Decompression sickness

Decompression sickness (DCS; also known as divers' disease, the bends, aerobullosis, or caisson disease) describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurisation. DCS most commonly refers to problems arising from underwater diving decompression (i.e., during ascent), but may be experienced in other depressurisation events such as emerging from a caisson, flying in an unpressurised aircraft at altitude, and extravehicular activity from spacecraft. DCS and arterial gas embolism are collectively referred to as decompression illness.

Since bubbles can form in or migrate to any part of the body, DCS can produce many symptoms, and its effects may vary from joint pain and rashes to paralysis and death. Individual susceptibility can vary from day to day, and different individuals under the same conditions may be affected differently or not at all. The classification of types of DCS by its symptoms has evolved since its original description over a hundred years ago.

Risk of DCS caused by diving can be managed through proper decompression procedures and contracting it is now uncommon. Its potential severity has driven much research to prevent it and divers almost universally use dive tables or dive computers to limit their exposure and to control their ascent speed. If DCS is suspected, it is treated by hyperbaric oxygen therapy in a recompression chamber. If treated early, there is a significantly higher chance of successful recovery.

Decompression sickness
Other namesDivers' disease, the bends, aerobullosis, caisson disease
Photograph of the cramped interior of a cylinder containing two benches
Two United States Navy sailors prepare for training inside a decompression chamber.
SpecialtyEmergency medicine


DCS is classified by symptoms. The earliest descriptions of DCS used the terms: "bends" for joint or skeletal pain; "chokes" for breathing problems; and "staggers" for neurological problems.[1] In 1960, Golding et al. introduced a simpler classification using the term "Type I ('simple')" for symptoms involving only the skin, musculoskeletal system, or lymphatic system, and "Type II ('serious')" for symptoms where other organs (such as the central nervous system) are involved.[1] Type II DCS is considered more serious and usually has worse outcomes.[2] This system, with minor modifications, may still be used today.[3] Following changes to treatment methods, this classification is now much less useful in diagnosis,[4] since neurological symptoms may develop after the initial presentation, and both Type I and Type II DCS have the same initial management.[5]

Decompression illness and dysbarism

The term dysbarism encompasses decompression sickness, arterial gas embolism, and barotrauma, whereas decompression sickness and arterial gas embolism are commonly classified together as decompression illness when a precise diagnosis cannot be made.[6] DCS and arterial gas embolism are treated very similarly because they are both the result of gas bubbles in the body.[5] The U.S. Navy prescribes identical treatment for Type II DCS and arterial gas embolism.[7] Their spectra of symptoms also overlap, although the symptoms from arterial gas embolism are generally more severe because they often arise from an infarction (blockage of blood supply and tissue death).

Signs and symptoms

While bubbles can form anywhere in the body, DCS is most frequently observed in the shoulders, elbows, knees, and ankles. Joint pain ("the bends") accounts for about 60% to 70% of all altitude DCS cases, with the shoulder being the most common site. Neurological symptoms are present in 10% to 15% of DCS cases with headache and visual disturbances being the most common symptom. Skin manifestations are present in about 10% to 15% of cases. Pulmonary DCS ("the chokes") is very rare in divers and has been observed much less frequently in aviators since the introduction of oxygen pre-breathing protocols.[8] The table below shows symptoms for different DCS types.[9]

Signs and symptoms of decompression sickness
DCS type Bubble location Signs & symptoms (clinical manifestations)
Musculoskeletal Mostly large joints

(elbows, shoulders, hip, wrists, knees, ankles)

  • Localized deep pain, ranging from mild to excruciating. Sometimes a dull ache, but rarely a sharp pain.
  • Active and passive motion of the joint aggravates the pain.
  • The pain may be reduced by bending the joint to find a more comfortable position.
  • If caused by altitude, pain can occur immediately or up to many hours later.
Cutaneous Skin
  • Itching, usually around the ears, face, neck, arms, and upper torso
  • Sensation of tiny insects crawling over the skin (formication)
  • Mottled or marbled skin usually around the shoulders, upper chest and abdomen, with itching
  • Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema)
Neurologic Brain
  • Altered sensation, tingling or numbness (paresthesia), increased sensitivity (hyperesthesia)
  • Confusion or memory loss (amnesia)
  • Visual abnormalities
  • Unexplained mood or behaviour changes
  • Seizures, unconsciousness
Neurologic Spinal cord
Constitutional Whole body
  • Headache
  • Unexplained fatigue
  • Generalised malaise, poorly localised aches
Audiovestibular Inner ear [10][a]
Pulmonary Lungs


The relative frequencies of different symptoms of DCS observed by the U.S. Navy are as follows:[11]

Symptoms by frequency
Symptoms Frequency
local joint pain 89%
arm symptoms 70%
leg symptoms 30%
dizziness 5.3%
paralysis 2.3%
shortness of breath 1.6%
extreme fatigue 1.3%
collapse/unconsciousness 0.5%


Although onset of DCS can occur rapidly after a dive, in more than half of all cases symptoms do not begin to appear for at least an hour. In extreme cases, symptoms may occur before the dive has been completed. The U.S. Navy and Technical Diving International, a leading technical diver training organization, have published a table that documents time to onset of first symptoms. The table does not differentiate between types of DCS, or types of symptom.[12][13]

Onset of DCS symptoms
Time to onset Percentage of cases
within 1 hour 42%
within 3 hours 60%
within 8 hours 83%
within 24 hours 98%
within 48 hours 100%


DCS is caused by a reduction in ambient pressure that results in the formation of bubbles of inert gases within tissues of the body. It may happen when leaving a high-pressure environment, ascending from depth, or ascending to altitude.

Ascent from depth

DCS is best known as a diving disorder that affects divers having breathed gas that is at a higher pressure than the surface pressure, owing to the pressure of the surrounding water. The risk of DCS increases when diving for extended periods or at greater depth, without ascending gradually and making the decompression stops needed to slowly reduce the excess pressure of inert gases dissolved in the body. The specific risk factors are not well understood and some divers may be more susceptible than others under identical conditions.[14][15] DCS has been confirmed in rare cases of breath-holding divers who have made a sequence of many deep dives with short surface intervals; and it may be the cause of the disease called taravana by South Pacific island natives who for centuries have dived by breath-holding for food and pearls.[16]

Two principal factors control the risk of a diver suffering DCS:

  1. the rate and duration of gas absorption under pressure – the deeper or longer the dive the more gas is absorbed into body tissue in higher concentrations than normal (Henry's Law);
  2. the rate and duration of outgassing on depressurization – the faster the ascent and the shorter the interval between dives the less time there is for absorbed gas to be offloaded safely through the lungs, causing these gases to come out of solution and form "micro bubbles" in the blood.[17]

Even when the change in pressure causes no immediate symptoms, rapid pressure change can cause permanent bone injury called dysbaric osteonecrosis (DON). DON can develop from a single exposure to rapid decompression.[18]

Leaving a high-pressure environment

Caisson Schematic
The principal features of a caisson are the workspace, pressurised by an external air supply, and the access tube with an airlock

When workers leave a pressurized caisson or a mine that has been pressurized to keep water out, they will experience a significant reduction in ambient pressure.[14][19] A similar pressure reduction occurs when astronauts exit a space vehicle to perform a space-walk or extra-vehicular activity, where the pressure in their spacesuit is lower than the pressure in the vehicle.[14][20][21][22]

The original name for DCS was "caisson disease". This term was introduced in the 19th century, when caissons under pressure were used to keep water from flooding large engineering excavations below the water table, such as bridge supports and tunnels. Workers spending time in high ambient pressure conditions are at risk when they return to the lower pressure outside the caisson if the pressure is not reduced slowly. DCS was a major factor during construction of Eads Bridge, when 15 workers died from what was then a mysterious illness, and later during construction of the Brooklyn Bridge, where it incapacitated the project leader Washington Roebling.[23] On the other side of the Manhattan island during construction of the Hudson River Tunnel contractor's agent Ernest William Moir noted in 1889 that workers were dying due to decompression sickness and pioneered the use of an airlock chamber for treatment.[24]

