Altitude illness refers to a group of syndromes that result from hypoxia. Acute mountain sickness (AMS) and high-altitude cerebral edema (HACE) are manifestations of the brain pathophysiology, while high-altitude pulmonary edema (HAPE) is that of the lung. Everyone traveling to altitude is at risk, regardless of age, level of physical fitness, prior medical history, or previous altitude experience.
The high-altitude environment generally refers to elevations over 1,500 m (4,800 ft). Moderate altitude, 2,000-3,500 m (6,400-11,200 ft), includes the elevation of many US ski resorts. Although arterial oxygen saturation is well maintained at these altitudes, low PO2 results in mild tissue hypoxia, and altitude illness is common.
Very high altitude refers to elevations of 3,500-5,600 m (11,200-18,000 ft). Arterial oxygen saturation is not maintained in this range, and extreme hypoxemia can occur during sleep, with exercise, or with illness. HACE and HAPE are most common at these altitudes.
Extreme altitude is over 5,600 m. At these elevations, successful long-term acclimatization is not possible; in fact, deterioration ensues. Individuals must progressively acclimatize to intermediate altitudes to reach extreme altitude.