People with prior history of AMS and ascending to 8,200–9,200 ft (2,500–2,800 m) or higher in 1 day.People taking ≥2 days to arrive at 8,200–9,800 ft (2,500–3,000 m), with subsequent increases in sleeping elevation less than 1,600 ft (500 m) per day, and an extra day for acclimatization every 3,300 ft (1,000 m)Īcetazolamide prophylaxis generally not indicated.People with no prior history of altitude illness and ascending to less than 9,000 ft (2,750 m).Risk categories for acute mountain sickness In some cases, such as Cusco and La Paz, the traveler can descend to elevations much lower than the airport to sleep. Chemoprophylaxis may be necessary for these travelers, in addition to 2–4 days of acclimatization before going higher. Some common destinations (such as the ones mentioned above) require rapid ascent by airplane to >3,400 meters, placing travelers in the high-risk category ( Table 3-04). The goal for the traveler may not be to avoid all symptoms of altitude illness but to have no more than mild illness. Creating an itinerary to avoid any occurrence of altitude illness is difficult because of variations in individual susceptibility, as well as in starting points and terrain.
Given a baseline susceptibility, 3 factors largely influence the risk of a traveler developing altitude illness: elevation at destination, rate of ascent, and exertion ( Table 3-04).
How a traveler has responded to high elevations previously is the most reliable guide for future trips if the elevation and rate of ascent are similar, although this is not an infallible predictor. Children are equally susceptible as adults people aged >50 years slightly so. Training or physical fitness do not affect risk. Susceptibility and resistance to altitude illness are genetic traits, and no simple screening tests are available to predict risk. Inadequate acclimatization may lead to altitude illness in any traveler going to 8,000 ft (2,500 m) or higher, and sometimes even at lower elevations. Expanded red-cell production does not play a role in acute acclimatization, although hemoglobin concentration is increased within 48 hours because of diuresis and decreased plasma volume. Increase in ventilation is the most important factor in acute acclimatization therefore, respiratory depressants must be avoided. Acclimatization prevents altitude illness, improves sleep and cognition, and increases comfort and well-being, although exercise performance will always be reduced compared to what it would be at lower elevations. The process of acute acclimatization to high elevation takes 3–5 days therefore, acclimatizing for a few days at 8,000–9,000 ft (2,500–2,750 m) before proceeding to a higher elevation is ideal. The human body adjusts very well to moderate hypoxia,but requires time to do so ( Box 3-05). Sleeping at high elevation produces the most hypoxemia day trips to high elevations with return to low elevation are much less stressful on the body. The magnitude of hypoxic stress depends on elevation, rate of ascent, and duration of exposure. At an elevation of 10,000 ft (3,000 m) above sea level, for example, the inspired PO2 is a little more than two-thirds (69%) what it is at sea level. The biggest concern, however, is hypoxia. Environments significantly above sea level expose travelers to cold, low humidity, increased ultraviolet radiation, and decreased air pressure, all of which can cause problems.
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