High Altitude Related-Conditions

Aka: High Altitude Related-Conditions, Atmospheric Pressure at High Altitude, Inspired Oxygen Pressure at High Altitude, Fraction of Inspired Oxygen at High Altitude, FIO2 and PiO2 at Altitude

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II. Pathophysiology

  1. See High Altitude Related-Conditions
  2. Ambient air pressure (atmospheric pressure) decreases with increasing altitude, allowing for an increase in Gas Volume
    1. Allows trapped air (e.g. Pneumothorax) to expand
    2. A helicopter transport with 5000 foot rise from sea level
      1. Results in a pressure drop (14.7 psi to 12.7 psi)
      2. Results in a Gas Volume increase of 15-20%
        1. Boyles Law states P1*V1=P2*V2
  3. Oxygen Partial Pressure decreases with altitude related drop in total Ambient air pressure (atmospheric pressure)
    1. Oxygen maintains its percentage (21%) of overall gas constituents in atmospheric pressure (78% Nitrogen, 1% Argon)
    2. However, Oxygen Partial Pressure drops in relation to total atmospheric pressure decrease
      1. DRY Sea level Partial Pressure: 160 mmHg (21% of total atmospheric pressure, 760 mmHg)
      2. DRY Partial Pressure at 5000 feet: 132 mmHg (21% of total atmospheric pressure, 630 mmHg)
    3. Partial Pressure is lowered by the humidified, heated air we inspire through our upper airways
      1. Saturated vapor pressure at Body Temperature (98.6 F or 37 C) is 47 mmHg
      2. Alveolar Partial Pressure of oxygen at sea level = 0.21 * (760 - 47 mmHg) = 0.21 * 713 = 150 mmHg
    4. Decreased oxygen Partial Pressure results in fewer oxygen molecules available on inspiration of a given Lung Volume
      1. Hypoxia risk increases

