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16. 7. 2012.

Health risks of acute exposure to altitude

Apart from cold, wind, and solar radiation that confront those who ascend to moderate and high altitudes, some people can experience symptoms of acute altitude(mountain) sickness. This disorder is characterized by symptoms such as headache, nausea, vomiting, dyspnea(difficult breathing), and insomnia. These symptoms can begin anywhere from 6 to 48 after arrival at high altitude and are most severe on days 2 and 3. Although not life threatening, acute altitude sickness can be incapacitating for several days or longer. In some cases, the condition can worsen. The victim can develop the more lethal altitude-related illnesses of high-altitude pulmonary edema or high-altitude cerebral edema.

Acute altitude sickness

The incidence of acute altitude sickness varies with the altitude, the rate of ascent, and the individual’s susceptibility. Several studies have been conducted to determine the incidence of acute altitude sickness in groups of hikers and climbers. Results vary widely, ranging from a frequency of less than 1% to 53% at altitudes of 3,000 to 5,500m (9,840 – 18,045 ft). Forster, however, reported that 80% of those who ascended to the top of Mauna Kea(4,205m, or 13,976ft) on the island of Hawaii experienced some symptoms of acute altitude sickness. At elevations of 2,500 to 3,500m(8,202 – 11,483ft), altitudes commonly experienced by recreational skiers and hikers, the incidence of acute altitude sickness is about 7% for men and 22% for women, but the reason for this sex difference is unclear.
Although the precise underlying cause of acute altitude sickness is not fully understood, it appears that those people who experience the greatest distress also have a low ventilatory response to hypoxia. This inadequate ventilation allows PO2 to decrease further and carbon dioxide to accumulate in the tissues, and these two factors may induce most of the symptoms associated with altitude sickness.
Headache is the most common symptom associated with ascent to high altitude. Headache is rarely experienced below 2,500m(under 8,000ft), but ascent to 3,600m(12,000ft) results in headache in the majority of people. The headache at altitude, which many sufferers describe as continuous and throbbing, is typically worse in the morning and after exercise. Alcohol consumption worsens the symptoms. The precise mechanism is unknown, but hypoxia causes dilation of the cerebral blood vessels, so stretching of pain receptors in these structures is a likely cause. Related problems associated with central nervous systemhypoxia include impaired visual acuity and night vision. At extreme altitudes, example Mount Everest, verbal expression, reasoning, and mood may all be affected. There is no evidence of permanent mental impairment associated with cerebral hypoxia.
Another side effect of acute altitude sickness is an inability to sleep despite marked fatique. Studies have shown that the inability to achieve satisfying sleep at altitude is associated with an interruption in the sleep stages. In addition, some people suffer a pattern of interrupted breathing, called Cheyne-Stokes breathing, which prevents them from falling to sleep and remaining asleep. Cheyne-Stokes breathing is characterized by alternate rapid breathing and slow, shallow breathing, usually including intermittent periods in which breathing completely stops. The incidence of this irregular breathing pattern increases with altitude, occurring 24% of the time at 2,440m(8,005ft), 40% of the time at 4,270m(14,009ft), and 100% of the time at altitudes above 6,300m(20,669ft).
How can athletes avoid acute altitude sickness? No evidence indicates that superior physical conditioning prevents the symptoms of altitude sickness. Even athletes who are highly endurance trained before altitude exposure seem to have little protection against the effects of hypoxia, and some data suggest that young, fit individuals may be more prone to experiencing symptoms. Currently, it is difficult to determine which athletes may be susceptible to these symptoms, unless suggested by a prior history of acute altitude sickness.
People can usually prevent acute altitude sickness by gradually ascending to altitude, spending periods of a few days at several lower elevations. A gradual ascent of no more than 300m(984ft) per day at elevations above 3,000m(9,840ft) has been suggested to minimize the risks of altitude sickness. Of the drugs that have been used to reduce the symptoms of those who develop acute altitude sickness, acetazolamide started the day before ascent is the only established preventive measure. Acetazolamide is sometimes combined with steroids such as dexamethazone. Both drugs must be used with medical supervision. Of course, the definitive treatment for severe acute mountain sickness is a retreat to lower altitude, but high-flow oxygen and the use of hyperbaric rescue bags are also effective in severe cases.

High-altitude pulmonary edema

Unlike acute mountain sickness, high-altitude pulmonary edema(HAPE), which is the accumulation of fluids in the lungs, is life threating. The cause of HAPE is unknown, but may be related to the pulmonary vasoconstriction resulting from hypoxia, causing blood clots to form in the lungs. Remaining tissue becomes overperfused, and fluid and protein leak out of the capillaries. This seems to occur most frequently in unacclimatized people who rapidly ascend to altitudes above 2,500m(8,202ft). The disorder occurs in otherwise healthy people and has been reported more often in children and young adults. The fluid accumulation interferes with air movement into and out of the lungs, leading to shortness of breath, a persistent cough, chest tightness, and excessive fatique. Disruption of normal breathing impairs blood oxygenation, causing cyanosis(a bluish tint) of the lips and fingernails, mental confusion, and loss of consciuousness. High-altitude pulmonary edema is treated via administration of supplemental oxygen and movement of the victim to a lower altitude.

High-altitude cerebral edema

Rare cases of high-altitude cerebral edema(HACE), which is fluid accumulation in the cranial cavity, have been reported. The condition is often a subsequent complication of HAPE. This neurological condition is characterized by mental confusion, lethargy, and ataxia(difficulty walking), progressing to unconsciousness and death. Most cases have been reported at altitudes greater than 4,300m(14,108ft). The cause of HACE is unknown, but the treatment is administration of supplemental oxygen, a hyperbaric bag, and prompt descent to a lower altitude. If descent is delayed, permanent impairment may ensue.

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