1 when all individuals were analyzed. When only AMS-susceptible individuals were analyzed, the mean ΔPV was positive in the active ascent group (+7.23 ± 6.6%) with 10/11 demonstrating an increase in PV while the mean ΔPV was negative in the passive ascent group (-0.04 ± 6.9%) with 3/7 demonstrating a decrease in PV. CONCLUSION: Strenuous exercise increased PV during ascent to 3600 m and the resulting fluid retention likely contributed to an increased prevalence of AMS on the first day at HA. Authors’ views not official U.S. Army or DoD policy. Funding: USAMRDC