High altitude medicine

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Altitude Stages

Stage Altitude Physiology
Intermediate Altitude 5000-8000ft
  • Decreased exercise performance without major impairment in SaO2
High Altitude 8000-12,000ft
  • Decreased SaO2 with marked impairment during exercise and sleep
Very High Altitude 12,000-18,000ft
  • Abrupt ascent can be dangerous; acclimatization is required to prevent illness
Extreme Altitude >18,000ft
  • Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia
  • Sustained human habitation is impossible
  • RV strain, intestinal malabsorption, impaired renal function, polycythemia

Physiology of Acclimatization


  • Increased elevation → decreased partial pressure of O2 → decreased PaO2
    • Hypoxic ventilatory response results in ↑ ventilation to maintain PaO2
    • Vigor of this inborn response relates to successful acclimatization
  • Initial hyperventilation is attenuated by respiratory alkalosis
    • As renal excretion of bicarb compensates for respiratory alkalosis, pH returns toward normal
  • Process of maximizing ventilation culminates within 4-7 days at a given altitude
    • With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
    • Completeness of acclimatization can be gauged by partial pressure of arterial CO2
    • Acetazolamide, which results in bicarb diuresis, can facilitate this process


  • Erythropoietin level begins to rise within 2 days of ascent to altitude
  • Takes days to weeks to significantly increase red cell mass
    • This adaptation is not important for the initial initial acclimatization process

Fluid Balance

  • Peripheral venoconstriction on ascent to altitude causes increase in central blood volume
    • This leads to decreased ADH → diuresis
    • This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
      • One of the hallmarks of AMS is antidiuresis

Cardiovascular System

  • SV decreases initially while HR increases to maintain CO
  • Cardiac muscle in healthy patients can withstand extreme hypoxemia with out ischemic events
  • Pulmonary circulation constricts with exposure to hypoxia
    • Degree of pulmonary hypertension varies; a hyper-reactive response is associated with HAPE

Differential Diagnosis

High Altitude Illnesses

High Altitude Syndromes

  • All caused by hypoxia
  • All are seen in rapid ascent in unacclimatized patients
    • Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
    • Above 10,000ft rule of thumb is to sleep no higher than 1000 additional ft/day
  • All respond to O2/descent

See Also