What Causes Altitude Sickness?
High altitudes have decreased atmospheric pressure, a condition under
which humans inhale a decreased amount of oxygen*. Altitude sickness
is caused by a lack of oxygen supply in the blood, a physiological state
known as hypoxemia. The body attempts to compensate for this decrease
in oxygen by hyperventilating, or breathing in rapid, shallow breaths.
Hyperventilating, which is part of a physical acclimatization process,
allows more oxygen to be delivered to body tissues. However, hyperventilation
also causes abnormalities in body pH that take time to bring
back to normal. The body increases the number of red blood cells to
increase oxygen carrying capacity and alters red blood cells to increase
their efficiency for dropping oxygen off to body tissues per unit of time.
It generally takes a few days for this process to occur at altitudes up
to 10,000 feet. The higher the altitude, the longer the acclimatization
What Are the Risk Factors for Altitude Sickness?
Altitude sickness can affect anyone ascending to a high altitude, especially
if the ascent is done too quickly for the body to adapt accordingly. Rate
of ascent to the higher altitude is a key factor. The geographical point
above sea level at which an individual experiences altitude sickness varies
from person to person. Altitude sickness usually starts to affect people at
an elevation of 7,000 feet to 9,000 feet above sea level, especially when
ascent occurs within one day. However, some people experience effects as low as 5,000 feet. Physical condition and level of fitness are factors that
affect the onset of altitude sickness, although being physically fit will not
prevent an individual from developing altitude sickness. The amount of
physical exertion placed on the body affects how much oxygen the body
needs, making greater exertion a risk factor for altitude sickness when
individuals are ascending. Altitude sickness is often seen in mountain
climbers, trekkers, and skiers who travel to high-altitude areas. Previous
episodes of altitude sickness and living at low altitudes predispose
individuals for an episode. Young children are most susceptible to altitude
sickness. Maintaining hydration, eating frequent high-carbohydrate
meals, avoiding high salt intake, and alcohol avoidance help prevent the
occurrence of altitude sickness.
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What Are the Symptoms and Treatment of Altitude Sickness?
Altitude sickness may occur in varying degrees of severity. The general set
of symptoms includes a headache, as well as fatigue, nausea or vomiting,
or dizziness. There may also be irritability, muscle aches and pains, and
sleep disturbances. Symptoms are exacerbated by physical exertion and
dehydration*. Upon reaching a high altitude, individuals usually take
six to ten hours to develop symptoms. Given a halt in ascent and rest
from physical exertion, symptoms usually either resolve in a day or so or
become worse. If altitude sickness increases in severity, the individual may
experience confusion, difficulty walking normally, and even development
of a stupor or coma*. Difficulty breathing and other respiratory problems may develop.
Treatment of altitude sickness involves stopping the ascent and resting
from physical exertion until symptoms resolve. Other treatments
include increasing fluid intake and taking painkillers for aches and
pain. If symptoms are severe or not resolving, a descent to lower altitude
may be necessary. In some cases, patients may be given oxygen therapy.
Acetazolamide is a drug sometimes used to prevent or treat occurrences of
altitude sickness. Acetazolamide increases ventilation and thereby helps
maintain the amount of oxygen present in the blood available to tissues.
Physiological Adaptation to Altitude Sickness
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Acute Adaptations When first confronted with an increased altitude,
the body automatically changes some of its normal processes to
adapt in a process called acclimatization. The goal of adaptation to high
altitude is to increase the availability of oxygen to the body tissues. One
of the first adaptations is ventilatory acclimatization, which involves a
change in breathing patterns. There is an increase in respiratory rate
(the number of breaths taken per minute). This type of breathing, called
hyperventilation, increases the amount of oxygen available to the tissues
and increases the amount of carbon dioxide expired from the body. As
the body releases more and more carbon dioxide in greater amounts than with normal breathing, a change occurs in the blood pH. As the blood
pH becomes higher than normal, the brain receives signals that tell it to
keep the hyperventilation in check and not let it go on for too long. In
the meantime, the kidneys excrete substances that help keep the body pH
normal. This process may take up to four days and is improved by the use
Another form of adaptation involves the circulation. When individuals
ascend to a high altitude, their heart rate increases in order to increase
the amount of blood pumped out of the heart to the body tissues. Blood
flow also increases to the brain in order to maintain proper brain function.
Circulatory acclimatization takes place within minutes of ascent.
Chronic Adaptations In order to maximize the ability of the blood
to carry oxygen to the tissues, the amount of hemoglobin present in the
blood increases. This process requires weeks to accomplish. Red blood
cells are made in the bone marrow and used to transport oxygen to
the tissues. The more red blood cells present in the blood, the greater the
oxygen-carrying capacity of the blood. In addition to an increase in the
number of red blood cells, the change in pH of the blood also triggers an
alteration in the red blood cell so that it has a greater than normal capacity
to unload oxygen into the tissues.
How Can Altitude Sickness Be Prevented?
The best way to prevent altitude sickness is to ascend to high altitude
gradually. For mountain climbers and hikers, a slow ascent allows the
body time to acclimatize. When people drive or fly to a high-altitude
location making a slow ascent impossible, they should minimize physical
activity for the first few days during the acclimatization process.
Maintaining hydration, eating frequent high-carbohydrate meals, avoiding
high salt intake, and alcohol avoidance help prevents the occurrence of
altitude sickness. Acetazolamide is sometimes used preventatively to avoid
the occurrence of altitude sickness. The narrowing of blood vessels caused
by lack of oxygen to tissue can be helped in part by endothelin and may
potentially be prevented or greatly improved by endothelin antagonists*.