Altitude sickness, also known as
acute
mountain sickness (
AMS),
altitude
illness,
hypobaropathy, or
soroche, is a pathological
effect of high altitude on
humans (and animals), caused by acute exposure to low
partial pressure of
oxygen at
high altitude. It
commonly occurs above 2,400 metres (approximately 8,000 feet).
Acute mountain sickness can progress to
high altitude pulmonary edema
(
HAPE) or
high altitude cerebral edema
(
HACE).
The causes of altitude sickness are not fully understood. The
percentage of oxygen in air remains essentially constant with
altitude at 21% up until 70,000 feet (21,330 m), but the air
pressure (and therefore the number of oxygen molecules) drops as
altitude increases — consequently, the available amount of
oxygen to sustain mental and physical alertness decreases above
10,000 feet (3,050 m). Altitude sickness usually does not affect
persons traveling in
aircraft because the
cabin altitude in modern passenger aircraft is kept to 8,000 feet
(2,440 m) or lower.
A superficially related condition is
chronic mountain sickness, also
known as Monge's disease, occurring only after prolonged exposure
to high altitude.
An unrelated condition, often confused with altitude sickness, is
dehydration, due to the higher rate of
water vapor lost from the lungs at higher altitudes.
Introduction
High altitude or mountain sickness is defined as a collection of
nonspecific symptoms that can
resemble a case of flu, carbon monoxide poisoning or a hangover
caused by high altitudes. It is hard to determine who will be
affected by altitude-sickness, as there are no specific factors
that compare with this susceptibility to altitude sickness.
However, most people can climb up to 2500 meters (8000 ft)
normally.
Generally, different people have different susceptibilities to
altitude sickness. For some otherwise healthy people, Acute
Mountain Sickness (AMS) can begin to appear at around 2000 meters
(6,500 ft) above sea level, such as at many mountain ski resorts,
equivalent to a pressure of 80 kPa. AMS is the most frequent type
of altitude sickness encountered. Symptoms often manifest
themselves six to ten hours after ascent and generally subside in
one to two days, but they occasionally develop into the more
serious conditions. Symptoms include headache, fatigue, stomach
illness, dizziness, and sleep disturbance. Exertion aggravates the
symptoms.
High altitude pulmonary
edema (
HAPE) and cerebral edema (
HACE) are the most ominous of these symptoms, while
AMS, retinal hemorrhage, and peripheral edema are less severe forms
of the disease. The rate of ascent, altitude attained, amount of
physical activity at high altitude, as well as individual
susceptibility, are contributing factors to the onset and severity
of high-altitude illness.
Altitude sickness usually occurs following a rapid ascent and can
usually be prevented by ascending slowly. In most of these cases,
the symptoms are temporary and usually abate as altitude
acclimatisation occurs. However, in extreme cases, altitude
sickness can be fatal.
The word "soroche" came from
South
America and originally meant "
ore", because
of an old, incorrect belief that it was caused by toxic emanations
of ores in the
Andes mountains.
[11566]
Signs and symptoms
Headaches are a 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 meters (8000 feet = 76 kPa), combined with
any one or more of the following symptoms, can indicate altitude
sickness:
Symptoms that may indicate life-threatening altitude sickness
include:
- pulmonary edema (fluid in the
lungs):
- persistent dry cough
- fever
- shortness of breath even when resting
- cerebral edema (swelling of the
brain):
- headache that does not respond to analgesics
- unsteady gait
- increased vomiting
- gradual loss of consciousness.
Severe cases
The most serious symptoms of altitude sickness are due to
edema (fluid accumulation in the tissues of the body).
At very high altitude, humans can get either
high altitude pulmonary edema
(HAPE), or
high altitude
cerebral edema (HACE). The physiological cause of
altitude-induced edema is not conclusively established. It is
currently believed, however, that HACE is caused by local
vasodilation of cerebral blood vessels in response to hypoxia,
resulting in greater blood flow and, consequently, greater
capillary pressures. On the other hand, HAPE may be due to general
vasoconstriction in the pulmonary circulation (normally a response
to regional ventilation-perfusion mismatches) which, with constant
or increased cardiac output, also leads to increases in capillary
pressures. For those suffering HACE,
dexamethasone may provide temporary relief
from symptoms in order to keep descending under their own
power.
