Altitude sickness

Altitude sickness, the mildest form being acute mountain sickness, is a harmful effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation. People's bodies can respond to high altitude in different ways. Symptoms of altitude sickness may include headaches, vomiting, tiredness, confusion, trouble sleeping, and dizziness. Acute mountain sickness can progress to high-altitude pulmonary edema with associated shortness of breath or high-altitude cerebral edema with associated confusion. Chronic mountain sickness may occur after long-term exposure to high altitude.
Altitude sickness typically occurs only above, though some people are affected at lower altitudes. Risk factors include a prior episode of altitude sickness, a high degree of activity, and a rapid increase in elevation. Being physically fit does not decrease the risk. Diagnosis is based on symptoms and is supported for those who have more than a minor reduction in activities. It is recommended that at high altitude any symptoms of headache, nausea, shortness of breath, or vomiting be assumed to be altitude sickness.
Sickness is prevented by gradually increasing elevation by no more than per day. Generally, descent and sufficient fluid intake can treat symptoms. Mild cases may be helped by ibuprofen, acetazolamide, or dexamethasone. Severe cases may benefit from oxygen therapy and a portable hyperbaric bag may be used if descent is not possible. The only definite and reliable treatment for severe AMS, HACE, and HAPE is to descend immediately until symptoms resolve. Other treatment efforts have not been well studied.
AMS occurs in about 20% of people after rapidly going to and in 40% of people after going to. While AMdS and HACE occurs equally frequently in males and females, HAPE occurs more often in males. The earliest description of altitude sickness is attributed to a Chinese text from around 30 BCE that describes "Big Headache Mountains", possibly referring to the Karakoram Mountains around Kilik Pass.

Signs and symptoms

People have different susceptibilities to altitude sickness; for some otherwise healthy people, acute altitude sickness can begin to appear at around above sea level, such as at many mountain ski resorts, equivalent to a pressure of. This is the most frequent type of altitude sickness encountered. Symptoms often manifest within ten hours of ascent and generally subside within two days, though they occasionally develop into the more serious conditions. Symptoms include headache, confusion, fatigue, stomach illness, dizziness, and sleep disturbance. Exertion may aggravate the symptoms.
Those individuals with the lowest initial partial pressure of end-tidal pCO₂ and corresponding high oxygen saturation levels tend to have a lower incidence of acute mountain sickness than those with high end-tidal pCO₂ and low oxygen saturation levels.

Primary symptoms

s are the primary symptom used to diagnose altitude sickness, although a headache is also a symptom of dehydration. A headache occurring at an altitude above a pressure of combined with any one or more of the following symptoms, may indicate altitude sickness:

Severe symptoms

Symptoms that may indicate life-threatening altitude sickness include:
; Pulmonary edema
; Cerebral edema
The most serious symptoms of altitude sickness arise from edema. At very high altitude, humans can get either high-altitude pulmonary edema, or high-altitude cerebral edema. 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 which, with constant or increased cardiac output, also leads to increases in capillary pressures. For those with HACE, dexamethasone may provide temporary relief from symptoms in order to keep descending under their own power.
HAPE 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. 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 affected by HACE.

Cause

Altitude sickness can first occur at, with the effects becoming severe at extreme altitudes. Only brief trips above are possible and supplemental oxygen is needed to avert sickness.
As altitude increases, the available amount of oxygen to sustain mental and physical alertness decreases with the overall air pressure, though the relative percentage of oxygen in air, at about 21%, remains practically unchanged up to. The RMS velocities of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen until stratospheric heights.
Dehydration due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.
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 acclimatization occurs. However, in extreme cases, altitude sickness can be fatal.
High altitude illness can be classified according to the altitude: high, very high and extreme.

High altitude

At high altitude,, the onset of physiological effects of diminished inspiratory oxygen pressure includes decreased exercise performance and increased ventilation. While arterial oxygen transport may be only slightly impaired the arterial oxygen saturation generally stays above 90%. Altitude sickness is common between because of the large number of people who ascend rapidly to these altitudes.

Very high altitude

At very high altitude,, maximum SaO₂ falls below 90% as the arterial PO₂ falls below 60mmHg. Extreme hypoxemia may occur during exercise, during sleep, and in the presence of high altitude pulmonary edema or other acute lung conditions. Severe altitude illness occurs most commonly in this range.

Extreme altitude

Above, marked hypoxemia, hypocapnia, and alkalosis are characteristic of extreme altitudes. Progressive deterioration of physiologic function eventually outstrips acclimatization. As a result, no permanent human habitation occurs above. A period of acclimatization is necessary when ascending to extreme altitude; abrupt ascent without supplemental oxygen for other than brief exposures invites severe altitude sickness.

Mechanism

The physiology of altitude sickness centres around the alveolar gas equation; the atmospheric pressure is low, but there is still 20.9% oxygen. Water vapour still occupies the same pressure too—this means that there is less oxygen pressure available in the lungs and blood. Compare these two equations comparing the amount of oxygen in blood at altitude:

Type	At Sea Level	At 8400 m	Formula
Pressure of oxygen in the alveolus
Oxygen carriage in the blood

The hypoxia leads to an increase in minute ventilation, Hb increases through haemoconcentration and erythrogenesis. Alkalosis shifts the haemoglobin dissociation constant to the left, 2,3-BPG increases to counter this. Cardiac output increases through an increase in heart rate.
The body's response to high altitude includes the following:

↑ Erythropoietin → ↑ hematocrit and haemoglobin
↑ 2,3-BPG
↑ kidney excretion of bicarbonate
Chronic hypoxic pulmonary vasoconstriction

People with high-altitude sickness generally have reduced hyperventilator response, impaired gas exchange, fluid retention or increased sympathetic drive. There is thought to be an increase in cerebral venous volume because of an increase in cerebral blood flow and hypocapnic cerebral vasoconstriction causing oedema.

Diagnosis

Altitude sickness is typically self-diagnosed since symptoms are consistent: nausea, vomiting, headache, and can generally be deduced from a rapid change in altitude or oxygen levels. However, some symptoms may be confused with dehydration. Some severe cases may require professional diagnosis which can be assisted with multiple different methods such as using an MRI or CT scan to check for abnormal buildup of fluids in the lung or brain.

Prevention

Ascending slowly is the best way to avoid altitude sickness. Avoiding strenuous activity such as skiing, hiking, etc. in the first 24 hours at high altitude may reduce the symptoms of AMS. Alcohol and sleeping pills are respiratory depressants, and thus slow down the acclimatization process and should be avoided. Alcohol also tends to cause dehydration and exacerbates AMS. Thus, avoiding alcohol consumption in the first 24–48 hours at a higher altitude is optimal.

Pre-acclimatization

Pre-acclimatization is when the body develops tolerance to low oxygen concentrations before ascending to an altitude. It significantly reduces risk because less time has to be spent at altitude to acclimatize in the traditional way. Additionally, because less time has to be spent on the mountain, less food and supplies have to be taken up. Several commercial systems exist that use altitude tents, so called because they mimic altitude by reducing the percentage of oxygen in the air while keeping air pressure constant to the surroundings. Examples of pre-acclimation measures include remote ischaemic preconditioning, using hypobaric air breathing in order to simulate altitude, and positive end-expiratory pressure.