Acute Mountain Sickness and SaO2 in Moderate Altitude Versus Sea-level Residents Ascending to 4300 m

Abstract

When sea-level residents (SLR) rapidly ascend to ∼4300 m, arterial oxygen saturation (SaO2) is low, and the incidence and severity of acute mountain sickness (AMS) is typically high, especially following heavy exertion in the early hours of altitude exposure. On the other hand, moderate-altitude residents (MAR) have a higher initial SaO2 upon arrival at 4300 m (Muza 2004) compared to SLR, but it is unknown whether they also exhibit a lower incidence and severity of AMS. PURPOSE: To determine in MAR, compared to SLR, whether ventilatory acclimatization, obtained during residence at 1800–2200 m (21 ±3 mo, ± SD), reduces the incidence and severity of AMS for up to 72 h of exposure to 4300 m with heavy exertion in the early hours of altitude exposure. METHODS: Sixteen MAR (9 men, 7 women; 30±3 yr, 69±9 kg) and eighteen male SLR (25±5 yr, 78±9 kg;) completed an Environmental Symptoms Questionnaire (ESQ) and resting SaO (pulse oximeter) measurement in the morning at their residence altitude (Colorado Springs, CO (CS), and Palo Alto, CA (PA), respectively) and after ∼10, 24, 48, and 72 h residence on Pikes Peak (PP, 4300 m, 458 mmHg). AMS was assessed using the validated “AMS-Cerebral” (AMS-C) factor score in which a score of > 0.7 is indicative of AMS. Each subject completed a 2–4 h cycle exercise bout (50–60% VO2peak) within the first 5 h of altitude exposure. RESULTS: The incidence of AMS was lower (P < 0.05) in MAR compared to SLR, respectively, at PP10 (13% vs. 72%), PP24 (0% vs. 61 %), PP48 (0% vs. 56%), and PP72 (0% vs. 44%). The severity of AMS was also lower (P < 0.05) in MAR compared to SLR, respectively at PP10 (0.4 ± 0.2 vs. 1.5±1.3), PP24 (0.2 ± 0.2 vs. 1.4±1.2), PP48 (0.1 ± 0.2 vs. 1.4±1.5), andPP72 (0.1 ± 0.1 vs. 0.7 ± 0.7). The PP3 exercise SaO2 was higher (P < 0.01) in MAR compared to SLR (80±3% vs.74±6%). Resting SaO2 was similar in MAR and SLR at PP24 (86±2% vs. 83±6%), but higher (P < 0.05) in MAR at PP48 (89±3% vs. 83±6%) and PP72 (89±3% vs. 84±4%). CONCLUSION:These results show that compared to residence at sea level, residence at moderate altitude is very effective in decreasing the incidence and severity of AMS upon exposure to a higher altitude (4300 m) due, in part, to a greater degree of ventilatory acclimatization.