Altitude illness refers to physiological changes that occur at higher elevations in a body that has not adapted (acclimated) to the higher altitude. Typical symptoms of one type of altitude illness, known as acute mountain sickness, include a severe headache, nausea, and physical weakness. However, much more severe, even life-threatening altitude illnesses—called high-altitude cerebral edema and high-altitude pulmonary edema—can develop.
Altitude is measured as the distance above sea level. High altitude is considered to be 8,000-12,000 ft (2,400-3,700 m) above sea level. As examples, the coastal city of Los Angeles is at sea level, while the city of Denver, Colorado, is located at an elevation of 5,000 ft (1,575 m) above sea level.
A plane trip from a coastal city such as Los Angeles to Denver represents a rapid elevation change. A further elevation change, such as a hike into the surrounding mountains, can place someone in an environment to which he or she has not been acclimated. Altitude illness can result from this rapid altitude change.
The principal reason for altitude illness is the reduced intake of oxygen that occurs at altitude. At both sea level and higher altitudes, oxygen constitutes about 21% of the gases that comprise air. However, the number of oxygen molecules present in a set volume of air (measured as the barometric pressure, which is the weight of the atmosphere over a set portion of the Earth) decreases with increasing altitude. At sea level, for example, the barometric pressure is 760 millimeters (mm) of mercury. But at an elevation of 12,000 ft (3,700 m), the barometeric pressure is only 483 mm of mercury.
Put another way, there are less oxygen molecules in each breath of air at the higher altitude. For example, at the summit of Mount Everest (approximately 29,000 ft [8,839 m] above sea level), the available oxygen is only one-third that at sea level. While individuals such as seasoned mountain guides have climbed Mount Everest without the aid of oxygen, most climbers will need the assistance of oxygen tanks at this altitude.
At higher altitudes, the body will attempt to compensate for the reduced oxygen level by increasing the heart rate and by breathing faster. Yet, even these increases will not restore the oxygen level in the body to the oxygen level at the lower altitude. As a consequence, physical adaptation to a reduced amount of oxygen must occur.
If the change from a lower to a higher altitude is gradual, the physiological adaptations will occur in plenty of time. These include the production of more oxygen-carrying red blood cells, increased production of an enzyme that releases oxygen from the blood cells, and an increased pressure in the pulmonary arteries (which essentially forces air into regions of the lung that are nor normally required for breathing at sea level).
Symptoms of altitude illness vary from mild to life-threatening. In the majority of people who are not acclimated, elevations over 10,000 ft (3,000 m) will produce headache, shortness of breath, a feeling of fatigue, dizziness, nausea, loss of appetite, and disrupted sleep. These symptoms of acute mountain sickness usually begin from 12 to 24 hours after the elevation change, and they tend to increase over the first few days, as part of the body's acclimation process.
As acclimation occurs or with descent to a lower altitude, the symptoms subside. A drug called aceta-zolamide speeds up breathing by blocking an enzyme in the kidney, which makes the blood more acidic. In turn, this is interpreted by the brain as a signal to increase blood alkalinity by more rapid breathing. The increased breathing rate increases the intake of oxygen. However, allergic reactions to the sulfona-mide drug can occur.
More moderate symptoms of weakness, shortness of breath upon exertion, and decreased coordination (ataxia) may not be relieved by routine medication. Descent to a lower altitude is the typical course of treatment. Time spent at the lower altitude can produce acclimation, and ascent to higher altitudes can recommence.
Severe altitude illness, which most often occurs when the ascent to higher altitudes is done quickly, manifest with the same moderate, as well as shortness of breath when resting, the complete inability to walk, mental confusion, buildup of fluid in the lungs (pulmonary edema), and fluid leakage in the brain that produces swelling (cerebral edema). An immediate descent to a lower altitude is necessary to prevent persistent or permanent neurological injury or death (which can occur within several days). During descent, administering dexamethasone or nifedipine can lessen the symptoms of cerebral edema and pulmonary edema, respectively. The drugs do not help in acclimation, however.
Altitude illness can also be treated on the mountainside using a Gamow bag. A person is placed inside the bag, which is then inflated and sealed. By pumping air into the bag, an atmosphere enriched in oxygen is created, simulating a lower altitude. After several hours inside a Gamow bag, acclimation to the artificial lower altitude will have occurred. This acclimation will persist for some hours after exiting the bag, providing enough time for a real descent to the safety of a lower altitude.