Decompression (altitude)

Altitude decompression or hypobaric decompression is the reduction in ambient pressure below the normal range of sea level atmospheric pressure. Altitude decompression is the natural consequence of unprotected elevation to altitude, while hypobaric decompression is due to intentional or unintentional release of pressurisation of a pressure suit or pressurised compartment, vehicle or habitat, and may be controlled or uncontrolled, or the reduction of pressure in a hypobaric chamber.

Altitude decompression may occur as a decompression from saturation at a lower altitude, or as decompression from an excursion to a lower altitude, in the case of people living at high altitude, making a short duration trip to low altitude, and returning, or a person decompressing from a dive at altitude, which is a special case of diving decompression.[1]

Decompression has physical effects on gas filled spaces and on liquids, particularly when they contain dissolved gases. Physiological effects of decompression are due to these physical effects and the consequential effects on the living tissues, mostly as a result of the formation and growth of bubbles, and the expansion of gas filled spaces. Formation and growth of bubbles due to reduced pressure can be due to reduction in solubility of dissolved gases as described by Henry's Law, with nucleation and growth of bubbles in supersaturated liquids, or due to boiling of liquids when the pressure is reduced below the vapour pressure for the temperature of the liquid.

Physiological effects
Main article: Physiology of decompression

There are three principal physiological effects arising from decompression at altitude:

  • Decompression illness, which includes
    • decompression sickness due to bubble formation in the tissues similar to those caused by decompression after exposure to pressures higher than sea level atmospheric pressure. There is little evidence of altitude decompression sickness occurring among healthy individuals at altitudes below 18,000 feet (5,500 m),[2] but it can occur at lower altitudes in underwater divers with sufficient residual inert gas tissue loading after recent diver.[1]
    • Barotrauma caused by the over-expansion of gas-filled spaces.
  • Altitude sickness, also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, or soroche, is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen and respiratory alkalosis arising from low partial pressure of blood carbon dioxide caused by hyperventilation.[3]

Decompression sickness
Main article: Decompression sickness

Abrupt excursions from sea level to altitudes above 15,000 feet (4,600 m) without oxygen prebreathing may induce venous gas bubbles, with a 5% probability of symptoms developing at about 21,200 feet (6,500 m), at which altitude there is over 50% probability of venous bubbles. By 22,500 feet (6,900 m) the incidence of venous bubbles exceeds 70%, with a 55% incidence of DCS.[4] These effects may be prevented or delayed by more gradual decompression or by flushing some of the nitrogen from the tissues before decompression by prebreathing a high percentage of oxygen before and during decompression.[5] Altitude decompression sickness often resolves on return to the saturation altitude, but sometimes treatment on elevated concentrations of oxygen is indicated, usually 100% at surface pressre. In more severe cases hyperbaric oxygen treatment may be indicated.[6]

See also

References
  1. US Navy (2008). "9". US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. p. 61. Retrieved 2008-06-15.
  2. "Altitude-induced Decompression Sickness" (PDF). Federal Aviation Administration. Retrieved 2012-02-21.
  3. Brown JP, Grocott MP (2013-02-01). "Humans at altitude: Physiology and Pathophysiology". Continuing Education in Anaesthesia, Critical Care & Pain. 13 (1): 17–22. doi:10.1093/bjaceaccp/mks047. ISSN 1743-1816.
  4. Webb, JT; Pilmanis, AA; O'Connor, RB (April 1998). "An abrupt zero-preoxygenation altitude threshold for decompression sickness symptoms". Aviat Space Environ Med. 69 (4): 335–40. PMID 9561279.
  5. "Prebreathing". Oxford Reference. Retrieved 17 December 2021.
  6. de la Cruz, Richard A.; Clemente Fuentes, Roselyn W.; Wonnum, Sundonia J.; Cooper, Jeffrey S. (27 June 2022). "Aerospace Decompression Illness". National Library of Medicine. Retrieved 2 October 2022.

 This article incorporates public domain material from Altitude-induced Decompression Sickness (PDF). Federal Aviation Administration.

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