high altitude pulmonary oedema (HAPO)

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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


Last reviewed 10/2020