Lessons from high-altitude physiology

Educational aims

1. To review the physiological response to high-altitude exposure.

2. To discuss the three major high-altitude diseases: acute mountain sickness, high-altitude cerebral oedema and high-altitude pulmonary oedema.

3. To provide information on prevention and treatment of high-altitude diseases.

Summary High-altitude exposure causes a series of normal physiological responses, termed acclimatisation, which mitigate the effects of hypobaric hypoxia. Hypoxic ventilatory stimulation results in improved oxygen uptake but is associated with respiratory alkalosis that may trigger periodic breathing, particularly during sleep, thereby impairing sleep quality. As travelling to high altitude is popular, high-altitude related illnesses that affect subjective wellbeing, reduce physical performance and alter mental status are also frequently observed. They encompass acute mountain sickness (AMS), high-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE). Depending on ascent rate and individual susceptibility, symptoms usually occur at altitudes above 2,500 m. Therapeutic options include descent accompanied by administration of oxygen and drugs as required. Prevention is based on appropriate acclimatisation, moderate ascent rate, low sleeping altitude and drugs, including acetazolamide, dexamethasone and nifedipine.