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High Altitude sickness

Acute Mountain Sickness (AMS)

Some people are more susceptible to altitude sickness than others. If you suffer from a case of altitude sickness it does not mean that you can never go to high altitudes again. However, it does mean in the future, you should pay attention. Awareness of altitude sickness has caused some trekkers to be unnecessarily anxious as they trek. The progression of symptoms is usually gradual, and you will have plenty of time to react appropriately. Design your itineraries to allow plenty of time for acclimatization so that you will be able to adjust to the increase in altitude. Human bodies have the ability to adjust to higher altitudes when given enough time. If a person travels up to high altitudes more rapidly than his or her body is able to adjust, AMS symptoms develop.

The treatments of AMS are first and foremost not to ascend with symptoms and if symptoms are severe, to descend. In rare cases where the descent is difficult or impossible a portable pressure chamber is effective. Three medications have also been proven useful for treating and preventing AMS: Acetazolamide (Diamox), Dexamethasone (Decadron), Nifedipine. Your physician and local Public Health Service are the best sources for further information.

Altitude sickness—also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, 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)   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". 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 meters (8,000 ft) without difficulty.

Acute mountain sickness can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE), which is potentially fatal.

Chronic mountain sickness, also known as Monge's disease, is a different condition that only occurs after very prolonged exposure to high altitude. 

Oxygen Level in Altitude

Altitude (in meters)

Oxygen Rate

Altitude (in meters)

Oxygen Rate


33 %


57 %


36 %


60 %


41 %


64 %


47 %


68 %


50 %


73 %


52 %


88 %


53 %

Sea level

100 %



The percentage of oxygen in air, at 21%, remains almost unchanged up to 70,000 feet (21,000 m). 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 level, which drops as altitude increases. Consequently, the available amount of oxygen to sustain mental and physical alertness decreases above 10,000 feet (3,000 m). Although the cabin altitude in modern passenger aircraft is kept to 8,000 feet (2,400 m) or lower, some passengers on long-haul flights may experience some symptoms of altitude sickness.

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, altitudes 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.

Signs and symptoms

Primary symptoms:

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:

  • Lack of appetite, nausea, or vomiting
  • Fatigue or weakness
  • Dizziness or lightheadedness
  • Insomnia
  • Pins and needles
  • Shortness of breath upon exertion
  • Nosebleed
  • Persistent rapid pulse
  • Drowsiness
  • General malaise
  • Peripheral edema (swelling of hands, feet, and face).
  • Severe symptoms
  • Symptoms that may indicate life-threatening altitude sickness include:
  • Pulmonary edema (fluid in the lungs)
  • Symptoms similar to bronchitis
  • 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
  • Gradual loss of consciousness
  • Increased nausea
  • Retinal hemorrhage

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.


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 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

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 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 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 meters (1,000 ft) per day to sleep. That is, one can climb from 3,000 (10,000 feet = 70 kPa) to 4,500 meters (15,000 feet = 58 kPa) in one day, but one should then descend back to 3,300 meters (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) acclimatizing 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

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.

Medical treatment

The drug acetazolamide 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. TheEverest 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 125–250 mg twice daily for prophylaxis, starting from 24 hours before ascending until a few days at the highest altitude or on descending;] with 250 mg twice daily recommended for treatment of AMS. The Centers for Disease Control and Prevention suggests a lower dose for prevention of 125 mg acetazolamide every 12 hours. An undesirable side-effect of acetazolamide is a reduction in aerobic endurance performance. 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 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. 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 preventative treatment for altitude sickness is myo-inositol trispyrophosphate (ITPP), which increases the amount of oxygen released by hemoglobin.

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.

Oxygen enrichment

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.

Other methods

Increased water intake may 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 cause dangerous 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 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:

Oxygen may be used for mild to moderate AMS below 12,000 feet (3,700 m) 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 acclimatization.
  • and prevents further impairment of pulmonary gas exchange."

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.

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Pristine Mountain Adventure (P.) Ltd

Dristi Marga , Lakeside -6, Pokhara 

P.O. Box No: 574 , Nepal

Phone No: +977- 61-465929 

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