Ascent to altitude

The most common health risk on ascent to altitude is not decompression sickness but altitude sickness, or acute mountain sickness (AMS), which has an entirely different and unrelated set of causes and symptoms. AMS results not from the formation of bubbles from dissolved gasses in the body but from exposure to a low partial pressure of oxygen and alkalosis. However, passengers in unpressurized aircraft at high altitude may also be at some risk of DCS.[14][20][21][25]

Altitude DCS became a problem in the 1930s with the development of high-altitude balloon and aircraft flights but not as great a problem as AMS, which drove the development of pressurized cabins, which coincidentally controlled DCS. Commercial aircraft are now required to maintain the cabin at or below a pressure altitude of 2,400 m (7,900 ft) even when flying above 12,000 m (39,000 ft). Symptoms of DCS in healthy individuals are subsequently very rare unless there is a loss of pressurization or the individual has been diving recently.[26][27] Divers who drive up a mountain or fly shortly after diving are at particular risk even in a pressurized aircraft because the regulatory cabin altitude of 2,400 m (7,900 ft) represents only 73% of sea level pressure.[14][20][28]

Generally, the higher the altitude the greater the risk of altitude DCS but there is no specific, maximum, safe altitude below which it never occurs. There are very few symptoms at or below 5,500 m (18,000 ft) unless patients had predisposing medical conditions or had dived recently. There is a correlation between increased altitudes above 5,500 m (18,000 ft) and the frequency of altitude DCS but there is no direct relationship with the severity of the various types of DCS. A US Air Force study reports that there are few occurrences between 5,500 m (18,000 ft) and 7,500 m (24,600 ft) and 87% of incidents occurred at or above 7,500 m (24,600 ft). [29] High altitude parachutists may reduce the risk of altitude DCS if they flush nitrogen from the body by pre-breathing pure oxygen.[30]

Predisposing factors

Although the occurrence of DCS is not easily predictable, many predisposing factors are known. They may be considered as either environmental or individual. Decompression sickness and arterial gas embolism in recreational diving are associated with certain demographic, environmental, and dive style factors. A statistical study published in 2005 tested potential risk factors: age, gender, body mass index, smoking, asthma, diabetes, cardiovascular disease, previous decompression illness, years since certification, dives in the last year, number of diving days, number of dives in a repetitive series, last dive depth, nitrox use, and drysuit use. No significant associations with risk of decompression sickness or arterial gas embolism were found for asthma, diabetes, cardiovascular disease, smoking, or body mass index. Increased depth, previous DCI, larger number of consecutive days diving, and being male were associated with higher risk for decompression sickness and arterial gas embolism. Nitrox and drysuit use, greater frequency of diving in the past year, increasing age, and years since certification were associated with lower risk, possibly as indicators of more extensive training and experience.[31]


The following environmental factors have been shown to increase the risk of DCS:

  • the magnitude of the pressure reduction ratio – a large pressure reduction ratio is more likely to cause DCS than a small one.[20][28][32]
  • repetitive exposures – repetitive dives within a short period of time (a few hours) increase the risk of developing DCS. Repetitive ascents to altitudes above 5,500 metres (18,000 ft) within similar short periods increase the risk of developing altitude DCS.[20][32]
  • the rate of ascent – the faster the ascent the greater the risk of developing DCS. The US Navy Dive Manual indicates that ascent rates greater than about 20 m/min (66 ft/min) when diving increase the chance of DCS, while recreational dive tables such as the Bühlmann tables require an ascent rate of 10 m/min (33 ft/min) with the last 6 m (20 ft) taking at least one minute.[33] An individual exposed to a rapid decompression (high rate of ascent) above 5,500 metres (18,000 ft) has a greater risk of altitude DCS than being exposed to the same altitude but at a lower rate of ascent.[20][32]
  • the duration of exposure – the longer the duration of the dive, the greater is the risk of DCS. Longer flights, especially to altitudes of 5,500 m (18,000 ft) and above, carry a greater risk of altitude DCS.[20]
  • underwater diving before flying – divers who ascend to altitude soon after a dive increase their risk of developing DCS even if the dive itself was within the dive table safe limits. Dive tables make provisions for post-dive time at surface level before flying to allow any residual excess nitrogen to outgas. However, the pressure maintained inside even a pressurized aircraft may be as low as the pressure equivalent to an altitude of 2,400 m (7,900 ft) above sea level. Therefore, the assumption that the dive table surface interval occurs at normal atmospheric pressure is invalidated by flying during that surface interval, and an otherwise-safe dive may then exceed the dive table limits.[34][35][36]
  • diving before travelling to altitude – DCS can occur without flying if the person moves to a high-altitude location on land immediately after diving, for example, scuba divers in Eritrea who drive from the coast to the Asmara plateau at 2,400 m (7,900 ft) increase their risk of DCS.[37]
  • diving at altitude – diving in water whose surface altitude is above 300 m (980 ft) — for example, Lake Titicaca is at 3,800 m (12,500 ft) — without using versions of decompression tables or dive computers that are modified for high-altitude.[34][38]


Atrial septal defect-en
Atrial septal defect (PFO) showing left-to-right shunt. A right-to-left shunt may allow bubbles to pass into the arterial circulation.

The following individual factors have been identified as possibly contributing to increased risk of DCS:

  • dehydration – Studies by Walder concluded that decompression sickness could be reduced in aviators when the serum surface tension was raised by drinking isotonic saline,[39] and the high surface tension of water is generally regarded as helpful in controlling bubble size.[32] Maintaining proper hydration is recommended.[40]
  • patent foramen ovale – a hole between the atrial chambers of the heart in the fetus is normally closed by a flap with the first breaths at birth. In about 20% of adults the flap does not completely seal, however, allowing blood through the hole when coughing or during activities that raise chest pressure. In diving, this can allow venous blood with microbubbles of inert gas to bypass the lungs, where the bubbles would otherwise be filtered out by the lung capillary system, and return directly to the arterial system (including arteries to the brain, spinal cord and heart).[41] In the arterial system, bubbles (arterial gas embolism) are far more dangerous because they block circulation and cause infarction (tissue death, due to local loss of blood flow). In the brain, infarction results in stroke, and in the spinal cord it may result in paralysis.[42]
  • a person's age – there are some reports indicating a higher risk of altitude DCS with increasing age.[14][32]
  • previous injury – there is some indication that recent joint or limb injuries may predispose individuals to developing decompression-related bubbles.[14][43]
  • ambient temperature – there is some evidence suggesting that individual exposure to very cold ambient temperatures may increase the risk of altitude DCS.[14][32] Decompression sickness risk can be reduced by increased ambient temperature during decompression following dives in cold water.[44]
  • body type – typically, a person who has a high body fat content is at greater risk of DCS.[14][32] This is due to nitrogen's five times greater solubility in fat than in water, leading to greater amounts of total body dissolved nitrogen during time at pressure. Fat represents about 15–25 percent of a healthy adult's body, but stores about half of the total amount of nitrogen (about 1 litre) at normal pressures.[45]
  • alcohol consumption – although alcohol consumption increases dehydration and therefore may increase susceptibility to DCS,[32] a 2005 study found no evidence that alcohol consumption increases the incidence of DCS.[46]


Preparing for recompression
This surfacing diver must enter a decompression chamber for surface decompression, a standard operating procedure to avoid decompression sickness after long or deep bounce dives.

