Time requirements of pre-acclimatization at simulated altitude to prevent acute mountain sickness

Travelling to high altitudes (sleeping above 2500 m) carries a risk of acute mountain sickness (AMS). Proper pre-acclimatization strategies, such as time spent at simulated altitude, can significantly lower this risk. However, the optimal duration of pre-acclimatization remains unknown. Therefore, we examined published research on how different types and durations of pre-acclimatization at simulated altitude influence the AMS risk.

A literature search was conducted to find controlled studies investigating the effects of normobaric or hypobaric simulated altitude exposures. Inclusion criteria required sufficient details on study design, exposure duration and outcomes, specifically the incidence of AMS on subsequent ascent. The simulated or real altitude exposure following pre-acclimatization had to last at least eight hours to accurately assess AMS incidence.

Findings from seven controlled studies using repeated intermittent exposures to simulated altitude were included. The cumulative pre-acclimatization time ranged from 7 to 104 hours; altitudes ranged from 2200 to 5000 m. The AMS risk after pre-acclimatization decreased by 12-73% compared to controls, with the degree of reduction depending mainly on the duration at simulated altitude. Extrapolating from these data indicates that, within the analysed altitude range, ~200 hours of pre-acclimatization may reduce the subsequent AMS risk to nearly zero.

Our analysis suggests that accumulating 200 hours in a hypoxic simulated-altitude environment substantially reduces the risk of AMS when subsequently exposed to terrestrial altitude. The effectiveness of this strategy depends on the intervals between pre-acclimatization exposures, the method used and the target altitude. Observational studies indicate that time spent at simulated altitude can be complemented by time spent at terrestrial altitude. Importantly, while pre-acclimatization exposures can effectively prevent AMS, they do not, per se, enhance endurance performance or mountaineering skills, and factors such as co-existing mental or physical health conditions may influence risk and the effectiveness of prevention.