Author(s): Peter Hackett, David Shlim
Typical high-elevation travel destinations include Colorado ski resorts with lodgings at 8,000-10,000 ft (≈2,440-3,050 m); Cusco, Peru (11,000 ft; ≈3,350 m); La Paz, Bolivia (12,000 ft; ≈3,650 m); Lhasa, Tibet Autonomous Region (12,100 ft; ≈3,700 m); Everest base camp, Nepal (17,700 ft; ≈5,400 m); and Mount Kilimanjaro, Tanzania (19,341 ft; ≈5,900 m). High-elevation environments expose travelers to cold, low humidity, increased ultraviolet radiation, and decreased air pressure, all of which can cause health problems. The biggest concern, however, is hypoxia, due to the decreased partial pressure of oxygen (PO2). At 10,000 ft (≈3,050 m), for example, the inspired PO2 is only 69% of that at sea level; acute exposure to this reduced PO2 can lower arterial oxygen saturation to 88%-91%.
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The magnitude and consequences of hypoxic stress depend on the elevation, rate of ascent, and duration of exposure; host genetic factors may also contribute. Hypoxemia is greatest during sleep; day trips to high-elevation destinations with an evening return to a lower elevation are much less stressful on the body. Because of the key role of ventilation, travelers must avoid taking respiratory depressants at high elevations.
Acclimatization
The human body can adjust to moderate hypoxia at elevations ≤17,000 ft (≈5,200 m) but requires time to do so. Some acclimatization to high elevation continues for weeks to months, but the acute process, which occurs over the first 3-5 days following ascent, is crucial for travelers. The acute phase is associated with a steady increase in ventilation, improved oxygenation, and changes in cerebral blood flow. Increased red cell production does not play a role in acute acclimatization, although a decrease in plasma volume over the first few days does increase hemoglobin concentration.
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Altitude illness can develop before the acute acclimatization process is complete, but not afterwards. In addition to preventing altitude illness, acclimatization improves sleep, increases comfort and sense of well-being, and improves submaximal endurance; maximal exercise performance at high elevation will always be reduced compared to that at low elevation.
Travelers can optimize acclimatization by adjusting their itineraries to avoid going “too high too fast” (see Box 4-08). Gradually ascending to elevation or staging the ascent provides crucial time for the body to adjust. For example, acclimatizing for a minimum of 2-3 nights at 8,000-9,000 ft (≈2,450-≈2,750 m) before proceeding to a higher elevation is markedly protective against acute mountain sickness (AMS). The Wilderness Medical Society recommends avoiding ascent to a sleeping elevation of ≥9,000 ft (≈2,750 m) in a single day; ascending at a rate of no greater than 1,650 ft (≈500 m) per night in sleeping elevation once above 9,800 ft (≈3,000 m); and allowing an extra night to acclimatize for every 3,300 ft (≈1,000 m) of sleeping elevation gain. These reasonable recommendations can still be too fast for some travelers and annoyingly slow for others.
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