Depressurisation causes inert gases, which were dissolved under higher pressure, to come out of physical solution and form gas bubbles within the body. These bubbles produce the symptoms of decompression sickness.[14][47] Bubbles may form whenever the body experiences a reduction in pressure, but not all bubbles result in DCS.[48] The amount of gas dissolved in a liquid is described by Henry's Law, which indicates that when the pressure of a gas in contact with a liquid is decreased, the amount of that gas dissolved in the liquid will also decrease proportionately.

On ascent from a dive, inert gas comes out of solution in a process called "outgassing" or "offgassing". Under normal conditions, most offgassing occurs by gas exchange in the lungs.[49][50] If inert gas comes out of solution too quickly to allow outgassing in the lungs then bubbles may form in the blood or within the solid tissues of the body. The formation of bubbles in the skin or joints results in milder symptoms, while large numbers of bubbles in the venous blood can cause lung damage.[51] The most severe types of DCS interrupt — and ultimately damage — spinal cord function, leading to paralysis, sensory dysfunction, or death. In the presence of a right-to-left shunt of the heart, such as a patent foramen ovale, venous bubbles may enter the arterial system, resulting in an arterial gas embolism.[5][52] A similar effect, known as ebullism, may occur during explosive decompression, when water vapour forms bubbles in body fluids due to a dramatic reduction in environmental pressure.[53]

Inert gases

The main inert gas in air is nitrogen, but nitrogen is not the only gas that can cause DCS. Breathing gas mixtures such as trimix and heliox include helium, which can also cause decompression sickness. Helium both enters and leaves the body faster than nitrogen, so different decompression schedules are required, but, since helium does not cause narcosis, it is preferred over nitrogen in gas mixtures for deep diving.[54] There is some debate as to the decompression requirements for helium during short-duration dives. Most divers do longer decompressions; however, some groups like the WKPP have been pioneering the use of shorter decompression times by including deep stops.[55]

Any inert gas that is breathed under pressure can form bubbles when the ambient pressure decreases. Very deep dives have been made using hydrogen-oxygen mixtures (hydrox),[56] but controlled decompression is still required to avoid DCS.[57]

Isobaric counterdiffusion

DCS can also be caused at a constant ambient pressure when switching between gas mixtures containing different proportions of inert gas. This is known as isobaric counterdiffusion, and presents a problem for very deep dives.[58] For example, after using a very helium-rich trimix at the deepest part of the dive, a diver will switch to mixtures containing progressively less helium and more oxygen and nitrogen during the ascent. Nitrogen diffuses into tissues 2.65 times slower than helium, but is about 4.5 times more soluble. Switching between gas mixtures that have very different fractions of nitrogen and helium can result in "fast" tissues (those tissues that have a good blood supply) actually increasing their total inert gas loading. This is often found to provoke inner ear decompression sickness, as the ear seems particularly sensitive to this effect.[59]

Bubble formation

The location of micronuclei or where bubbles initially form is not known.[60] The most likely mechanisms for bubble formation are tribonucleation, when two surfaces make and break contact (such as in joints), and heterogeneous nucleation, where bubbles are created at a site based on a surface in contact with the liquid. Homogeneous nucleation, where bubbles form within the liquid itself is less likely because it requires much greater pressure differences than experienced in decompression.[60] The spontaneous formation of nanobubbles on hydrophobic surfaces is a possible source of micronuclei, but it is not yet clear if these can grow large enough to cause symptoms as they are very stable.[60]

Once microbubbles have formed, they can grow by either a reduction in pressure or by diffusion of gas into the gas from its surroundings. In the body, bubbles may be located within tissues or carried along with the bloodstream. The speed of blood flow within a blood vessel and the rate of delivery of blood to capillaries (perfusion) are the main factors that determine whether dissolved gas is taken up by tissue bubbles or circulation bubbles for bubble growth.[60]


The primary provoking agent in decompression sickness is bubble formation from excess dissolved gases. Various hypotheses have been put forward for the nucleation and growth of bubbles in tissues, and for the level of supersaturation which will support bubble growth. The earliest bubble formation detected is subclinical intravascular bubbles detectable by doppler ultrasound in the venous systemic circulation. The presence of these "silent" bubbles is no guarantee that they will persist and grow to be symptomatic.[61]

Vascular bubbles formed in the systemic capillaries may be trapped in the lung capillaries, temporarily blocking them. If this is severe, the symptom called "chokes" may occur.[62] If the diver has a patent foramen ovale (or a shunt in the pulmonary circulation), bubbles may pass through it and bypass the pulmonary circulation to enter the arterial blood. If these bubbles are not absorbed in the arterial plasma and lodge in systemic capillaries they will block the flow of oxygenated blood to the tissues supplied by those capillaries, and those tissues will be starved of oxygen. Moon and Kisslo (1988) concluded that "the evidence suggests that the risk of serious neurological DCI or early onset DCI is increased in divers with a resting right-to-left shunt through a PFO. There is, at present, no evidence that PFO is related to mild or late onset bends."[63] Bubbles form within other tissues as well as the blood vessels.[62] Inert gas can diffuse into bubble nuclei between tissues. In this case, the bubbles can distort and permanently damage the tissue.[64] As they grow, the bubbles may also compress nerves, causing pain.[65][66]Extravascular or autochthonous[a] bubbles usually form in slow tissues such as joints, tendons and muscle sheaths. Direct expansion causes tissue damage, with the release of histamines and their associated affects. Biochemical damage may be as important as, or more important than mechanical effects.[65][62][67]

Bubble size and growth may be affected by several factors - gas exchange with adjacent tissues, the presence of surfactants, coalescence and disintegration by collision.[61] Vascular bubbles may cause direct blockage, aggregate platelets and red blood cells, and trigger the coagulation process, causing local and downstream clotting.[64]

Arteries may be blocked by intravascular fat aggregation. Platelets accumulate in the vicinity of bubbles. Endothelial damage may be a mechanical effect of bubble pressure on the vessel walls, a toxic effect of stabilised platelet aggregates and possibly toxic effects due to the association of lipids with the air bubbles.[61] Protein molecules may be denatured by reorientation of the secondary and tertiary structure when non-polar groups protrude into the bubble gas and hydrophilic groups remain in the surrounding blood, which may generate a cascade of pathophysiological events with consequent production of clinical signs of decompression sickness.[61]

The physiological effects of a reduction in environmental pressure depend on the rate of bubble growth, the site, and surface activity. A sudden release of sufficient pressure in saturated tissue results in a complete disruption of cellular organelles, while a more gradual reduction in pressure may allow accumulation of a smaller number of larger bubbles, some of which may not produce clinical signs, but still cause physiological effects typical of a blood/gas interface and mechanical effects. Gas is dissolved in all tissues, but decompression sickness is only clinically recognised in the central nervous system, bone, ears, teeth, skin and lungs.[68]

Necrosis has frequently been reported in the lower cervical, thoracic, and upper lumbar regions of the spinal cord. A catastrophic pressure reduction from saturation produces explosive mechanical disruption of cells by local effervescence, while a more gradual pressure loss tends to produce discrete bubbles accumulated in the white matter, surrounded by a protein layer.[68] Typical acute spinal decompression injury occurs in the columns of white matter. Infarcts are characterised by a region of oedema, haemorrhage and early myelin degeneration, and are typically centred on small blood vessels. The lesions are generally discrete. Oedema usually extends to the adjacent grey matter. Microthrombi are found in the blood vessels associated with the infarcts.[68]

Following the acute changes there is an invasion of lipid phagocytes and degeneration of adjacent neural fibres with vascular hyperplasia at the edges of the infarcts. The lipid phagocytes are later replaced by a cellular reaction of astrocytes. Vessels in surrounding areas remain patent but are collagenised.[68] Distribution of spinal cord lesions may be related to vascular supply. There is still uncertainty regarding the aetiology of decompression sickness damage to the spinal cord.[68]