III. Background: Atmospheric Pressure (Patm), Inspired Oxygen Pressure (PiO2), and Fraction of Inspired Oxygen (FIO2)

  1. Caveats
    1. Fraction of inspired oxygen (FIO2) does not change in relation to other air gas constituents on earth
      1. Earths atmosphere is composed primarily of Nitrogen (78%) and Oxygen (21%)
      2. Regardless of altitude, the fraction of inspired oxygen remains constant at 21%
    2. However, oxygen Partial Pressure does decrease with altitude and the associated drop in atmospheric pressure
      1. Oxygen Partial Pressure 160 mmHg at sea level and 132 mmHg at 5000 feet (see above)
      2. Drop in Partial Pressure is relative to the drop in atmospheric pressure (760 to 630 mmHg)
        1. Despite the Partial Pressure, the Ambient air FIO2 remains constant at 21%
    3. FIO2 is re-calculated for the purposes of this chart to relay the relative impact on oxygenation with altitude
      1. Reflects the inverse of providing supplemental oxgen (removing available oxygen with altitude gain)
  2. 0 m (sea level) Patm: 760 mmHg, PiO2: 150 mmHg, FIO2: 21%
    1. PiO2 = (760 mmHg - 47 mmHg) * 0.21 = 150 mmHg
    2. Where Atmospheric Pressure at sea level = 760 mmHg
    3. Where Fully saturated Water Vapor Pressure = 47 mmHg
    4. Where FiO2 (fraction of inspired air that is Oxygen) = 0.21
      1. FIO2 is constant in earth's atmosphere (regardless of altitude)
      2. Most of remaining inspired air is Nitrogen (0.78)
  3. 1500 m (4920 ft) Patm: 641 mmHg, PiO2: 124 mmHg (as if the FIO2 dropped to 17% at sea level)
    1. Albuquerque, New Mexico (1500 m, 4920 ft)
    2. Low altitude is defined as <1500 meters, a level at which Acute Mountain Sickness (AMS) does not occur
    3. High altitude (1500 to 3500 meters) definition starts here, and is the level at which most AMS occurs
      1. High Incidence of rapid ascent to this altitude range
      2. AMS risk at at this high altitude range: 25%
  4. 2000 m (6562 ft) Patm: 604 mmHg, PiO2: 116 mmHg (as if the FIO2 dropped to 16% at sea level)
    1. Sante Fe, New Mexico (2194 m or 7200 ft)
  5. 2500 m (8200 ft) Patm 570 mmHg, PiO2: 110 mmHg (as if the FIO2 dropped to 15% at sea level)
    1. Risk of Altitude Sickness increases above 2500 meters
    2. Bogota, Columbia (2620 m or 8596 ft)
  6. 3000 m (9843 ft) Patm 537 mmHg, PiO2: 102 mmHg (as if the FIO2 dropped to 14% at sea level)
    1. Ski altitude (3048 m or 10,000 ft)
    2. Cusco, Peru (3,399 m or 11,152 ft)
  7. 3500 m (11482 ft) Patm 505 mmHg, PiO2: 96 mmHg
    1. La Paz, Bolivia (3,640 m or 11,942 ft)
    2. Very high altitude (3500 to 5500 m) definition starts here
      1. Blood Oxygen Saturation is often <90%, even in healthy persons at very high altitude
      2. This very high range is also associated with severe AMS, HACE and HAPE
  8. 4000 m (13123 ft) Patm 475 mmHg, PiO2: 90 mmHg (as if the FIO2 dropped to 13% at sea level)
    1. Longs Peak in Colorado, U.S. (4345 m or 14,255 ft)
  9. 4500 m (14764 ft) Patm 447 mmHg, PiO2: 84 mmHg (as if the FIO2 dropped to 12% at sea level)
    1. Mont Blanc in Alps, France\Italy (4810 m or 15,781 ft)
  10. 5000 m (16464 ft) Patm 420 mmHg, PiO2: 78 mmHg (as if the FIO2 dropped to 11% at sea level)
  11. 5500 m (18045 ft) Patm 394 mmHg, PiO2: 73 mmHg (as if the FIO2 dropped to 10% at sea level)
    1. Mt. Kilimanjaro in Tanzania (5895 m or 19341 ft)
    2. Extreme altitude (>5500 meters) definition starts here
      1. Permanent human habitation is impossible at this altitude (unable to acclimate)
  12. 6000 m (19685 ft) Patm 369 mmHg, PiO2: 68 mmHg
    1. Mt. McKinley in Alaska, U.S. (6190 m or 20,308 ft)
    2. AMS risk at altitude >6000 meters: 50%
  13. 6500 m (21325 ft) Patm 346 mmHg, PiO2: 63 mmHg (as if the FIO2 dropped to 9% at sea level)
  14. 7000 m (23000 ft) Patm 324 mmHg, PiO2: 58 mmHg (as if the FIO2 dropped to 8% at sea level)
  15. 7500 m (24606 ft) Patm 303 mmHg, PiO2: 54 mmHg
  16. 8000 m (26247 ft) Patm 284 mmHg, PiO2: 50 mmHg (as if the FIO2 dropped to 7% at sea level)
    1. Annapurna 1 Main in Himalayas (8091 m or 26,545 ft)
  17. 8500 m (27887 ft) Patm 265 mmHg, PiO2: 46 mmHg (as if the FIO2 dropped to 6% at sea level)
    1. Mount Everest (8849 m or 29,035 feet)
  18. 9000 m (29528 ft) Patm 248 mmHg, PiO2: 42 mmHg
  19. References
    1. Roach, Lawley and Hackett in Auerback, Cushing and Harris (2016) Auerbach's Wilderness Medicine, p. 4

IV. Associated Conditions: General

  1. Acute Mountain Sickness
  2. Altered Sleep at high altitude
    1. Fragmented Sleep Stages
    2. Frequent awakenings
    3. Sleep Apnea
    4. Periodic breathing occurs normally above 3000 meters
  3. Physical wasting above 18,000 feet
    1. Weight loss
    2. Increased lethargy
    3. Muscle Weakness
    4. Polycythemia
    5. Acclimatization above 18,000 feet offers no benefit
  4. Peripheral Edema
    1. Self limited with spontaneous resolution
    2. Consider Diuretics if needed
  5. Thrombosis (higher Incidence at altitude)
  6. Immune Suppression
    1. Infections common and difficult to treat at altitude
    2. T Lymphocyte depression
    3. Descend for refractory infectious symptoms and signs

V. Associated Conditions: Neurologic

  1. High Altitude Cerebral Edema

VI. Associated Conditions: Respiratory

  1. High Altitude Pulmonary Edema
  2. High altitude Pharyngitis or Bronchitis
    1. Dry cough results from respiratory irritation at altitude (multi-factorial)
    2. Reduced with throat lozenges, oxygen Face Mask breathing
    3. Cough may be severe enough to cause Rib Fractures (Cough fracture)

VII. Associated Conditions: Ocular

  1. Ultraviolet Keratitis
  2. High altitude Retinopathy
    1. Retinal Hemorrhage at high altitude
    2. Resolves spontaneously in 1-2 weeks at sea level
    3. Predictive of more severe High Altitude Sickness including HACE and HAPE
  3. Vision change (refractive shift)
    1. Occurs in those with Radial Keratotomy history

VIII. References

  1. Comp and Rogich (2021) Crit Dec Emerg Med 35(4): 3-8
  2. Comp and Rogich (2026) Crit Dec Emerg Med 40(9): 29-35

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