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Decompression sickness (DCS), the diver’s disease, the bends, or caisson disease is the name given to a variety of symptoms suffered by a person exposed to a decrease (nearly always after a big increase) in the pressure around the body. It is a type of diving hazard and dysbarism.
Additional recommended knowledge
Decompression sickness can happen in these situations:
These situations cause inert gases, generally nitrogen, which are normally dissolved in body fluids and tissues, to come out of physical solution (i.e., outgas) and form gas bubbles.
According to Henry’s Law, when the pressure of a gas over a liquid is decreased, the amount of gas dissolved in that liquid will also decrease. One of the best practical demonstrations of this law is offered by opening a soft drink can or bottle. When you remove the cap from the bottle, you can clearly hear gas escaping and see bubbles forming in the soda. This is carbon dioxide gas coming out of solution as a result of the pressure inside the container reducing to atmospheric pressure.
Similarly, nitrogen is an inert gas normally stored throughout the human body, such as tissues and fluids, in physical solution. When the body is exposed to decreased pressures, such as when flying an un-pressurized aircraft to altitude or during a scuba ascent through water, the nitrogen dissolved in the body outgases. If nitrogen is forced to come out of solution too quickly, bubbles form in parts of the body causing the signs and symptoms of the "bends" which can be itching skin and rashes, joint pain, sensory system failure, paralysis, and death.
Wikisource has the text of the 1911 Encyclopædia Britannica article Caisson Disease.
Signs and symptoms
Bubbles can form anywhere in the body, but symptomatic sensation is most frequently observed in the shoulders, elbows, knees, and ankles.
This table gives symptoms for the different DCS types. The "bends" (joint pain) accounts for about 60 to 70 percent of all altitude DCS cases, with the shoulder being the most common site. These types are classifed medically as DCS I. Neurological symptoms are present in 10 to 15 percent of all DCS cases with headache and visual disturbances the most common. DCS cases with neurological symptoms are generally classified as DCS II. The "chokes" are rare and occur in less than two-percent of all DCS cases. Skin manifestations are present in about 10 to 15 percent of all DCS cases.
Recompression is the only effective treatment for severe DCS, although rest and oxygen (increasing the percentage of oxygen in the air being breathed via a tight fitting oxygen mask) applied to lighter cases can be effective. Recompression is normally carried out in a recompression chamber. In diving, a high-risk alternative is in-water recompression.
Oxygen first aid treatment is useful for suspected DCS casualties or divers who have made fast ascents or missed decompression stops. Most fully closed-circuit rebreathers can deliver sustained high concentrations of oxygen-rich breathing gas and could be used as an alternative to pure open-circuit oxygen resuscitators.
Common pressure reductions that cause DCS
The main cause of DCS is a reduction in the pressure surrounding the body. Common ways in which the required reduction in pressure occur are:
Leaving a high pressure environment
The original name for DCS was caisson disease; this term was used in the 19th century, when large engineering excavations below the water table, such as with the piers of bridges and with tunnels, had to be done in caissons under pressure to keep water from flooding the excavations. Workers who spend time in high pressure atmospheric pressure conditions are at risk when they return to the lower pressue outside the caisson without slowly and gradually reducing the pressure surrounding them.
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.
Ascent through water during a dive
DCS is best known as an injury that affects underwater divers who breathe gas which is at a higher pressure than surface pressure. The pressure of the surrounding water increases as the diver descends and reduces as the diver ascends. The risk of DCS increases by diving long or deep without slowly ascending and making the decompression stops needed to eliminate the inert gases normally, although the specific risk factors are not well understood. Some divers seem more susceptible than others under identical conditions.
There have been known cases of bends in snorkellers who have made many deep dives in succession. DCS may be the cause of the disease taravana which affects South Pacific island natives who for centuries have dived without equipment for food and pearls.
Two linked factors contribute to divers' DCS, although the complete relationship of causes is not fully understood:
The physiologist John Haldane studied this problem in the early 20th century, eventually devising the method of staged, gradual decompression, whereby the pressure on the diver is released slowly enough that the nitrogen comes gradually out of solution without leading to DCS. Bubbles form after every dive: slow ascent and decompression stops simply reduce the volume and number of the bubbles to a level at which there is no injury to the diver.
Even when the change in pressure causes no immediate symptoms, rapid pressure change can cause permanent bone injury called dysbaric osteonecrosis (DON) "bone cell death from bad pressure". DON can develop from a single exposure to rapid decompression. DON is diagnosed from lesions visible in X-ray images of the bones. Unfortunately, X-rays appear normal for at least 3 months after the permanent damage has occurred; it may take 4 years after the damage has occurred for its effects to become visible in the X-ray images. 
Decompression tables and dive computers have been developed that help the diver choose depth and duration of decompression stops for a particular dive profile at depth.
Avoiding decompression sickness is not an exact science. Accidents can occur after relatively shallow and short dives. To reduce the risks, divers should avoid long and deep dives and should ascend slowly. Also, dives requiring decompression stops and dives with less than a 16 hour interval since the previous dive increase the risk of DCS. There are many additional risk factors, such as age, obesity, fatigue, use of alcohol, dehydration and a patent foramen ovale. In addition, flying at high altitude less than 24 hours after a deep dive can be a precipitating factor for decompression illness.
Astronauts aboard the International Space Station preparing for Extra-vehicular activity "camp out" at low atmospheric pressure (approximately 10 psi = 700 mbar) spending 8 sleeping hours in the airlock chamber before their spacewalk. Their spacesuits can operate at 4.7 psi = 330 mbar for maximum flexibility.
Nitrogen is not the only breathing gas that causes DCS. Gas mixtures such as trimix and heliox include helium, which can also be implicated in decompression sickness.
