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Medical condition due to rapid exposure to low oxygen at high altitude

Sickness is prevented by gradually increasing elevation by no more than per day. Treatment efforts, however, have not been well studied. AMS occurs in about 20% of people after rapidly going to and in 40% of people after going to .

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 (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.

At high altitude, , the onset of physiological effects of diminished inspiratory oxygen pressure (PiO2) includes decreased exercise performance and increased ventilation (lower arterial partial pressure of carbon dioxide: PCO2). While arterial oxygen transport may be only slightly impaired the arterial oxygen saturation (SaO2) generally stays above 90%. Altitude sickness is common between because of the large number of people who ascend rapidly to these altitudes.

Ascending slowly is the best way to avoid altitude sickness. Once above approximately a pressure of 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. Once the climber has acclimatized to a given altitude, the process is repeated with camps placed at progressively higher elevations. The rule of thumb is to ascend no more than per day to sleep. That is, one can climb from to in one day, but one should then descend back to to sleep. This process cannot safely be rushed, and this is why climbers need to spend days (or even weeks at times) acclimatizing before attempting to climb a high peak. Simulated altitude equipment such as altitude tents provide hypoxic (reduced oxygen) air, and are designed to allow partial pre acclimation to high altitude, reducing the total time required on the mountain itself. Altitude acclimatization is necessary for some people who move rapidly from lower altitudes to higher altitudes.

The drug acetazolamide (trade name Diamox) may help some people making a rapid ascent to sleeping altitude above , and it may also be effective if started early in the course of AMS. Acetazolamide can be taken before symptoms appear as a preventive measure at a dose of 125 mg twice daily. 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. The Centre suggests a dosage of 125 mg twice daily for prophylaxis, starting from 24 hours before ascending until a few days at the highest altitude or on descending; The Centers for Disease Control and Prevention (CDC) suggest the same dose for prevention of 125 mg acetazolamide every 12 hours. Acetazolamide, a mild diuretic, works by stimulating the kidneys to secrete more bicarbonate in the urine, thereby acidifying the blood. This change in pH stimulates the respiratory center to increase the depth and frequency of respiration, thus speeding the natural acclimatization process. 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. Although a sulfonamide, acetazolamide is a non antibiotic and has not been shown to cause life threatening allergic cross reactivity in those with a self reported sulfonamide allergy. Dosage of 1000 mg/day will produce a 25% decrease in performance, on top of the reduction due to high altitude exposure. The CDC advises that Dexamethasone be reserved for treatment of severe AMS and HACE during descents, and notes that Nifedipine may prevent HAPE. Despite their popularity, antioxidant treatments have not been found to be effective medications for prevention of AMS. Interest in phosphodiesterase inhibitors such as sildenafil has been limited by the possibility that these drugs might worsen the headache of mountain sickness. A promising possible preventive for altitude sickness is myo inositol trispyrophosphate (ITPP), which increases the amount of oxygen released by hemoglobin. Prior to the onset of altitude sickness, ibuprofen is a suggested non steroidal anti inflammatory and painkiller that can help alleviate both the headache and nausea associated with AMS. It has not been studied for the prevention of cerebral edema (swelling of the brain) associated with extreme symptoms of AMS.

Herbal supplements and traditional medicines are sometimes suggested to prevent high altitude sickness including ginkgo biloba, R crenulata, minerals such as iron, antacids, and hormonal based supplements such as medroxyprogesterone and erythropoietin. In Chinese and Tibetan traditional medicine, an extract of the root tissue of Radix rhodiola is often taken in order to prevent the symptoms of high altitude sickness, however, no clear medical studies have confirmed the effectiveness or safety of this extract.

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 , raising the oxygen concentration level by 5% via an oxygen concentrator and an existing ventilation system provides an effective altitude of , which is more tolerable for those unaccustomed to high altitudes. 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 use 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 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).

Increased water intake may also help in acclimatization 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.

The only reliable treatment, and in many cases the only option available, is to descend. Attempts to treat or stabilize the patient in situ (at altitude) are 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 to 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 A Gamow bag is generally used only as an aid to evacuate severe AMS patients, not to treat them at altitude. Acetazolamide 250 mg twice daily dosing assists in AMS treatment by quickening altitude acclimatization. A study by the Denali Medical Research Project concluded: "In established cases of acute mountain sickness, treatment with acetazolamide relieves symptoms, improves arterial oxygenation, and prevents further impairment of pulmonary gas exchange." The folk remedy for altitude sickness in Ecuador, Peru and Bolivia is a tea made from the coca plant. See mate de coca. Steroids can be used to treat the symptoms of pulmonary or cerebral edema, but do not treat the underlying AMS. Two studies in 2012 showed that ibuprofen 600 milligrams three times daily was effective at decreasing the severity and incidence of AMS; it was not clear if HAPE or HACE was affected. Paracetamol (acetaminophen) has also shown to be as good as ibuprofen for altitude sickness when tested on climbers ascending Everest.