Dysbaric osteonecrosis lesions are typically bilateral and usually occur at both ends of the femur and at the proximal end of the humerus Symptoms are usually only present when a joint surface is involved, which typically does not occur until a long time after the causative exposure to a hyperbaric environment. The initial damage is attributed to the formation of bubbles, and one episode can be sufficient, however incidence is sporadic and generally associated with relatively long periods of hyperbaric exposure and aetiology is uncertain. Early identification of lesions by radiography is not possible, but over time areas of radiographic opacity develop in association with the damaged bone.[69]


Decompression sickness should be suspected if any of the symptoms associated with the condition occurs following a drop in pressure, in particular, within 24 hours of diving.[70] In 1995, 95% of all cases reported to Divers Alert Network had shown symptoms within 24 hours.[71] An alternative diagnosis should be suspected if severe symptoms begin more than six hours following decompression without an altitude exposure or if any symptom occurs more than 24 hours after surfacing.[72] The diagnosis is confirmed if the symptoms are relieved by recompression.[72][73] Although MRI or CT can frequently identify bubbles in DCS, they are not as good at determining the diagnosis as a proper history of the event and description of the symptoms.[3]


Underwater diving

The display of a basic personal dive computer shows depth, dive time, and decompression information.

To prevent the excess formation of bubbles that can lead to decompression sickness, divers limit their ascent rate—the recommended ascent rate used by popular decompression models is about 10 metres (33 ft) per minute—and carry out a decompression schedule as necessary.[74] This schedule requires the diver to ascend to a particular depth, and remain at that depth until sufficient gas has been eliminated from the body to allow further ascent.[75] Each of these is termed a "decompression stop", and a schedule for a given bottom time and depth may contain one or more stops, or none at all. Dives that contain no decompression stops are called "no-stop dives", but divers usually schedule a short "safety stop" at 3 m (10 ft), 4.6 m (15 ft), or 6 m (20 ft), depending on the training agency.[74][b]

The decompression schedule may be derived from decompression tables, decompression software, or from dive computers, and these are commonly based upon a mathematical model of the body's uptake and release of inert gas as pressure changes. These models, such as the Bühlmann decompression algorithm, are designed to fit empirical data and provide a decompression schedule for a given depth and dive duration.[76]

Since divers on the surface after a dive still have excess inert gas in their bodies, decompression from any subsequent dive before this excess is fully eliminated needs to modify the schedule to take account of the residual gas load from the previous dive. This will result in a shorter allowable time under water without obligatory decompression stops, or an increased decompression time during the subsequent dive. The total elimination of excess gas may take many hours, and tables will indicate the time at normal pressures that is required, which may be up to 18 hours.[77]

Decompression time can be significantly shortened by breathing mixtures containing much less inert gas during the decompression phase of the dive (or pure oxygen at stops in 6 metres (20 ft) of water or less). The reason is that the inert gas outgases at a rate proportional to the difference between the partial pressure of inert gas in the diver's body and its partial pressure in the breathing gas; whereas the likelihood of bubble formation depends on the difference between the inert gas partial pressure in the diver's body and the ambient pressure. Reduction in decompression requirements can also be gained by breathing a nitrox mix during the dive, since less nitrogen will be taken into the body than during the same dive done on air.[78]

Following a decompression schedule does not completely protect against DCS. The algorithms used are designed to reduce the probability of DCS to a very low level, but do not reduce it to zero.[79]

Exposure to altitude

One of the most significant breakthroughs in the prevention of altitude DCS is oxygen pre-breathing. Breathing pure oxygen significantly reduces the nitrogen loads in body tissues by reducing the partial pressure of nitrogen in the lungs, which induces diffusion of nitrogen from the blood into the breathing gas, and this effect eventually lowers the concentration of nitrogen in the other tissues of the body. If continued for long enough, and without interruption, this provides effective protection upon exposure to low-barometric pressure environments.[20][21] However, breathing pure oxygen during flight alone (ascent, en route, descent) does not decrease the risk of altitude DCS as the time required for ascent is generally not sufficient to significantly desaturate the slower tissues.[20][21]

Pure aviator oxygen which has moisture removed to prevent freezing of valves at altitude is readily available and routinely used in general aviation mountain flying and at high altitudes. Most small general aviation aircraft are not pressurized, therefore oxygen use is an FAA requirement at higher altitudes.

Although pure oxygen pre-breathing is an effective method to protect against altitude DCS, it is logistically complicated and expensive for the protection of civil aviation flyers, either commercial or private. Therefore, it is currently used only by military flight crews and astronauts for protection during high-altitude and space operations. It is also used by flight test crews involved with certifying aircraft, and may also be used for high-altitude parachute jumps.

Astronauts aboard the International Space Station preparing for extra-vehicular activity (EVA) "camp out" at low atmospheric pressure, 10.2 psi (0.70 bar), spending eight sleeping hours in the Quest airlock chamber before their spacewalk. During the EVA they breathe 100% oxygen in their spacesuits, which operate at 4.3 psi (0.30 bar),[80] although research has examined the possibility of using 100% O2 at 9.5 psi (0.66 bar) in the suits to lessen the pressure reduction, and hence the risk of DCS.[81]


Nasa decompression chamber
The recompression chamber at the Neutral Buoyancy Lab.

All cases of decompression sickness should be treated initially with 100% oxygen until hyperbaric oxygen therapy (100% oxygen delivered in a high-pressure chamber) can be provided.[82] Mild cases of the "bends" and some skin symptoms may disappear during descent from high altitude; however, it is recommended that these cases still be evaluated. Neurological symptoms, pulmonary symptoms, and mottled or marbled skin lesions should be treated with hyperbaric oxygen therapy if seen within 10 to 14 days of development.[83]

Recompression on air was shown to be an effective treatment for minor DCS symptoms by Keays in 1909.[84] Evidence of the effectiveness of recompression therapy utilizing oxygen was first shown by Yarbrough and Behnke,[85] and has since become the standard of care for treatment of DCS.[86] Recompression is normally carried out in a recompression chamber. At a dive site, a riskier alternative is in-water recompression.[87][88][89]

Oxygen first aid has been used as an emergency treatment for diving injuries for years.[90] If given within the first four hours of surfacing, it increases the success of recompression therapy as well as decreasing the number of recompression treatments required.[91] Most fully closed-circuit rebreathers can deliver sustained high concentrations of oxygen-rich breathing gas and could be used as a means of supplying oxygen if dedicated equipment is not available.[92]

It is beneficial to give fluids, as this helps reduce dehydration. It is no longer recommended to administer aspirin, unless advised to do so by medical personnel, as analgesics may mask symptoms. People should be made comfortable and placed in the supine position (horizontal), or the recovery position if vomiting occurs.[70] In the past, both the Trendelenburg position and the left lateral decubitus position (Durant's maneuver) have been suggested as beneficial where air emboli are suspected,[93] but are no longer recommended for extended periods, owing to concerns regarding cerebral edema.[90][94]

The duration of recompression treatment depends on the severity of symptoms, the dive history, the type of recompression therapy used and the patient's response to the treatment. One of the more frequently used treatment schedules is the US Navy Table 6, which provides hyperbaric oxygen therapy with a maximum pressure equivalent to 60 feet (18 m) of seawater for a total time under pressure of 288 minutes, of which 240 minutes are on oxygen and the balance are air breaks to minimise the possibility of oxygen toxicity.[95]

A multiplace chamber is the preferred facility for treatment of decompression sickness as it allows direct physical access to the patient by medical personnel, but monoplace chambers are more widely available and should be used for treatment if a multiplace chamber is not available or transportation would cause significant delay in treatment, as the interval between onset of symptoms and recompression is important to the quality of recovery.[96] It may be necessary to modify the optimum treatment schedule to allow use of a monoplace chamber, but this is usually better than delaying treatment. A US Navy treatment table 5 can be safely performed without air breaks if a built-in breathing system is not available.[96] In most cases the patient can be adequately treated in a monoplace chamber at the receiving hospital.[96]


