Hypobaric decompression

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Hypobaric decompression is the reduction in ambient pressure below the normal range of sea level atmospheric pressure. Altitude decompression is hypobaric decompression which is the natural consequence of unprotected elevation to altitude, while other forms of hypobaric decompression are due to intentional or unintentional release of pressurization of a pressure suit or pressurized compartment, vehicle or habitat, and may be controlled or uncontrolled, or the reduction of pressure in a hypobaric chamber.[ citation needed ]

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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, the expansion of gas filled spaces, and adverse reactions in the injured tissues. 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.[ citation needed ]

Both rate of decompression and pressure difference affect the type of injury likely and the severity of the consequences. Barotrauma is more likely to occur for rapid decompression, while decompression sickness is more likely with a large pressure drop, but both can occur simultaneously. Hypoxia risk depends mainly on the oxygen partial pressure after decompression.[ citation needed ]

Physiological effects

There are three principal physiological effects arising from decompression at altitude: 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, barotrauma caused by the over-expansion of gas-filled spaces, and altitude sickness, a manifestation of hypoxia due to the naturally low partial pressure of oxygen in the air at altitude. At higher altitudes, more severe, and potentially fatal hypoxia will occur. Decompression sickness and barotrauma are considered aspects of decompression illness. [2]

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. [3] 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. [4]

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 pressure. In more severe cases hyperbaric oxygen treatment may be indicated. [5] There is little evidence of altitude decompression sickness occurring among healthy individuals at altitudes below 18,000 feet (5,500 m), [6] but it can occur at lower altitudes in underwater divers with sufficient residual inert gas tissue loading after recent diver. [1]

Barotrauma

Barotrauma is physical damage to body tissues caused by a difference in pressure between a gas space inside, or in contact with, the body, and the surrounding gas or liquid. [7] [8] The initial damage is usually due to over-stretching the tissues in tension or shear, either directly by expansion of the gas in the closed space or by pressure difference hydrostatically transmitted through the tissue. Tissue rupture may be complicated by the introduction of gas into the local tissue or circulation through the initial trauma site, which can cause blockage of circulation at distant sites or interfere with normal function of an organ by its presence.[ citation needed ]

Decompression may be intentional or uncontrolled. Intentional decompression includes controlled unpressurised ascent to altitude. Uncontrolled decompression is an unplanned drop in the pressure of a sealed system, such as an aircraft cabin or hyperbaric chamber, and typically results from human error, material fatigue, engineering failure, or impact, causing a pressure vessel to vent into its lower-pressure surroundings or fail to pressurize at all. [9] [10]

Such decompression may be classed as explosive, rapid, or slow: [10]

Altitude sickness

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. [11] Altitude sickness is primarily a consequence of hypoxia. Altitude sickness can be avoided and treated by breathing supplementary oxygen, within limits.

Above the Armstrong limit, the atmospheric pressure is sufficiently low that exposed water boils at normal human body temperature. At altitudes above about 50,000 feet (15 km), the time of useful consciousness is 9 to 12 seconds. Loss of consciousness is due to hypoxia and is followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored in 60–90 seconds. [2] On Earth, the Armstrong limit is around 18–19 km (11–12 mi; 59,000–62,000 ft) above sea level, [2] [12] above which atmospheric air pressure drops below 0.0618 atm (6.3 kPa, 47 mmHg, or about 1 psi). The U.S. Standard Atmospheric model sets the Armstrong pressure at an altitude of 63,000 feet (19,202 m).

See also

Related Research Articles

<span class="mw-page-title-main">Decompression sickness</span> Disorder caused by dissolved gases forming bubbles in tissues

Decompression sickness is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompression. DCS most commonly occurs during or soon after a decompression ascent from underwater diving, but can also result from other causes of depressurisation, such as emerging from a caisson, decompression from saturation, flying in an unpressurised aircraft at high altitude, and extravehicular activity from spacecraft. DCS and arterial gas embolism are collectively referred to as decompression illness.

<span class="mw-page-title-main">Air embolism</span> Vascular blockage by air bubbles

An air embolism, also known as a gas embolism, is a blood vessel blockage caused by one or more bubbles of air or other gas in the circulatory system. Air can be introduced into the circulation during surgical procedures, lung over-expansion injury, decompression, and a few other causes. In flora, air embolisms may also occur in the xylem of vascular plants, especially when suffering from water stress.