HAPE occurs in about 2% of those who are adjusting to altitudes of
about 3000 m (10,000 feet = 70 kPa) or more. It can progress
rapidly and is often fatal. Symptoms include fatigue, severe
dyspnea at rest, and cough that is initially
dry but may progress to produce pink, frothy
sputum. Descent to lower altitudes alleviates the
symptoms of HAPE.
HACE is a life threatening condition that can lead to coma or
death. It occurs in about 1% of people adjusting to altitudes above
2700 m (9,000 feet = 73 kPa). Symptoms include headache, fatigue,
visual impairment, bladder dysfunction, bowel dysfunction, loss of
coordination, paralysis on one side of the body, and confusion.
Descent to lower altitudes may save those afflicted with
HACE.
A person suffering from serious symptoms of altitude sickness has a
relatively short period of time of useful consciousness in which
corrective action can be taken. The following is a correlation of
approximate altitude to the amount of time that a person will have
useful consciousness:
- 20,000
ft / 6,100 m = 5-12 minutes (peak of Mount McKinley
or Mount Kilimanjaro
)
- 25,000 ft / 7,620 m = 3-5 minutes;
- 29,000
ft / 8,840 m = 1-2 minutes; (peak of Mount Everest
)
- 40,000 ft / 12,200 m = 9-15 seconds (represents the oxygen that
was in a person's system before the exposure)
Prevention
As alcohol tends to dehydrate, avoidance in the first 24 hours at a
higher altitude is optimal.
Strenuous activity
People with recurrent AMS note that by avoiding strenuous activity
such as skiing, hiking, etc. in the first 24 hours at altitude
reduces their problems.
Altitude acclimatization
Altitude acclimatization is the process of adjusting to decreasing
oxygen levels at higher elevations, in order
to avoid altitude sickness. Once above approximately 3,000 metres
(10,000 feet = 70 kPa), most climbers and high altitude trekkers
follow the "golden rule" - climb high, sleep low. For high altitude
climbers, a typical acclimatization regime might be to stay a few
days at a
base camp, climb up to a higher
camp (slowly), then return to base camp. A subsequent climb to the
higher camp would then include an overnight stay. This process is
then repeated a few times, each time extending the time spent at
higher altitudes to let the body adjust to the oxygen level there,
a process that involves the production of additional
red blood cells . Once the climber has
acclimatised to a given altitude, the process is repeated with
camps placed at progressively higher elevations. The general rule
of thumb is to not ascend more than 300 metres (1,000 ft) per day
to sleep. That is, one can climb from 3,000 (10,000 feet = 70 kPa)
to 4,500 metres(15,000 feet = 58 kPa) in one day, but one should
then descend back to 3,300 metres (11,000 feet = 67.5 kPa) to
sleep. This process cannot safely be rushed, and this explains why
climbers need to spend days (or even weeks at times) acclimatising
before attempting to climb a high peak. Simulated altitude
equipment that produce hypoxic (reduced oxygen) air can be used to
acclimate to altitude, reducing the total time required on the
mountain itself.
Altitude acclimatization is necessary for some people who rapidly
move from lower altitudes to more moderate altitudes, usually by
aircraft and ground transportation over a few hours, such as from
sea level to of many Colorado, USA mountain resorts. Stopping at an
intermediate altitude overnight can reduce or eliminate a repeat
episode of AMS.
Drugs
Acetazolamide may help some people
making a rapid ascent to sleeping altitude above 2750 metres and
also be effective if started early in the course of AMS. The
Everest Base Camp Medical Centre
cautions against its routine use as a substitute for a reasonable
ascent schedule, except where rapid ascent is forced by flying into
high altittude locations or due to terrain considerations. The
centre suggests a dosage of 125-250 mg twice daily for
prophylaxis, starting from 24 hours before ascending until a few
days at the highest altitude or on descending; with 250mg twice
daily recommended for treatment of AMS. The
Centers for Disease
Control and Prevention suggest the lower value for prevention
of 125 mg acetazolamide every 12 hours. The CDC advises that
Dexamethasone be reserved for
treatment of AMS and HACE during descents and notes that
Nifedipine may prevent HAPE.
A single
randomized
controlled trial found that
sumatriptan may help prevent altitude sickness..