Immediate treatment with 100% oxygen, followed by recompression in a hyperbaric chamber, will in most cases result in no long-term effects. However, permanent long-term injury from DCS is possible. Three-month follow-ups on diving accidents reported to DAN in 1987 showed 14.3% of the 268 divers surveyed had ongoing symptoms of Type II DCS, and 7% from Type I DCS.[97][98] Long-term follow-ups showed similar results, with 16% having permanent neurological sequelae.[99]


The incidence of decompression sickness is rare, estimated at 2.8 cases per 10,000 dives, with the risk 2.6 times greater for males than females.[3] DCS affects approximately 1,000 U.S. scuba divers per year.[70] In 1999, the Divers Alert Network (DAN) created "Project Dive Exploration" to collect data on dive profiles and incidents. From 1998 to 2002, they recorded 50,150 dives, from which 28 recompressions were required — although these will almost certainly contain incidents of arterial gas embolism (AGE) — a rate of about 0.05%.[2][100]

Around 2013, Honduras had the highest number of decompression-related deaths and disabilities in the world, caused by unsafe practices in lobster diving among the indigenous Miskito people, who face great economic pressures.[101] At that time it was estimated that in the country over 2000 divers had been injured and 300 others had died since the 1970s.[101]


  • 1670: Robert Boyle demonstrated that a reduction in ambient pressure could lead to bubble formation in living tissue. This description of a bubble forming in the eye of a viper subjected to a near vacuum was the first recorded description of decompression sickness.[102]
  • 1769: Giovanni Morgagni described the post mortem findings of air in cerebral circulation and surmised that this was the cause of death.[103]
  • 1840: Charles Pasley, who was involved in the recovery of the sunken warship HMS Royal George, commented that, of those having made frequent dives, "not a man escaped the repeated attacks of rheumatism and cold".[104]
  • 1841: First documented case of decompression sickness, reported by a mining engineer who observed pain and muscle cramps among coal miners working in mine shafts air-pressurized to keep water out.
  • 1854: Decompression sickness reported and one resulting death of caisson workers on the Royal Albert Bridge.[105]
  • 1867: Panamanian pearl divers using the revolutionary Sub Marine Explorer submersible repeatedly suffered "fever" due to rapid ascents. Continued sickness led to the vessel's abandonment in 1869.[106]
  • 1870: Bauer published outcomes of 25 paralyzed caisson workers.
    From 1870 to 1910, all prominent features were established. Explanations at the time included: cold or exhaustion causing reflex spinal cord damage; electricity cause by friction on compression; or organ congestion; and vascular stasis caused by decompression.[103]
    Eads Bridge panorama 20090119
    The Eads Bridge where 42 workers were injured by caisson disease
  • 1871: The Eads Bridge in St Louis employed 352 compressed air workers including Alphonse Jaminet as the physician in charge. There were 30 seriously injured and 12 fatalities. Jaminet developed decompression sickness and his personal description was the first such recorded.[23]
  • 1872: The similarity between decompression sickness and iatrogenic air embolism as well as the relationship between inadequate decompression and decompression sickness was noted by Friedburg.[103] He suggested that intravascular gas was released by rapid decompression and recommended: slow compression and decompression; four-hour working shifts; limit to maximum pressure of 44.1 psig (4 atm); using only healthy workers; and recompression treatment for severe cases.
  • 1873: Andrew Smith first utilized the term "caisson disease" describing 110 cases of decompression sickness as the physician in charge during construction of the Brooklyn Bridge.[23][107] The project employed 600 compressed air workers. Recompression treatment was not used. The project chief engineer Washington Roebling suffered from caisson disease,[23] and endured the after-effects of the disease for the rest of his life. During this project, decompression sickness became known as "The Grecian Bends" or simply "the bends" because afflicted individuals characteristically bent forward at the hips: this is possibly reminiscent of a then popular women's fashion and dance maneuver known as the Grecian Bend.[23][108]
  • 1890 During construction of the Hudson River Tunnel contractor's agent Ernest William Moir pioneered the use of an airlock chamber for treatment.[24]
  • 1900: Leonard Hill used a frog model to prove that decompression causes bubbles and that recompression resolves them.[103][109] Hill advocated linear or uniform decompression profiles.[103][109] This type of decompression is used today by saturation divers. His work was financed by Augustus Siebe and the Siebe Gorman Company.[103]
  • 1904: Tunnel building to and from Manhattan Island caused over 3,000 injuries and over 30 deaths which led to laws requiring PSI limits and decompression rules for "sandhogs" in the United States.[110]
  • 1904: Seibe and Gorman in conjunction with Leonard Hill developed and produced a closed bell in which a diver can be decompressed at the surface.[111]
    Early diving recompression chamber at Broome, Western Australia
    An early recompression chamber (door removed for public safety)
  • 1908: "The Prevention of Compressed Air Illness" was published by JS Haldane, Boycott and Damant recommending staged decompression.[112] These tables were accepted for use by the Royal Navy.[103]
  • 1914–16: Experimental decompression chambers were in use on land and aboard ship.[113][114][115]
  • 1924: The US Navy published the first standardized recompression procedure.[116]
  • 1930s: Albert R Behnke separated the symptoms of Arterial Gas Embolism (AGE) from those of DCS.[103]
  • 1935: Behnke et al. experimented with oxygen for recompression therapy.[103][116][117]
  • 1937: Behnke introduced the "no-stop" decompression tables.[103]
  • 1941: Altitude DCS is treated with hyperbaric oxygen for the first time.[118]
  • 1957: Robert Workman established a new method for calculation of decompression requirements (M-values).[119]
  • 1959: The "SOS Decompression Meter", a submersible mechanical device that simulated nitrogen uptake and release, was introduced.[120]
  • 1960: FC Golding et al. split the classification of DCS into Type 1 and 2.[121]
  • 1982: Paul K Weathersby, Louis D Homer and Edward T Flynn introduce survival analysis into the study of decompression sickness.[122]
  • 1983: Orca produced the "EDGE", a personal dive computer, using a microprocessor to calculate nitrogen absorption for twelve tissue compartments.[120]
  • 1984: Albert A Bühlmann released his book "Decompression–Decompression Sickness," which detailed his deterministic model for calculation of decompression schedules.[123]
Eads Bridge panorama 20090119
The Eads Bridge where 42 workers were injured by caisson disease
Early diving recompression chamber at Broome, Western Australia
An early recompression chamber (door removed for public safety)

Society and culture


In the United States, it is common for medical insurance not to cover treatment for the bends that is the result of recreational diving. This is because scuba diving is considered an elective and "high-risk" activity and treatment for decompression sickness is expensive. A typical stay in a recompression chamber will easily cost several thousand dollars, even before emergency transportation is included. As a result, groups such as Divers Alert Network (DAN) offer medical insurance policies that specifically cover all aspects of treatment for decompression sickness at rates of less than $100 per year.[124]

In the United Kingdom, treatment of DCS is provided by the National Health Service. This may occur either at a specialised facility or at a hyperbaric centre based within a general hospital.[125][126]

Other animals

Animals may also contract DCS, especially those caught in nets and rapidly brought to the surface. It has been documented in loggerhead turtles and likely in prehistoric marine animals as well.[127][128] Modern reptiles are susceptible to DCS, and there is some evidence that marine mammals such as cetaceans and seals may also be affected.[129][130][131] AW Carlsen has suggested that the presence of a right-left shunt in the reptilian heart may account for the predisposition in the same way as a patent foramen ovale does in humans.[128]


  1. ^ Inner ear counter diffusion is a rare form of DCS sometimes experienced by divers engaged in extreme deep diving, caused by switching from a helium-rich gas to a nitrogen-rich gas at the start of a decompression stop. Although nitrogen diffuses more slowly than helium, nitrogen is much more soluble than helium and the total inert gas load in some tissues can temporarily exceed the critical supersaturation limit, resulting in bubble formation. The inner ear is particularly susceptible to this effect. Two of the best-recorded instances of it both occurred at Boesmansgat, South Africa — once to Nuno Gomes in an early world record attempt, and later to Don Shirley when he tried to rescue David Shaw on his fateful dive trying to recover the body of Deon Dreyer, who had been one of Gomes's support divers.
  2. ^ Tables based on US Navy tables, such as the NAUI tables have a safety stop at 15 feet (5 m);(Lippmann & Mitchell, p. 219) BSAC tables have a safety stop at 6 metres (20 ft); Bühlmann tables have a safety stop at 3 metres (10 ft).

