Oxygen saturation and acute mountain sickness during repeated altitude exposures simulating high-altitude working schedules

This study aimed to quantify the effect of two consecutive prolonged, intermittent exposures to high and very high altitudes on oxygen saturation (SpO₂) and acute mountain sickness (AMS). For this, healthy lowlanders (N = 21), aged 18-30 years, underwent two 7-day sojourns at the ALMA observatory, Chile (6 h/day at 5050 m, 18 h/day at 2900 m), separated by 1-week at 520 m. SpO₂ (pulse oximetry) and AMS severity (AMSc, Environmental Symptom Questionnaire cerebral score) diagnosing AMS (AMSc ≥ 0.7) were assessed daily at both altitudes. The study was registered at www.ClinicalTrials.gov: NCT02730143. SpO₂ at 2900 m and 5050 m on arrival was mean ± SD 93.6 ± 0.5% and 79.9 ± 1.0% (P < 0.05 between altitudes), whereas the AMSc scores were 0.43 ± 0.08 and 0.97 ± 0.11 (P < 0.05 between altitudes), respectively. At 2900 m during a 7-day intermittent hypoxic exposure, SpO₂ increased by a mean (95% CI) 0.3 %/day (0.1;0.4) and by 0.9 %/day (0.4;1.3) at 5050 m. Similarly, AMSc decreased by 0.05 points/day (0.01;0.08) at 2900 m and by 0.16 points/day (0.11;0.21) at 5050 m. During the second sojourn (vs. 1st sojourn), day 1, SpO₂ at 2900 m was unchanged but higher at 5050 m by 2.9% (0.6;5.3). AMSc was lower at 2900 m and 5050 m by 0.37 (0.16;0.59) and 0.37 (0.11;0.63) (P < 0.05 both comparisons vs 1st sojourn), respectively. Acclimatization with the 2nd sojourn increased SpO₂ at 2900 m by 0.3%/day (0.1;0.4) and 5050 m by 0.5%/day (0.1;0.8). AMSc remained unchanged with acclimatization at 2900 m but decreased at 5050 m by 0.08 points/day (0.04;0.11). In conclusion, in healthy lowlanders, a 7-day intermittent hypobaric hypoxic exposure improved SpO₂ and AMS severity at 2900 m, with larger improvements at 5050 m. During a second identical sojourn, initial AMS severity was reduced despite comparable SpO₂ at 2900 m compared to the 1st sojourn. No further acclimatization effects were observed in SpO₂ but in AMS symptoms at 2900 m. In contrast, re-exposure to 5050 m showed higher initial SpO₂ and lower AMSc values with further improvement with intermittent re-exposures. These findings highlight altitude-dependent acclimatization patterns and confirm pre-conditioning's effectiveness in preventing AMS.