Altitude sickness—also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, or soroche—is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude. It commonly occurs above 2,400 metres (8,000 feet). It presents as a collection of nonspecific symptoms, acquired at high altitude or in low air pressure, resembling a case of "flu, carbon monoxide poisoning, or a hangover". It is hard to determine who will be affected by altitude sickness, as there are no specific factors that correlate with a susceptibility to altitude sickness. However, most people can ascend to 2,400 meters (8,000 ft) without difficulty. Acute mountain sickness can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE), which are potentially fatal.
High altitude 5,000 to 11,500 feet (1,500 to 3,500 m) - The onset of physiologic 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 8,000 and 11,500 ft, high-altitude illness is common in this range.
Very high altitude 11,500 to 18,000 feet (3,500 to 5,500 m) - 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 18,000 feet (5,500 m) - 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 18,000 ft. 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.
Headaches are the primary symptom used to diagnose altitude sickness, although a headache is also a symptom of dehydration. A headache occurring at an altitude above 2,400 metres (8,000 feet = 76 kPa), combined with any one or more of the following symptoms, may indicate altitude sickness:
Lack of appetite, nausea, or vomiting
Fatigue or weakness
Dizziness or lightheadedness
Pins and needles
Shortness of breath upon exertion
Persistent rapid pulse
Peripheral edema (swelling of hands, feet, and face).
Symptoms that may indicate life-threatening altitude sickness include:
Pulmonary edema (fluid in the lungs)
Symptoms similar to bronchitis
Persistent dry cough
Shortness of breath even when resting
Cerebral edema (swelling of the brain)
Headache that does not respond to analgesics
Gradual loss of consciousness