Acclimatization effects on partial pressure of arterial oxygen at high altitude: A systematic review and meta-analysis

During acclimatization to high altitude with reduced barometric pressure, the rate of increase in PaO 2 after its initial drop has not been quantitatively described in healthy lowlanders. We therefore performed a systematic search on PubMed and Embase up to 4 October 2025, identifying 39 prospective studies that reported at least two PaO 2 measurements at > 1500 m in initially unacclimatized healthy adults. Additionally, changes in other blood gas parameters were investigated. The primary outcome was the mean effect and 95%CI for changes in PaO 2 during high altitude exposure in healthy, unacclimatized lowlanders. Data from 490 participants (mean[SD] age 30.5[5.4] years; 76.1% male), comprising 1109 measurements at a mean altitude of 4140 m (range 1600–5334 m), were pooled using random-effects meta-analysis. PaO 2 showed an increase of 0.1 kPa/day (95%CI − 0.02;0.22; p = 0.10). Across all studies with the first measurement not restricted to day one, the increase was 0.06 kPa/day (95%CI − 0.02;0.15; p = 0.14). Neither altitude nor duration of exposure significantly influenced these estimates. Concurrently, SaO 2 increased, PaCO 2 decreased, and pH remained unchanged. Although not reaching statistical significance, the PaO 2 showed a clear trend of increasing by 0.06 to 0.1 kPa/day at high altitude over an observation period of several weeks. These values may serve as reference estimates for physiological acclimatization in healthy individuals at high altitude.