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High altitude pulmonary oedema (HAPO)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

Acute altitude sickness occurs when an individual who is accustomed to low altitudes rapidly climbs to high altitude. Altitude sickness is a potentially lethal complication of climbing to altitudes above 8,000 feet.

Three main syndromes of altitude illness may affect travellers: acute mountain sickness, high altitude cerebral oedema (HACO), and high altitude pulmonary oedema (HAPO)

  • risk of dying from altitude related illnesses is low, at least for tourists. For trekkers to Nepal the death rate from all causes was 0.014% and from altitude illness 0.0036%
  • soldiers posted to altitude had an altitude related death rate of 0.16%

Clinical features of mild altitude sickness are (1):

  • headache
  • loss of appetite
  • nausea
  • fatigue
  • dizziness
  • insomnia
  • extremity oedema
  • dyspnoea
  • palpitations

There is an increased mortality in patients with acute altitude sickness.

Definitions of altitude and associated physiological changes

Intermediate altitude (1500-2500 metres)

  • physiological changes detectable
  • arterial oxygen saturation >90%
  • altitude illness possible but rare

High altitude (2500-3500 metres)

  • altitude illness common with rapid ascent
  • very high altitude (3500-5800 metres)
  • altitude illness common
  • arterial oxygen saturation <90%
  • marked hypoxaemia during exercise

Extreme altitude (>5800 metres)

  • marked hypoxaemia at rest
  • progressive deterioration, despite maximal acclimatisation
  • permanent survival cannot be maintained

Treatment of altitude related illness is to stop further ascent and, if symptoms are severe or getting worse, to descend

  • oxygen, drugs, and other treatments for altitude illness should be viewed as adjuncts to aid descent

Prevention of acute mountain sickness (AMS) (3)

  • acetazolamide can be used for preventing AMS according to the National Travel and Health Network Centre and Fit For Travel recommendations (not licensed for this this indication)
  • acetazolamide prevents AMS by mimicking the body naturally adjusting to a change in environment
  • a Cochrane review demonstrated acetazolamide reduced the risk of AMS vs placebo by a factor of 0.47 (n=2,301, 16 studies). Acetazolamide was administered one to five days prior to ascent with doses of up to 500mg/day to adults at risk of AMS
  • overall, evidence for the use of the medicines listed below to prevent AMS is inconclusive and for some, side effects are a concern:
    • aspirin
    • dexamethasone
      • Using dexamethasone has been suggested by some organisations to help prevent AMS. However, the Cochrane review (n=176) assessing four parallel studies comparing dexamethasone with placebo found dexamethasone does not prevent AMS at any dose and does not aid acclimatisation.
    • ibuprofen
    • iron supplements
    • magnesium citrate
    • spironolactone
    • sumatriptan


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