PRE-ACCLIMATIZATION

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.

NUTRITION & HYDRATION

HYDRATION: Bicarbonate diuresis in response to respiratory alkalosis as well as venous constriction with suppression of antidiuretic hormone and aldosterone are normal physiologic responses to ascent, and the resulting hemoconcentration can increase blood oxygen carrying capacity.3-4 These responses plus the lower humidity and the lower atmospheric pressure of higher altitude alpine environments predispose individuals to dehydration. Dehydration can be further exacerbated if patients are taking diuretic medications, such as acetazolamide. Signs and symptoms of dehydration can mimic those of AMS and HACE; however, dehydration itself has not been shown to increase the incidence of AMS.21,22 Thus, dehydration should be avoided and guided by thirst and appropriate urine output. Overhydration and forced hydration has not been shown to be helpful.7,23 Consider dehydration as a differential diagnosis or co-presentation when evaluating for AMS/HACE.

NUTRITION: Multiple studies have shown both men and women tend to reduce their energy intake after acute ascent to over 4,300m.24-26 Acute high-altitude exposure alone has been demonstrated to increase basal metabolic rate and energy demand by 30%.26 AMS can cause nausea, further exacerbating anorexia. One study showed that reduced calorie intake at altitude was independently associated with the presence of AMS symptoms.24 Data on any specific type of diet having a benefit in performance or decreasing incidence of HAI are mixed.28 Prolonged physical exertion at a caloric deficit will likely lead to performance deterioration. Considering the above, individuals should be encouraged to intentionally increase energy intake above sea-level when ascending to altitude; although the specific amount of caloric increase to optimize performance is likely determined by complex unique factors and there is not enough data to make a specific recommendation at this time.

IRON: Pulmonary hypertension from any cause is a strong risk factor for HAPE..4 Iron infusions in healthy adult males lowered pulmonary artery systolic pressures by 6 mmHg 3 days after moving from sea level to 4340m as well as a separate study showing decreased pulmonary vascular reactivity in acute hypoxia.29,30 Both those studies used one time dose of Iron Sucrose 200mg IV. Maximum oral iron absorption was shown to be limited at 25mg/day.31 A small single randomized, double-blinded, placebo-controlled study did show a lower rate of AMS with iron infusions at 24hrs of altitude exposure, but it did not reach significance (P = .097).21 Based on the above, consider screening personnel, especially those with a history of pulmonary arterial hypertension or HAPE, for iron deficiency prior to high altitude operations and treat iron deficiency with iron supplementation. Iron infusions are currently being studied at USAREIM at time of this CPG publication. Update required upon completion of their study.