High altitude medicine

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Relationship between total atmospheric pressure and altitude above sea level.

Altitude Stages

Stage Altitude Physiology
Intermediate Altitude 5,000 - 8,000 ft
(1,524 - 2,438 meters)
  • Decreased exercise performance without major impairment in SaO2
High Altitude 8,000 - 12,000 ft
(2,438 - 3,658 meters)
  • Decreased SaO2 with marked impairment during exercise and sleep
Very High Altitude 12,000-18,000 ft
(3,658 - 5,487 meters)
  • Abrupt ascent can be dangerous; acclimatization is required to prevent illness
Extreme Altitude >18,000 ft
(>5,500 meters)
  • Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia
  • Sustained human habitation is impossible
  • RV strain, intestinal malabsorption, impaired renal function, polycythemia

Height of Mount Everest (tallest in world): 29,035 feet (8,850 meters)

Height of Mount Whitney (tallest in contiguous US): 14,505 feet (4,421 meters)

Conversion: 1 meter = ~3.28 feet (calculator)

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 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 without 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

High altitude management algorithm.
  • 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 1,000 additional ft/day
  • All respond to O2/descent

Expected SpO2 and PaO2 levels at altitude[1]

Altitude SpO2 PaO2 (mm Hg)
1,500 to 3,500 m (4,900 to 11,500 ft) about 90% 55-75
3,500 to 5,500 m (11,500 to 18,000 ft) 75-85% 40-60
5,500 to 8,850 m (18,000 to 29,000 ft) 58-75% 28-40

See Also


  1. Gallagher, MD, Scott A.; Hackett, MD, Peter (August 28, 2018). "High altitude pulmonary edema". UpToDate. Retrieved May 2, 2019.