Definitions
- High altitude 1,500 to 3,500 metres (4,900 to 11,500 ft) - The onset of physiological effects of diminished inspiratory oxygen pressure (PiO2) includes decreased exercise performance and increased ventilation (lower arterial PCO2). Minor impairment exists in arterial oxygen transport (arterial oxygen saturation (SaO2) at least 90%), but arterial PO2 is significantly diminished. Because of the large number of people who ascend rapidly to altitudes between 2,400 and 4,000 m, high-altitude illness is common in this range.
- Very high altitude 3,500 to 5,500 metres (11,500 to 18,000 ft) - Maximum SaO2 falls below 90% as the arterial PO2 falls below 60mmHg. Extreme hypoxemia may occur during exercise, during sleep, and in the presence of high altitude pulmonary edema or other acute lung conditions. Severe altitude illness occurs most commonly in this range.
- Extreme altitude (above 5,500 metres (18,000 ft) - Marked hypoxemia, hypocapnia, and alkalosis are characteristic of extreme altitudes. Progressive deterioration of physiologic function eventually outstrips acclimatization. As a result, no permanent human habitation occurs above 6,000 m. A period of acclimatization is necessary when ascending to extreme altitude; abrupt ascent without supplemental oxygen for other than brief exposures invites severe altitude sickness.
Read more about this topic: Altitude Sickness
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