Helium both enters and leaves the body faster than nitrogen, and for dives of three or more hours in duration, the body almost reaches saturation of helium. For such dives the decompression time is shorter than for nitrogen-based breathing gases such as air.
There is some debate as to the decompression effects of helium for shorter duration dives. Most divers do longer decompressions, whereas some groups like the WKPP have been pioneering the use of shorter decompression times by including deep stops.
Decompression time can be significantly shortened by breathing rich nitrox (or pure oxygen in very shallow water) during the decompression phase of the dive. The reason is that the nitrogen outgases at a rate proportional to the difference between the ppN2 (partial pressure of nitrogen) in the diver's body and the ppN2 in the gas that he or she is breathing; but the likelihood of bubbles is proportional to the difference between the ppN2 in the diver's body and the total surrounding air or water pressure.
Ascent to altitude in the atmosphere
People flying in un-pressurized aircraft at high altitude, such as stowaways, or passengers in a cabin that has experienced rapid decompression, or pilots in an open cockpit, can suffer from decompression sickness. Even Lockheed U-2 pilots experienced altitude DCS in the mid-'50s during the Cold War flying over their targets. Divers who dive and then fly in aircraft are at greater risk even in pressurized aircraft because the cabin air pressure is less than the air pressure at sea level. The same applies to divers going into higher elevations by land after diving.
Altitude DCS became a commonly observed problem associated with high-altitude balloon and aircraft flights in the 1930s. In modern-day transport aircraft that fly at high altitudes, cabin pressurization systems ensure that the pressure within the cabin does not fall below the pressure that would be experienced at an altitude of 8000 feet, no matter what the outside air pressure or altitude may actually be during the flight. DCS is very rare in healthy individuals who experience pressures equivalent to this altitude or less. However, since the pressure in the cabin is not actually maintained at sea-level pressure, there is still a small risk of DCS in susceptible individuals (such as recent divers).
There is no specific altitude threshold that can be considered safe for everyone below which it can be assured that no one will develop altitude DCS, but there is very little evidence of altitude DCS occurring among healthy individuals at pressure altitudes below 18,000 feet who have not been scuba diving. Individual exposures to pressure altitudes between 18,000 and 25,000 feet have shown a low occurrence of altitude DCS. Most cases of altitude DCS occur among individuals exposed to pressure altitudes of 25,000 feet or higher. A US Air Force study of altitude DCS cases reported that only 13 percent occurred below 25,000 feet The higher the altitude of exposure, the greater the risk of developing altitude DCS. It is important to clarify that although exposures to incremental altitudes above 18,000 feet show an incremental risk of altitude DCS they do not show a direct relationship with the severity of the various types of DCS (see Table 1).
Arterial gas embolism and DCS have very similar treatment because they are both the result of gas bubbles in the body. Their spectra of symptoms also overlap, although those from arterial gas embolism are more severe because they often cause infarction and tissue death as noted above. In a diving context, the two are joined under the general term of decompression illness. Another term, dysbarism, encompasses decompression sickness, arterial gas embolism, and barotrauma.
Ascent to altitude can happen without flying in places such as the Ethiopia and Eritrea highland (8000 feet = about 1.5 miles above sea level) and the Peru and Bolivia altiplano and Tibet (2 to 3 miles above sea level).
Mild cases of the "bends" and skin bends (excluding mottled or marbled skin appearance) may disappear during descent from high altitude but still require medical evaluation. If the signs and symptoms persist during descent or reappear at ground level, it is necessary to provide hyperbaric oxygen treatment immediately (100-percent oxygen delivered in a high-pressure chamber). Neurological DCS, the "chokes," and skin bends with mottled or marbled skin lesions (see Table 1) should always be treated with hyperbaric oxygenation. These conditions are very serious and potentially fatal if untreated.
Effects of breathing pure oxygen
One of the most significant breakthroughs in altitude DCS research was oxygen pre-breathing. Breathing pure oxygen before exposure to a low-barometric pressure decreases the risk of developing altitude DCS. Oxygen pre-breathing promotes the elimination or washout of nitrogen from body tissues. Pre-breathing pure oxygen for 30 minutes before starting ascent to altitude reduces the risk of altitude DCS for short exposures (10 to 30 minutes only) to altitudes between 18,000 and 43,000 feet. However, oxygen pre-breathing has to be continued without interruption with in-flight, pure oxygen to provide effective protection against altitude DCS. Furthermore, it is very important to understand that breathing pure oxygen only during flight (ascent, en route, descent) does not decrease the risk of altitude DCS, and should not be used instead of oxygen pre-breathing.
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 only used now by military flight crews and astronauts for their protection during high altitude and space operations. It is also used by flight test crews involved with certifying aircraft.
Scuba diving before flying
The risk of decompression sickness does not cease to increase at sea level (even though decompression tables stop at sea level), but continues to increase for altitudes above sea level when a diver ascends (as in an airplane or by other means) to these higher altitudes. Altitude DCS can occur during exposure to altitudes as low as 5,000 feet or less. This can happen in an airliner, since airliners do not actually maintain sea-level pressure in the cabin but instead allow it to drop to the equivalent of an altitude of 8000 feet (but no more), depending on the actual external altitude of the aircraft. It can happen when moving to high-altitude locations on land after scuba diving—for example, a scuba diver in Eritrea who travels to the country's main airport on the 8000-foot (2400-metre) Asmara plateau may be at risk of DCS. It can also happen during cave diving: "Torricellian chambers," found in some caves, are full of water at less than atmospheric pressure, and develop when the water level drops and there is no way for air to get into the chamber.
Decompression sickness in popular culture
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Decompression_sickness". A list of authors is available in Wikipedia.|