Although popular,
antioxidant treatments
have not been found to be effective. Recent interest in
phosphodiesterase inhibitors such as
sildenafil has been limited by the possibility
that these drugs might worsen the headache of mountain
sickness.
For
centuries, indigenous cultures of
the Altiplano
, such as the Aymaras, have
chewed coca leaves to try to alleviate the
symptoms of mild altitude sickness, but its efficacy has not been
studied.
Oxygen enrichment
In high-altitude conditions, oxygen enrichment can counteract the
effects of altitude sickness, or
hypoxia. A small amount of supplemental
oxygen reduces the equivalent altitude in climate-controlled rooms.
At 3,400 m (67 kPa), raising the oxygen concentration level by 5
percent via an
oxygen
concentrator and an existing ventilation system provides an
effective altitude of 3,000 m (70 kPa), which is more tolerable for
surface-dwellers. The most effective source of supplemental oxygen
at high altitude are oxygen concentrators that use
vacuum swing adsorption (VSA)
technology. As opposed to generators that use
pressure swing absorption (PSA),
VSA technology does not suffer from performance degradation at
increased altitude. The lower air density actually facilitates the
vacuum step process.
Other methods
Drinking plenty of water will also help in acclimatisation to
replace the fluids lost through heavier breathing in the thin, dry
air found at altitude, although consuming excessive quantities
("over-hydration") has no benefits and may lead to
hyponatremia.
Oxygen from gas bottles or liquid containers can be applied
directly via a nasal cannula or mask. Oxygen concentrators based
upon
pressure swing
adsorption (PSA), VSA, or vacuum-pressure swing adsorption
(VPSA) can be used to generate the oxygen if electricity is
available. Stationary oxygen concentrators typically use PSA
technology, which has performance degradations at the lower
barometric pressures at high altitudes. One way to compensate for
the performance degradation is to utilize a concentrator with more
flow capacity. There are also portable oxygen concentrators that
can be used on vehicle DC power or on internal batteries, and at
least one system commercially available measures and compensates
for the altitude effect on its performance up to 4,000 meters
(13,000 ft). The application of high-purity oxygen from one of
these methods increases the partial pressure of oxygen by raising
the
FIO2 (fraction of inspired oxygen).
Treatment
The only reliable treatment and in many cases the only option
available is to descend. Attempts to treat or stabilise the patient
in situ at altitude is dangerous unless highly controlled and with
good medical facilities. However, the following treatments have
been used when the patient's location and circumstances
permit:
- Oxygen may be used for mild to moderate AMS below and is
commonly provided by physicians at mountain resorts. Symptoms abate
in 12-36 hours without the need to descend.
- For more serious cases of AMS, or where rapid descent is
impractical, a Gamow bag, a portable
plastic hyperbaric chamber inflated with a foot pump, can be used
to reduce the effective altitude by as much as 1,500 meters (5,000
ft). A Gamow bag is generally used only as an aid to evacuate
severe AMS patients, not to treat them at altitude.
- Acetazolamide may assist in
altitude aclimatisation but is not a reliable treatment for
established cases of even mild altitude sickness.
- Some claim that mild altitude sickness can be controlled by
consciously taking 10-12 large, rapid breaths every 5 minutes,
(hyperventilation) but this claim
lacks both empirical evidence and a plausible medical reason as to
why this should be effective. If overdone, this can remove too much
carbon dioxide causing hypocapnia.
- The
folk remedy for altitude sickness in Ecuador
, Peru
and Bolivia
is a tea
made from the coca plant. See mate de
coca.
- Other treatments include injectable steroids to reduce pulmonary edema, this may buy
time to descend but treats a symptom, it does not treat the
underlying AMS.
See also
References
- — High altitude information for laypeople
- The High Altitude Medicine Handbook, 3rd Edition, Andrew J.
Pollard and David R. Murdoch.
- Altitude and Oxygen
- A.J. Giannini, H.R. Black, R.L. Goettsche. The Psychiatric,
Psychogenic and Somatopsychic Disorders Handbook. New Hyde Park,
NY. Medical Examination Publishing Co.,1978. pp.190,192. ISBN
0-87488-596-5.
- The Mountaineers. Mountaineering: The Freedom of the Hills, 7th
Edition. Seattle, WA: Mountaineers Books, 2003
- high-altitude.org: High Altitude Medicine
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