Pre-acclimatization to high altitude environments describes exposures to altitude or hypobaric hypoxic conditions prior to actual high-altitude operations. Individuals pre-acclimatized to high altitude environments have a lower incidence of HAI.7 Thus, if the mission requires rapid ascent to high altitudes, risk of HAI can be attenuated by pre-acclimatization strategies.

Pre-acclimatization success is proportional to length of exposure, altitude of pre-acclimatization, proximity in time from pre-acclimatization to re-exposure, and exercise capacity during pre-acclimatization periods.8-11 Complete acclimatization can take weeks, but 70-80% of ventilatory acclimatization is achieved in 4-7 days. Benefits of pre-acclimatization have been shown to persist up to 2 months.10 Methods of successful pre-acclimatization include staged ascent, gradual ascent, and daily intermittent hypoxic exposures (IHE).12-14

Staged ascent is the practice of spending a period of time at high altitude prior to a rapid ascent to very high or extreme altitude. Most studies have shown decreased symptoms of AMS with 3-7 days of prestaging at intermediate to high altitudes.10,14-15 Personnel inserting at extreme high altitude will benefit from staging at the highest possible altitude for as long a duration as possible, prior to rapid ascent. If personnel experience AMS during staging, sleeping at a lower altitude while spending waking hours at a higher altitude will be beneficial.

Gradual ascent has been defined as increasing sleeping elevation by no more than 500m/day when ascending above 3000m. Daily exercises and activities may extend beyond 500m from the previous night’s sleep altitude. This strategy is the most conservative and probably the most effective method of acclimatization.

IHE refers to intermittent exposures to hypoxic environments in an attempt to pre-acclimate to high altitude. Numerous studies have had varying protocols ranging from 1-hour exposures daily for 1 week to greater than 4-hour exposures for greater than 1 week. Results have been mixed, with some studies showing decreased incidence of AMS and others showing no change.12,16-19 Pre-acclimatization in a normobaric hypoxic environment does not appear to be as effective as the hypobaric hypoxic environment at preventing AMS.16,20 For small unit special operations, IHE can be a second- or third-line method of pre-acclimatization, although not enough data exists to provide any meaningful strong recommendation on specific protocol. Following general principles of pre-acclimatization, noting that longer duration and higher simulated altitude exposure will likely prove to be more effective.