See also

  • Decompression (diving) – The reduction of ambient pressure on underwater divers after hyperbaric exposure and the elimination of dissolved gases from the diver's tissues
  • Decompression illness – Disorders arising from ambient pressure reduction
  • Decompression theory – Theoretical modelling of decompression physiology
  • Taravana – Decompression sickness after breath-hold diving


1. ^a autochthonous: formed or originating in the place where found


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  • Calder, Ian M. (1986). "Dysbarism. A Review". Forensic Science International. 30 (4): 237–266. doi:10.1016/0379-0738(86)90133-7.
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  • Francis, T James R; Mitchell, Simon J (2003). "10.6: Manifestations of Decompression Disorders". In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th Revised ed.). United States: Saunders. pp. 578–599. ISBN 978-0-7020-2571-6. OCLC 51607923.
  • Hamilton, Robert W; Thalmann, Edward D (2003). "10.2: Decompression Practice". In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th Revised ed.). United States: Saunders. pp. 455–500. ISBN 978-0-7020-2571-6. OCLC 51607923.
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  • Nishi, Ron Y; Brubakk, Alf O; Eftedal, Olav S (2003). "10.3: Bubble Detection". In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th Revised ed.). United States: Saunders. p. 501. ISBN 978-0-7020-2571-6. OCLC 51607923.
  • Powell, Mark (2008). Deco for Divers. Southend-on-Sea: Aquapress. ISBN 978-1-905492-07-7.

External links

External resources
Brian Andrew Hills

Brian Andrew Hills, born 19 March 1934 in Cardiff, Wales, died 13 January 2006 in Brisbane, Queensland, was a physiologist who worked on decompression theory.

Early decompression work was done with Hugh LeMessurier's aeromedicine group at the department of Physiology, University of Adelaide. His "thermodynamic decompression model" was one of the first models in which decompression is controlled by the volume of gas bubbles coming out of solution. In this model, pain only DCS is modelled by a single tissue which is diffusion-limited for gas uptake, and bubble-formation during decompression causes "phase equilibration" of partial pressures between dissolved and free gases. The driving mechanism for gas elimination in this tissue is inherent unsaturation, also called partial pressure vacancy or the oxygen window, where oxygen metabolised is replaced by more soluble carbon dioxide. This model was used to explain the effectiveness of the Torres Strait Islands pearl divers' empirically developed decompression schedules, which used deeper decompression stops and less overall decompression time than the current naval decompression schedules. This trend to deeper decompression stops has become a feature of more recent decompression models.Hills made a significant contribution to the mainstream scientific literature of some 186 articles between 1967 and 2006. The first 15 years of this contribution are mostly related to decompression theory. Other contributions to decompression science include the development of two early decompression computers, a method to detect tissue bubbles using electrical impedance, the use of kangaroo rats as animal models for decompression sickness, theoretical and experimental work on bubble nucleation, inert gas uptake and washout, acclimatisation to decompression sickness, and isobaric counterdiffusion.


Caisson (French for "box") may refer to:

Caisson (Asian architecture), a spider web ceiling

Caisson (engineering), a sealed underwater structure

Caisson (lock gate), a gate for a dock or lock, constructed as a floating caisson

Caisson (pen name), of Edward Sperling

Caisson (western architecture), a type of coffer

Caisson disease, or decompression sickness

Caisson lock, a type of canal lock

Deep foundation, also called a caisson foundation

Limbers and caissons, a two-wheeled cart for carrying ammunition, also used in certain state and military funerals

Compression arthralgia

Compression arthralgia is pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression, experienced by underwater divers.

Also referred to in the US Navy diving Manual as compression pains.

Compression arthralgia has been recorded as deep aching pain in the knees, shoulders, fingers, back, hips, neck and ribs. Pain may be sudden and intense in onset and may be accompanied by a feeling of roughness in the joints.Onset commonly occurs around 60 msw (meters of sea water), and symptoms are variable depending on depth, compression rate and personal susceptibility. Intensity increases with depth and may be aggravated by exercise. Compression arthralgia is generally a problem of deep diving, particularly deep saturation diving, where at sufficient depth even slow compression may produce symptoms. Peter B. Bennett et al. showed that the use of trimix could reduce the symptoms.Fast compression (descent) may produce symptoms as shallow as 30 msw. Saturation divers generally compress much more slowly, and symptoms are unlikely at less than around 90 msw. At depths beyond 180m even very slow compression may produce symptoms. Spontaneous improvement may occur over time at depth, but this is unpredictable, and pain may persist into decompression. They may be distinguished from decompression sickness as they are present before starting decompression, and resolve with decreasing pressure, the opposite of decompression sickness. The pain may be sufficiently severe to limit the diver's capacity for work, and may also limit travel rate and depth of downward excursions.

Decompression (diving)

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 occurs during the ascent, 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.

When a diver descends in the water the hydrostatic pressure, and therefore the ambient pressure, rises. Because breathing gas is supplied at ambient pressure, some of this gas dissolves into the diver's blood and is transferred by the blood to other tissues. Inert gas such as nitrogen or helium continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, at which point the diver is saturated for that depth and breathing mixture, or the depth, and therefore the pressure, is changed. During ascent, the ambient pressure is reduced, and at some stage the inert gases dissolved in any given tissue will be at a higher concentration than the equilibrium state and start to diffuse out again. If the pressure reduction is sufficient, excess gas may form bubbles, which may lead to decompression sickness, a possibly debilitating or life-threatening condition. It is essential that divers manage their decompression to avoid excessive bubble formation and decompression sickness. A mismanaged decompression usually results from reducing the ambient pressure too quickly for the amount of gas in solution to be eliminated safely. These bubbles may block arterial blood supply to tissues or directly cause tissue damage. If the decompression is effective, the asymptomatic venous microbubbles present after most dives are eliminated from the diver's body in the alveolar capillary beds of the lungs. If they are not given enough time, or more bubbles are created than can be eliminated safely, the bubbles grow in size and number causing the symptoms and injuries of decompression sickness. The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to avoid complications due to sub-clinical decompression injury.

The mechanisms of bubble formation and the damage bubbles cause has been the subject of medical research for a considerable time and several hypotheses have been advanced and tested. Tables and algorithms for predicting the outcome of decompression schedules for specified hyperbaric exposures have been proposed, tested and used, and in many cases, superseded. Although constantly refined and generally considered acceptably reliable, the actual outcome for any individual diver remains slightly unpredictable. Although decompression retains some risk, this is now generally considered acceptable for dives within the well tested range of normal recreational and professional diving. Nevertheless, all currently popular decompression procedures advise a 'safety stop' additional to any stops required by the algorithm, usually of about three to five minutes at 3 to 6 metres (10 to 20 ft), even on an otherwise continuous no-stop ascent.

