出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/07/21 14:31:00」(JST)
| Altitude sickness | |
|---|---|
| Classification and external resources | |
| ICD-10 | T70.2 |
| ICD-9 | E902.0 |
| DiseasesDB | 8375 29615 |
| MedlinePlus | 000133 |
| eMedicine | med/3225 |
| MeSH | D000532 |
Altitude sickness—also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, "the altitude bends", or soroche—is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude. It commonly occurs above 2,400 metres (8,000 feet).[1][2] It presents as a collection of nonspecific symptoms, acquired at high altitude or in low air pressure, resembling a case of "flu, carbon monoxide poisoning, or a hangover".[3] It is hard to determine who will be affected by altitude sickness, as there are no specific factors that correlate with a susceptibility to altitude sickness. However, most people can ascend to 2,400 metres (8,000 ft) without difficulty.
Acute mountain sickness can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE), which are potentially fatal.[2][4]
Chronic mountain sickness, also known as Monge's disease, is a different condition that only occurs after very prolonged exposure to high altitude.[5]
The percentage of oxygen in air, at 21%, remains almost unchanged up to 21,000 metres (69,000 ft).[6] The RMS velocities of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen. However, it is the air density itself, the number of molecules (of both oxygen and nitrogen) per given volume, which drops as altitude increases. Consequently, the available amount of oxygen to sustain mental and physical alertness decreases with altitude.
Dehydration due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.[7]
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.[4] 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.
People have different susceptibilities to altitude sickness; for some otherwise healthy people, acute altitude sickness 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.[10] This 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.[4] Exertion aggravates the symptoms.
The Lake Louise assessment system of AMS is based on a self-report questionnaire as well as a quick clinical assessment.[11]
Those individuals with the lowest initial partial pressure of end-tidal pCO2 (the lowest concentration of carbon dioxide at the end of the respiratory cycle, a measure of a higher alveolar ventilation) 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.[12]
Headaches 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 2,400 metres (8,000 feet = 76 kPa), combined with any one or more of the following symptoms, may indicate altitude sickness:
Symptoms that may indicate life-threatening altitude sickness include:
The most serious symptoms of altitude sickness arise from 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 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 afflicted with HACE.
The physiology of altitude sickness is based on the following equation:
Vgas=A/TDk(P1-P2)
Where Vgas is the diffusion rate, A is the area of the lung, T is the thickness of the lung membranes, P1 and P2 are the differences in partial pressure of any gas-but most importantly CO2 and O2-where in high altitudes the partial pressure differences for O2 are low and the differences in partial pressures for CO2 are high. Thus CO2 will have a high diffusion out and O2 will have a low diffusion though the alveolar membranes and into the blood.
The body's response to high altitude includes the following:
Ascending slowly is the best way to avoid altitude sickness.[4] Avoiding strenuous activity such as skiing, hiking, etc. in the first 24 hours at high altitude reduces the symptoms of AMS. As alcohol tends to cause dehydration, which exacerbates AMS, avoiding alcohol consumption in the first 24-hours at a higher altitude is optimal.
Altitude acclimatization is the process of adjusting to decreasing oxygen levels at higher elevations, in order to avoid altitude sickness.[13] Once above approximately 3,000 metres (10,000 feet = 70 kPa), most climbers and high-altitude trekkers take the "climb-high, sleep-low" approach. For high-altitude climbers, a typical acclimatization regimen might be to stay a few days at a base camp, climb up to a higher camp (slowly), and then return to base camp. A subsequent climb to the higher camp then includes 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.[citation needed] 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 is why climbers need to spend days (or even weeks at times) acclimatising before attempting to climb a high peak. Simulated altitude equipment that produces hypoxic (reduced oxygen) air can be used to acclimate to high altitude, reducing the total time required on the mountain itself.[citation needed]
Altitude acclimatization is necessary for some people who move rapidly from lower altitudes to intermediate altitudes (e.g., by aircraft and ground transportation over a few hours), such as from sea level to 8,000 feet (2,400 m) as in many Colorado, USA mountain resorts. Stopping at an intermediate altitude overnight can alleviate or eliminate occurrences of AMS.
The drug acetazolamide (trade name Diamox) may help some people making a rapid ascent to sleeping altitude above 2,700 metres (9,000 ft), and it may also be effective if started early in the course of AMS.[14] Acetazolamide should be taken before symptoms appear — this is a preventative measure. 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 altitude locations or due to terrain considerations.[15] 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;[15] with 250 mg twice daily recommended for treatment of AMS.[16] The Centers for Disease Control and Prevention (CDC) suggest a lower dose for prevention of 125 mg acetazolamide every 12 hours.[17] An undesirable side-effect of acetazolamide is a reduction in aerobic endurance performance. Other minor side effects include a tingle-sensation in hands and feet, and it can make carbonated drinks taste odd. Dosage of 1000 mg/day will produce a 25% decrease in performance, on top of the reduction due to high-altitude exposure.[18] The CDC advises that Dexamethasone be reserved for treatment of AMS and HACE during descents, and notes that Nifedipine may prevent HAPE.[17]
A single randomized controlled trial found that sumatriptan may help prevent altitude sickness.[19] Despite their popularity, antioxidant treatments have not been found to be effective medications for prevention of AMS.[20] Interest in phosphodiesterase inhibitors such as sildenafil has been limited by the possibility that these drugs might worsen the headache of mountain sickness.[21]
A promising possible preventative for altitude sickness is myo-inositol trispyrophosphate (ITPP), which increases the amount of oxygen released by hemoglobin. Following the onset of altitude sickness, ibuprofen is a suggested anti-inflammatory and painkiller that can help alleviate both the headache and nausea associated with AMS, as well as in combating cerebral edema (swelling of the brain) associated with extreme symptoms of AMS.[22]
For centuries, indigenous peoples of the Americas such as the Aymaras of the Altiplano, have chewed coca leaves to try to alleviate the symptoms of mild altitude sickness. In Chinese and Tibetan traditional medicine, extract of root tissue of Radix rhodiola is often taken in order to prevent the same symptoms.
In high-altitude conditions, oxygen enrichment can counteract the hypoxia related effects of altitude sickness. A small amount of supplemental oxygen reduces the equivalent altitude in climate-controlled rooms. At 3,400 meters (11,155 feet = 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 metres (10,000 feet = 70 kPa), which is more tolerable for surface-dwellers.[23]
Increased water intake may also help in acclimatisation[24] 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 cause dangerous hyponatremia. It’s a good idea to limit alcohol intake the first day or so at higher elevation as well.
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 vehicular 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).
Also, the use of Nitric Oxide has been proven to address and relieve altitude sickness. 12 scientists recently released a study in which Mount Everest trekkers were allowed to naturally adapt to altitude during their ascent. They were tested for nitric oxide levels along the way. The scientists found that their bodies naturally produced more nitric oxide as they climbed higher. "Our results suggest that nitric oxide is an integral part of the human physiological response to hypoxia." Scientific Reports
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:
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| リンク元 | 「急性高山病」 |
| 関連記事 | 「acute」「sickness」 |