Acute high altitude illnesses range from acute mountain sickness (AMS
), which is merely unpleasant, to high altitude cerebral edema (HACE
) and high altitude pulmonary edema (HAPE
), which can be fatal. Acute high altitude illnesses can be prevented, primarily by gradual ascent and can be treated, usually by rapid descent. Patients who survive acute altitude illness generally make a complete clinical recovery. Wilderness EMS
) providers who are involved in operations at high altitude need to know how to prevent and treat high altitude illness for their own safety as well as for the benefit of their patients.
People living in areas at or near sea level, who travel to areas above 2,400 m (7,874 ft), are at risk of acute high altitude illness. Tens of millions of people visit high altitude areas worldwide every year. There are an estimated 40 million visitors annually to areas above 2,400 m (7,874 ft) in the western United States alone.1
Many subjects of high altitude illness are visitors to wilderness areas or other remote areas with little or no medical infrastructure. There are millions of cases of AMS
and an unknown number of fatal cases of HACE
Scope of Discussion
The emphasis in this chapter is prevention, recognition, and treatment of acute high altitude illnesses, primarily AMS
, and HACE
in wilderness and in other remote settings where there are limited medical resources. Other high altitude conditions include high altitude headaches, high altitude syncope, neurologic conditions, retinopathies, visual problems, high altitude pharyngitis and bronchitis, high altitude peripheral edema, high altitude flatus expulsion (HAFE
), disturbed sleep, and periodic breathing.
responders who are based at altitudes lower than that of the rescue scene are themselves at risk of high altitude illnesses, especially if they are able to reach the scene but are unable to descend immediately. WEMS
responders also run risks due to limitations of human and machine performance at high altitudes.
affect the brain, while HAPE
affects the lungs. AMS
may occur alone or in combination with HAPE
may also occur alone or in combination with AMS
frequently occur together.
is rare below 2,400 m (7,874 ft). HACE
is rare below 3,000 m (9,843 ft) and, in most settings, is uncommon, even above 3,000 m. HACE
most frequently occurs in very high mountains, such as the Himalayas. HAPE
is more common than HACE
in most settings, especially in areas below 4,000 m (13,123 ft). The incidence of HAPE
is increased by colder conditions.4 HAPE
is common above 3,000 m (9,843 ft), but may be seen well below 2,400 m (7,874 ft) in occasional patients with congenital absence of a pulmonary artery.5,6
In most high altitude areas, the incidence of high altitude illness can only be roughly estimated. This is especially true in wilderness and remote areas where the number of people at risk is often unknown. Many travelers who suffer from altitude illness, especially AMS
, do not seek medical attention. Medical care is often unavailable in wilderness and remote areas. Visitors to high altitude may blame the symptoms of acute altitude illness on other causes.
Risk is increased primarily with rapid ascent. The higher the altitude, the higher are the risks of both HACE
. At a given location, AMS
are more common during periods of low barometric pressure, which causes a higher effective altitude.4 HAPE
is more common during periods of cold weather.4 HAPE
also seems to be more common in travelers such as trekkers and climbers who are exerting themselves and seems to be less common in those who reach high altitude by passive means and who do not participate in strenuous activities once they arrive.
In the western United States, where tens of millions of visitors from lowland locations sleep above 2,400 m (7,874 ft), the estimated incidence of AMS
is about 22% for those sleeping at 2,500 m (8,202 ft) and 17% to 42% for those sleeping above 3,000 m (9,843 ft).7
The overall incidence of HAPE
is about 0.01%. Trekkers in the Khumbu (Mt. Everest) region of Nepal typically sleep at altitudes from 3,400 m (11,155 ft) to 5,100 m (16,732 ft) and reach altitudes of 5,400 to 5,700 m (17,717 to 18,701 ft). In a classic study, almost half of the trekkers who flew to 2,800 m (9,186 ft) and ascended in several days to 4,300 m (14,108 ft) developed AMS
, compared to about a quarter of those who spent an extra week or so walking from the lowlands.8
The incidence of HACE
was 1% to 2% of those who flew to 2,800 m (9,186 ft) and about 0.05% of those who walked from the lowlands.
Climbers and hikers attempting summits of high mountains are at risk of altitude illness. On Mt. Rainier (4,392 m or 14,410 ft), in the Cascade Mountains of Washington State, most climbers live near sea level and sleep one night at about 3,048 m (10,000 ft) on the way to the summit. The incidence of AMS
is estimated to be about 67%, with a negligible incidence of HACE
On Mt. Kilimanjaro (5,895 m or 19,341 ft), hikers typically sleep at altitudes ranging from 2,700 to 4,700 m (8,858 to 15,420 ft), taking 2 to 6 days to reach the summit. The incidence of AMS
is estimated to be 50% to 83%.10,11
Although the incidence of HACE
is not known, deaths from altitude illness are common. On Denali in Alaska, climbers typically take 3 days to a week or more to reach the summit (6,190 m or 20,310 ft) after reaching 3,000 m (9,843 ft), sleeping at 3,000 to 5,300 m (17,388 ft). The incidence of AMS
is about 30%.12
The incidence of HACE
is about 2% to 3%.
The highest incidences of AMS
are found in situations with very rapid ascents, especially in travelers who fly to places like Lhasa, Tibet (3,656 m or 11,995 ft), and La Paz, Bolivia, where the airport is at about 4,000 m (13,123 ft) and most travelers sleep at about 3,500 m (11,483 ft). These are neither wilderness nor remote locations and are low enough that HACE
are rare. The combined incidence of HACE
is below 1% in most settings, but the incidence of HACE
has been reported in an amazing 31% of pilgrims ascending to Gosainkund Lake (4,380 m or 14,370 ft) in Nepal.13
Most of the pilgrims
live at about 1,340 m (4,400 ft). They ascend to the lake in 1 to 2 days. The incidence of HAPE
was estimated to be about 5%.