Decompression may be continuous or staged. A staged decompression is interrupted by decompression stops at calculated depth intervals, but the entire ascent is actually part of the decompression and the ascent rate is critical to harmless elimination of inert gas. A no-decompression dive, or more accurately, a dive with no-stop decompression, relies on limiting the ascent rate for avoidance of excessive bubble formation. The elapsed time at surface pressure immediately after a dive is also an important part of decompression and can be thought of as the last decompression stop of a dive. It can take up to 24 hours for the body to return to its normal atmospheric levels of inert gas saturation after a dive. When time is spent on the surface between dives this is known as the "surface interval" and is considered when calculating decompression requirements for the subsequent dive.

Decompression illness

Decompression Illness (DCI) describes a range of symptoms arising from decompression of the body.

DCI can be caused by two different mechanisms, which result in overlapping sets of symptoms. The two mechanisms are:

Decompression sickness (DCS), which results from metabolically inert gas dissolved in body tissue under pressure precipitating out of solution and forming bubbles during decompression. It typically afflicts underwater divers on poorly managed ascent from depth or aviators flying in inadequately pressurised aircraft.

Arterial gas embolism (AGE), which is gas bubbles in the bloodstream. In the context of DCI these may form either as a result of bubble nucleation and growth by dissolved gas into the blood on depressurisation, which is a subset of DCS above, or by gas entering the blood mechanically as a result of pulmonary barotrauma. Pulmonary barotrauma is a rupturing of lung tissue by expansion of breathing gas held in the lungs during depressurisation. This may typically be caused by an underwater diver ascending while holding the breath after breathing at ambient pressure, ambient pressure escape from a submerged submarine without adequate exhalation during the ascent, or the explosive decompression of an aircraft cabin or other pressurised environment.In any situation that could cause decompression sickness, there is also potentially a risk of arterial gas embolism, and as many of the symptoms are common to both conditions, it may be difficult to distinguish between the two in the field, and first aid treatment is the same for both mechanisms.

Decompression practice

The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in the tissues as a result of breathing at ambient pressures greater than surface atmospheric pressure, the equipment available and appropriate to the circumstances of the dive, and the procedures authorized for the equipment and profile to be used. There is a large range of options in all of these aspects.

Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation. Staged decompression may include deep stops depending on the theoretical model used for calculating the ascent schedule. Omission of decompression theoretically required for a dive profile exposes the diver to significantly higher risk of symptomatic decompression sickness, and in severe cases, serious injury or death. The risk is related to the severity of exposure and the level of supersaturation of tissues in the diver. Procedures for emergency management of omitted decompression and symptomatic decompression sickness have been published. These procedures are generally effective, but vary in effectiveness from case to case.

The procedures used for decompression depend on the mode of diving, the available equipment, the site and environment, and the actual dive profile. Standardized procedures have been developed which provide an acceptable level of risk in the circumstances for which they are appropriate. Different sets of procedures are used by commercial, military, scientific and recreational divers, though there is considerable overlap where similar equipment is used, and some concepts are common to all decompression procedures.

Decompression theory

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.The term "decompression" derives from the reduction in ambient pressure experienced by the organism and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during and after this reduction in pressure. The uptake of gas by the tissues is in the dissolved state, and elimination also requires the gas to be dissolved, however a sufficient reduction in ambient pressure may cause bubble formation in the tissues, which can lead to tissue damage and the symptoms known as decompression sickness, and also delays the elimination of the gas.Decompression modeling attempts to explain and predict the mechanism of gas elimination and bubble formation within the organism during and after changes in ambient pressure, and provides mathematical models which attempt to predict acceptably low risk and reasonably practicable procedures for decompression in the field.

Both deterministic and probabilistic models have been used, and are still in use.

Diving chamber

A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of breathing gas for the occupants.

There are two main functions for diving chambers:

as a simple form of submersible vessel to transport divers underwater and to provide a temporary base and retrieval system in the depths;

as a land, ship or offshore platform-based hyperbaric chamber or system, to artificially reproduce the hyperbaric conditions under the sea. Internal pressures above normal atmospheric pressure are provided for diving-related applications such as saturation diving and diver decompression, and non-diving medical applications such as hyperbaric medicine.

Diving disorders

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 into 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.

Disorders particularly associated with diving include those caused by variations in ambient pressure, such as barotraumas of descent and ascent, decompression sickness and those caused by exposure to elevated ambient pressure, such as some types of gas toxicity. There are also non-dysbaric disorders associated with diving, which include the effects of the aquatic environment, such as drowning, which also are common to other water users, and disorders caused by the equipment or associated factors, such as carbon dioxide and carbon monoxide poisoning. General environmental conditions can lead to another group of disorders, which include hypothermia and motion sickness, injuries by marine and aquatic organisms, contaminated waters, man-made hazards, and ergonomic problems with equipment. Finally there are pre-existing medical and psychological conditions which increase the risk of being affected by a diving disorder, which may be aggravated by adverse side effects of medications and other drug use.

Treatment depends on the specific disorder, but often includes oxygen therapy, which is standard first aid for most diving accidents, and is hardly ever contra-indicated for a person medically fit to dive, and hyperbaric therapy is the definitive treatment for decompression sickness. Screening for medical fitness to dive can reduce some of the risk for some of the disorders.

Diving medicine

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 relationships of a diver's fitness to dive affect a diver's safety.

Hyperbaric medicine is a corollary field associated with diving, since recompression in a hyperbaric chamber is used as a treatment for two of the most significant diving-related illnesses, decompression sickness and arterial gas embolism.

Diving medicine deals with medical research on issues of diving, the prevention of diving disorders, treatment of diving accidents and diving fitness. The field includes the effect of breathing gases and their contaminants under high pressure on the human body and the relationship between the state of physical and psychological health of the diver and safety.

In diving accidents it is common for multiple disorders to occur together and interact with each other, both causatively and as complications.

Diving medicine is a branch of occupational medicine and sports medicine, and an important part of diver education.


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 (for example, caisson workers), and people who move between different altitudes.

History of decompression research and development

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.

When a diver descends in the water column the ambient pressure rises. Breathing gas is supplied at the same pressure as the surrounding water, and some of this gas dissolves into the diver's blood and other tissues. Inert gas continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, (see: "Saturation diving"), or the diver moves up in the water column and reduces the ambient pressure of the breathing gas until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again. Dissolved inert gases such as nitrogen or helium can form bubbles in the blood and tissues of the diver if the partial pressures of the dissolved gases in the diver gets too high when compared to the ambient pressure. These bubbles, and products of injury caused by the bubbles, can cause damage to tissues known as decompression sickness or the bends. The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to also avoid complications due to sub-clinical decompression injury.

The symptoms of decompression sickness are known to be caused by damage resulting from the formation and growth of bubbles of inert gas within the tissues and by blockage of arterial blood supply to tissues by gas bubbles and other emboli consequential to bubble formation and tissue damage. The precise mechanisms of bubble formation and the damage they cause has been the subject of medical research for a considerable time and several hypotheses have been advanced and tested. Tables and algorithms for predicting the outcome of decompression schedules for specified hyperbaric exposures have been proposed, tested, and used, and usually found to be of some use but not entirely reliable. Decompression remains a procedure with some risk, but this has been reduced and is generally considered to be acceptable for dives within the well-tested range of commercial, military and recreational diving.

The first recorded experimental work related to decompression was conducted by Robert Boyle, who subjected experimental animals to reduced ambient pressure by use of a primitive vacuum pump. In the earliest experiments the subjects died from asphyxiation, but in later experiments, signs of what was later to become known as decompression sickness were observed. Later, when technological advances allowed the use of pressurisation of mines and caissons to exclude water ingress, miners were observed to present symptoms of what would become known as caisson disease, the bends, and decompression sickness. Once it was recognized that the symptoms were caused by gas bubbles, and that recompression could relieve the symptoms, further work showed that it was possible to avoid symptoms by slow decompression, and subsequently various theoretical models have been derived to predict low-risk decompression profiles and treatment of decompression sickness.

