Passport Health Austin/San Antonio

The stresses of the high-altitude environment include cold, low humidity, increased ultraviolet (UV) radiation, and decreased air pressure, all of which can cause problems for travelers. The greatest concern, however, is hypoxia. At 10,000 ft (3,000 m), for example, the inspired PO₂ is only 69% of sea-level value. The degree of hypoxic stress depends upon altitude, rate of ascent, and duration of exposure. Sleeping at high altitude produces the greatest hypoxia; day trips to high altitude with return to low altitude are much less stressful on the body.

Acclimatization

The human body adjusts very well to moderate hypoxia, but requires time to do so. The process of acute acclimatization to high altitude takes 3–5 days; therefore, acclimatizing for a few days at 8,000–9,000 ft before proceeding to higher altitude is ideal. Acclimatization prevents altitude illness, improves sleep, and increases comfort and well-being, although exercise performance will always be reduced compared with low altitude. Increase in ventilation is the most important factor in acute acclimatization; therefore, respiratory depressants must be avoided. Increased red-cell production does not play a role in acute acclimatization.

Risk for Travelers

Inadequate acclimatization may lead to altitude illness in any traveler going to 8,000 ft (2,500 m) or higher. Susceptibility and resistance to altitude illness are genetic traits, and no screening tests are available to predict risk. Risk is not affected by training or physical fitness. Children are equally susceptible as adults; persons >50 years of age have slightly lower risk. How a traveler has responded to high altitude previously is the most reliable guide for future trips but is not infallible. However, given certain baseline susceptibility, risk is greatly influenced by rate of ascent and exertion.

Determining an itinerary that will avoid any occurrence of altitude illness is difficult because of variations in individual susceptibility, as well as in starting points and terrain. Itineraries with a high risk for altitude illness include flying directly to >9,000 ft or rapid hiking ascents, such as climbing Mt. Kilimanjaro. It is best to average no more than 1,000 ft (300 m) ft per day in altitude gain above 12,000 ft (3,660 m).

Examples of high-altitude cities with airports are Cuzco, Peru (11,000 ft; 3,326 m); La Paz, Bolivia (12,000 ft; 3,660 m); and Lhasa, Tibet (12,500 ft; 3,810 m). Travelers flying into these locations may require a period of acclimatization before proceeding higher, and drug prophylaxis may be indicated.

Tips for acclimatization

The following are helpful tips for people traveling to high altitude destinations.

• Ascend gradually, if possible. Try not to go directly from low altitude to >9,000 ft (2,750 m) sleeping altitude in one day.
• Consider using acetazolamide (Diamox) to speed acclimatization if abrupt ascent is unavoidable.
• Avoid alcohol for the first 48 hours.
• Participate in only mild exercise for the first 48 hours.
• Having a high-altitude exposure at >9,000 ft (2,750 m), for 2 nights or more within 30 days prior to the trip is useful.
• Treat an altitude headache with simple analgesics.

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