<span class="mw-page-title-main">Barotrauma</span> Injury caused by external fluid pressure

Barotrauma is physical damage to body tissues caused by a difference in pressure between a gas space inside, or in contact with, the body and the surrounding gas or liquid. The initial damage is usually due to over-stretching the tissues in tension or shear, either directly by an expansion of the gas in the closed space or by pressure difference hydrostatically transmitted through the tissue. Tissue rupture may be complicated by the introduction of gas into the local tissue or circulation through the initial trauma site, which can cause blockage of circulation at distant sites or interfere with the normal function of an organ by its presence. The term is usually applied when the gas volume involved already exists prior to decompression. Barotrauma can occur during both compression and decompression events.

Decompression Illness (DCI) comprises two different conditions caused by rapid decompression of the body. These conditions present similar symptoms and require the same initial first aid. Scuba divers are trained to ascend slowly from depth to avoid DCI. Although the incidence is relatively rare, the consequences can be serious and potentially fatal, especially if untreated.

Dysbarism refers to medical conditions resulting from changes in ambient pressure. Various activities are associated with pressure changes. Underwater diving is the most frequently cited example, but pressure changes also affect people who work in other pressurized environments, and people who move between different altitudes.

<span class="mw-page-title-main">Diving medicine</span> Diagnosis, treatment and prevention of disorders caused by underwater diving

Diving medicine, also called undersea and hyperbaric medicine (UHB), is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how aspects of a diver's fitness to dive affect the diver's safety. Diving medical practitioners are also expected to be competent in the examination of divers and potential divers to determine fitness to dive.

Diving disorders, or diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.

<span class="mw-page-title-main">Hypobaric chamber</span> Chamber for simulating high altitude

A hypobaric chamber, or altitude chamber, is a chamber used during aerospace or high terrestrial altitude research or training to simulate the effects of high altitude on the human body, especially hypoxia and hypobaria. Some chambers also control for temperature and relative humidity.

Ebullism is the formation of water vapour bubbles in bodily fluids due to reduced environmental pressure, usually at extreme high altitude. It occurs because a system of liquid and gas at equilibrium will see a net conversion of liquid to gas as pressure lowers; for example, liquids reach their boiling points at lower temperatures when the pressure on them is lowered. The injuries and disorder caused by ebullism is also known as ebullism syndrome. Ebullism will expand the volume of the tissues, but the vapour pressure of water at temperatures in which a human can survive is not sufficient to rupture skin or most other tissues encased in skin. Ebullism produces predictable injuries, which may be survivable if treated soon enough, and is often accompanied by complications caused by rapid decompression, such as decompression sickness and a variety of barotrauma injuries. Persons at risk are astronauts and high altitude aviators, for whom it is an occupational hazard.

<span class="mw-page-title-main">Armstrong limit</span> Altitude where water boils at body temperature

The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. On Earth, the limit is around 18–19 km above sea level, above which atmospheric air pressure drops below 0.0618 atm. The U.S. Standard Atmospheric model sets the Armstrong limit at an altitude of 63,000 feet (19,202 m).

<span class="mw-page-title-main">Decompression (diving)</span> Pressure reduction and its effects during ascent from depth

The decompression of a diver is the reduction in ambient pressure experienced during ascent from depth. It is also the process of elimination of dissolved inert gases from the diver's body which accumulate during ascent, largely during pauses in the ascent known as decompression stops, and after surfacing, until the gas concentrations reach equilibrium. Divers breathing gas at ambient pressure need to ascend at a rate determined by their exposure to pressure and the breathing gas in use. A diver who only breathes gas at atmospheric pressure when free-diving or snorkelling will not usually need to decompress. Divers using an atmospheric diving suit do not need to decompress as they are never exposed to high ambient pressure.

<span class="mw-page-title-main">Decompression practice</span> Techniques and procedures for safe decompression of divers

To prevent or minimize decompression sickness, divers must properly plan and monitor decompression. Divers follow a decompression model to safely allow the release of excess inert gases dissolved in their body tissues, which accumulated as a result of breathing at ambient pressures greater than surface atmospheric pressure. Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent.

<span class="mw-page-title-main">History of decompression research and development</span> Chronological list of notable events in the history of diving decompression.

Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure.