Hyperbaric medicine

Hyperbaric medicine is medical treatment in which an ambient pressure greater than sea level atmospheric pressure is a necessary component. The treatment comprises hyperbaric oxygen therapy (HBOT), the medical use of oxygen at an ambient pressure higher than atmospheric pressure, and therapeutic recompression for decompression illness, intended to reduce the injurious effects of systemic gas bubbles by physically reducing their size and providing improved conditions for elimination of bubbles and excess dissolved gas.

The equipment required for hyperbaric oxygen treatment consists of a pressure chamber, which may be of rigid or flexible construction, and a means of delivering 100% oxygen. Operation is performed to a predetermined schedule by trained personnel who monitor the patient and may adjust the schedule as required. HBOT found early use in the treatment of decompression sickness, and has also shown great effectiveness in treating conditions such as gas gangrene and carbon monoxide poisoning. More recent research has examined the possibility that it may also have value for other conditions such as cerebral palsy and multiple sclerosis, but no significant evidence has been found.

Therapeutic recompression is usually also provided in a hyperbaric chamber. It is the definitive treatment for decompression sickness and may also be used to treat arterial gas embolism caused by pulmonary barotrauma of ascent. In emergencies divers may sometimes be treated by in-water recompression if a chamber is not available and suitable diving equipment to reasonably secure the airway is available.

A number of hyperbaric treatment schedules have been published over the years for both therapeutic recompression and hyperbaric oxygen therapy for other conditions.

Hyperbaric treatment schedules

Hyperbaric treatment schedules or hyperbaric treatment tables, are planned sequences of events in chronological order for hyperbaric pressure exposures specifying the pressure profile over time and the breathing gas to be used during specified periods, for medical treatment. Hyperbaric therapy is based on exposure to pressures greater than normal atmospheric pressure, and in many cases the use of breathing gases with oxygen content greater than that of air.

A large number of hyperbaric treatment schedules are intended primarily for treatment of underwater divers and hyperbaric workers who present symptoms of decompression illness during or after a dive or hyperbaric shift, but hyperbaric oxygen therapy may also be used for other conditions.

Most hyperbaric treatment is done in hyperbaric chambers where environmental hazards can be controlled, but occasionally treatment is done in the field by in-water recompression when a suitable chamber cannot be reached in time. The risks of in-water recompression include maintaining gas supplies for multiple divers and people able to care for a sick patient in the water for an extended period of time.


Hypoesthesia is a common side effect of various medical conditions which manifests as a reduced sense of touch or sensation, or a partial loss of sensitivity to sensory stimuli. In everyday speech this is generally referred to as numbness.Hypoesthesia primarily results from damage to nerves, and from blockages in blood vessels, resulting in ischemic damage to tissues supplied by the blocked blood vessels. This damage is detectable through the use of various imaging studies. Damage in this way is caused by a variety of different illnesses and diseases. A few examples of the most common illnesses and diseases that can cause hypoesthesia as a side effect are as follows:

Decompression sickness

Trigeminal schwannoma


Intradrual extramedullary tuberculoma of the spinal cord

Cutaneous sensory disorder

BeriberiTreatment of hypoethesia are aimed at targeting the more broad disease or illnesses that has caused the side effect of sensation loss.

In-water recompression

In-water recompression (IWR) or underwater oxygen treatment is the emergency treatment of decompression sickness (DCS) of sending the diver back underwater to allow the gas bubbles in the tissues, which are causing the symptoms, to resolve. It is a risky procedure that should only ever be used when the time to travel to the nearest recompression chamber is too long to save the victim's life.Carrying out in-water recompression when there is a nearby recompression chamber or without special equipment and training is never a favoured option. The risk of the procedure comes from the fact that a diver suffering from DCS is seriously ill and may become paralysed, unconscious or stop breathing whilst under water. Any one of these events is likely to result in the diver drowning or further injury to the diver during a subsequent rescue to the surface.

List of signs and symptoms of diving disorders

Diving disorders are medical conditions specifically arising from underwater diving. The signs and symptoms of these may present during a dive, on surfacing, or up to several hours after a dive. Divers have to breathe a gas which is at the same pressure as their surroundings (ambient pressure), which can be much greater than on the surface. The ambient pressure underwater increases by 1 standard atmosphere (100 kPa) for every 10 metres (33 ft) of depth.The principal conditions are: decompression illness (which covers decompression sickness and arterial gas embolism); nitrogen narcosis; high pressure nervous syndrome; oxygen toxicity; and pulmonary barotrauma (burst lung). Although some of these may occur in other settings, they are of particular concern during diving activities.The disorders are caused by breathing gas at the high pressures encountered at depth, and divers will often breathe a gas mixture different from air to mitigate these effects. Nitrox, which contains more oxygen and less nitrogen, is commonly used as a breathing gas to reduce the risk of decompression sickness at recreational depths (up to about 40 metres (130 ft)). Helium may be added to reduce the amount of nitrogen and oxygen in the gas mixture when diving deeper, to reduce the effects of narcosis and to avoid the risk of oxygen toxicity. This is complicated at depths beyond about 150 metres (500 ft), because a helium–oxygen mixture (heliox) then causes high pressure nervous syndrome. More exotic mixtures such as hydreliox, a hydrogen–helium–oxygen mixture, are used at extreme depths to counteract this.


Taravana is a disease often found among Polynesian island natives who habitually dive deep without breathing apparatus many times in close succession, usually for food or pearls. These free-divers may make 40 to 60 dives a day, each of 30 or 40 metres (100 to 140 feet).

Taravana seems to be decompression sickness. The usual symptoms are vertigo, nausea, lethargy, paralysis and death. The word taravana is Tuamotu Polynesian for "to fall crazily".

Taravana is also used to describe someone who is "crazy because of the sea".


Vertigo is a symptom where a person feels as if they or the objects around them are moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties walking. It is typically worse when the head is moved. Vertigo is the most common type of dizziness.The most common diseases that result in vertigo are benign paroxysmal positional vertigo (BPPV), Ménière's disease, and labyrinthitis. Less common causes include stroke, brain tumors, brain injury, multiple sclerosis, migraines, trauma, and uneven pressures between the middle ears. Physiologic vertigo may occur following being exposed to motion for a prolonged period such as when on a ship or simply following spinning with the eyes closed. Other causes may include toxin exposures such as to carbon monoxide, alcohol, or aspirin. Vertigo typically indicates a problem in a part of the vestibular system. Other causes of dizziness include presyncope, disequilibrium, and non-specific dizziness.Benign paroxysmal positional vertigo is more likely in someone who gets repeated episodes of vertigo with movement and is otherwise normal between these episodes. The episodes of vertigo should last less than one minute. The Dix-Hallpike test typically produces a period of rapid eye movements known as nystagmus in this condition. In Ménière's disease there is often ringing in the ears, hearing loss, and the attacks of vertigo last more than twenty minutes. In labyrinthitis the onset of vertigo is sudden and the nystagmus occurs without movement. In this condition vertigo can last for days. More severe causes should also be considered. This is especially true if other problems such as weakness, headache, double vision, or numbness occur.Dizziness affects approximately 20–40% of people at some point in time, while about 7.5–10% have vertigo. About 5% have vertigo in a given year. It becomes more common with age and affects women two to three times more often than men. Vertigo accounts for about 2–3% of emergency department visits in the developed world.

Consequences of external causes (T66–T78, 990–995)
Adverse effect
skin conditions
resulting from
physical factors
Recreational diving
Diving equipment
Diving safety:
risks and
History of
underwater diving
Injury and treatment
Researchers in
diving medicine
and physiology
Diving medical

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