Residence at 1,200 m Induces a Three-Fold Reduction in the Risk of Acute Mountain Sickness Following Rapid Ascent to 3,600 m

Moderate-altitude residence (1,600-2,400 m) elicits altitude acclimatization and reduces acute mountain sickness (AMS) following rapid ascent to higher altitude (HA). Cross-sectional studies suggest lower altitude residence (800-1,200 m) provides similar benefits, but conclusions are unclear due to lack of control. Purpose: Determine whether moderate-altitude residents (MAR, 1,200 m) experience less AMS than low-altitude residents (LAR) following active or passive ascent to HA (3,600 m). Methods: 78 Soldiers (mean±SD; age=26±5yr; women=8) were tested at their baseline residence at 331 m (LAR; n=41) or 1,200 m (MAR; n=37), transported to Taos, NM (2,845 m), hiked (n=39) or were driven (n=39) to 3,600 m and stayed for 4 days. AMS was assessed using the Environmental Symptoms Questionnaire twice on day 1 (HA1), five times on days 2 and 3 (HA2, HA3) and once on day 4 (HA4). Daily peak cerebral factor score (AMS-C) was recorded; if AMS-C was ≥ 0.7 individuals were considered sick that day. Results: Ascent condition (active vs. passive) did not demonstrate a significant main or interaction effect on AMS. The MAR vs. LAR experienced lower AMS incidence on HA1 (16% vs. 44%, p=0.008) and HA2 (19% vs. 39%, p=0.05), similar incidence on HA3 (14% vs. 29%, p=0.08) and lower incidence on HA4 (0% vs. 17%, p=0.007). AMS-C was lower in MAR vs. LAR on HA1 (0.40±0.49 vs. 0.74±0.86, p=0.04), HA2 (0.30±0.34 vs. 0.86±0.88, p=0.001), HA3 (0.30±0.36 vs. 0.56±0.69, p=0.03) and HA4 (0.09±0.14 vs. 0.35±0.58, p=0.01). MAR exhibited an approximately 3-fold reduction in the odds of developing AMS at HA1 (odds ratio (OR)=0.25, p=0.01), HA2 (OR=0.37, p=0.05) and HA3 (OR=0.38, p=0.09) compared to LAR. Conclusions: Residence at 1,200 m induces a 3-fold reduction in the odds of developing AMS following rapid ascent to 3,600 m.