<span class="mw-page-title-main">Decompression theory</span> Theoretical modelling of decompression physiology

Decompression theory is the study and modelling of the transfer of the inert gas component of breathing gases from the gas in the lungs to the tissues and back during exposure to variations in ambient pressure. In the case of underwater diving and compressed air work, this mostly involves ambient pressures greater than the local surface pressure, but astronauts, high altitude mountaineers, and travellers in aircraft which are not pressurised to sea level pressure, are generally exposed to ambient pressures less than standard sea level atmospheric pressure. In all cases, the symptoms caused by decompression occur during or within a relatively short period of hours, or occasionally days, after a significant pressure reduction.

<span class="mw-page-title-main">Haldane's decompression model</span> Decompression model developed by John Scott Haldane

Haldane's decompression model is a mathematical model for decompression to sea level atmospheric pressure of divers breathing compressed air at ambient pressure that was proposed in 1908 by the Scottish physiologist, John Scott Haldane, who was also famous for intrepid self-experimentation.

<span class="mw-page-title-main">Physiology of decompression</span> The physiological basis for decompression theory and practice

The physiology of decompression is the aspect of physiology which is affected by exposure to large changes in ambient pressure. It involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues. Gas is breathed at ambient pressure, and some of this gas dissolves into the blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the tissues is in a state of equilibrium with the gas in the lungs, or the ambient pressure is reduced until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

Human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.

Inner ear decompression sickness, (IEDCS) or audiovestibular decompression sickness is a medical condition of the inner ear caused by the formation of gas bubbles in the tissues or blood vessels of the inner ear. Generally referred to as a form of decompression sickness, it can also occur at constant pressure due to inert gas counterdiffusion effects.

Middle ear barotrauma (MEBT), also known to underwater divers as ear squeeze and reverse ear squeeze, is an injury caused by a difference in pressure between the external ear canal and the middle ear. It is common in underwater divers and usually occurs when the diver does not equalise sufficiently during descent or, less commonly, on ascent. Failure to equalise may be due to inexperience or eustachian tube dysfunction, which can have many possible causes. Unequalised ambient pressure increase during descent causes a pressure imbalance between the middle ear air space and the external auditory canal over the eardrum, referred to by divers as ear squeeze, causing inward stretching, serous effusion and haemorrhage, and eventual rupture. During ascent internal over-pressure is normally passively released through the eustachian tube, but if this does not happen the volume expansion of middle ear gas will cause outward bulging, stretching and eventual rupture of the eardrum known to divers as reverse ear squeeze. This damage causes local pain and hearing loss. Tympanic rupture during a dive can allow water into the middle ear, which can cause severe vertigo from caloric stimulation. This may cause nausea and vomiting underwater, which has a high risk of aspiration of vomit or water, with possibly fatal consequences.

References

  1. 1 2 US Navy (2008). "9". US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. p. 61. Archived from the original on 2008-05-02. Retrieved 2008-06-15.
  2. 1 2 3 Geoffrey A. Landis. "Human Exposure to Vacuum". Archived from the original on 2009-07-21. Retrieved 2016-02-05.
  3. ↑ 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.
  4. ↑ "Prebreathing". Oxford Reference. Retrieved 17 December 2021.
  5. ↑ 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. PMID   28846248 . Retrieved 2 October 2022.
  6. ↑ "Altitude-induced Decompression Sickness" (PDF). Federal Aviation Administration . Retrieved 2012-02-21.
  7. ↑ US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. 2006. Archived from the original on 2008-05-02. Retrieved 2021-12-17.
  8. ↑ Brubakk, A.O.; Neuman, T.S. (2003). Bennett and Elliott's physiology and medicine of diving (5th Rev. ed.). United States: Saunders Ltd. p. 800. ISBN   978-0-7020-2571-6.
  9. 1 2 3 "AC 61-107A – Operations of aircraft at altitudes above 25,000 feet msl and/or mach numbers (MMO) greater than .75" (PDF). Federal Aviation Administration. 2007-07-15. Retrieved 2008-07-29.
  10. 1 2 Dehart, R. L.; J. R. Davis (2002). Fundamentals Of Aerospace Medicine: Translating Research Into Clinical Applications, 3rd Rev Ed. United States: Lippincott Williams And Wilkins. p. 720. ISBN   978-0-7817-2898-0.
  11. ↑ 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.
  12. ↑ "NASAexplores Glossary". Archived from the original on 2007-09-27.

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