Decompression sickness
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Decompression sickness, diver's disease, DCS, the bends, dysbarism, or caisson disease is the name given to a variety of symptoms suffered by a person exposed to reducing barometric pressure.
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Introduction
Decompression sickness can happen in any of the following situations:
- A diver ascends from a dive: Diving DCS.
- An aircraft flies upwards: Altitude DCS.
- A workman comes out of a pressurized caisson, or out of a mine which has been pressurized to keep water out.
This causes inert gases (mainly nitrogen), normally dissolved in body fluids and tissues, to come out of physical solution and form bubbles.
DCS is most well known as an injury that affects divers. 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.
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. When the cap is removed from the bottle, gas is heard escaping, and bubbles can be seen forming in the soda. This is carbon dioxide gas coming out of solution as a result of sudden exposure to lower barometric pressure. Similarly, nitrogen is an inert gas normally stored throughout the human body (tissues and fluids) in physical solution. When the body is exposed to decreased barometric pressures (as in flying an unpressurized aircraft to altitude, or during a rapid decompression), the nitrogen dissolved in the body comes out of solution. If nitrogen is forced to come out of solution too quickly, bubbles form in areas of the body, causing a variety of signs and symptoms. The most common symptom is joint pain, which is known as "the bends."
As Air embolism, caused by other processes, can have many of the same symptoms as DCS, the two conditions are grouped together under the name decompression illness or DCI.
Decompression sickness caused by diving
Two linked factors contribute to divers' DCS, although the complete relationship of causes is not fully understood:
- deep or long dives: inert gases in breathing gases, such as nitrogen and helium, are adsorbed into the tissues of the body in higher concentrations than normal (Henry's Law) when breathed at high pressure.
- fast ascents: reducing the ambient pressure, as happens during the ascent, causes the absorbed gases to come back out of solution, and form "micro bubbles" in the blood. Those bubbles will safely leave the body through the lungs if the ascent is slow enough that the volume of bubbles does not rise too high.
The physiologist J. B. S. 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.
Repeated cases of decompression sickness can lead to the death of cells in long-bones and brittle bones. Severe cases can lead to death because large bubbles can impede the flow of oxygen-rich blood to the brain, central nervous system and other vital organs.
Avoiding decompression sickness
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.
Helium
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 long dives, of around 3 hours or more, the body almost reaches saturation of Helium. For such dives, the decompression is shorter than for nitrogen based breathing gases such as air.
There is some debate as to the decompression effects of helium for shorter 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 nitrox (or pure oxygen if in very shallow water), during the decompression phase of the dive. The reason is that the nitrogen comes out of solution at a rate proportional to difference between the ppN2 (partial pressure of nitrogen) in the diver's body and the ppN2 in the gas that he is breathing; but the likelihood of bubbles is proportionate to the difference between the ppN2 in the diver's body and the total surrounding air or water pressure.
Decompression sickness caused by altitude
People flying in unpressurised aircraft at high altitude, such as stowaways in unpressurised parts of the aircraft, or passengers after failure of the cabin pressure vessel, or pilots in an unpressurized cockpit, can suffer from decompression sickness. Divers who dive and then travel in aircraft are at risk even in pressurised aircraft because the cabin air pressure is less than the air pressure at sea level. The same applies to going onto very high land after diving: e.g. the Asmara plateau in Eritrea is 8000 feet (2400 meters) above sea level, and going there after diving on the coast of Eritrea should be treated the same as flying after diving.
Altitude DCS became a commonly observed problem associated with high-altitude balloon and aircraft flights in the 1930s. In present-day aviation, technology allows civilian aircraft (commercial and private) to fly higher and faster than ever before. Though modern aircraft are safer and more reliable, occupants are still subject to the stresses of high altitude flight—and the unique problems that go with these lofty heights. A century and a half after the first DCS case was described, our understanding of DCS has improved, and a body of knowledge has accumulated; however, this problem is far from being solved. Altitude DCS is still a risk to the occupants of modern aircraft.
Arterial gas embolism and DCS have very similar symptoms and treatment because they are both the result of gas bubbles in the body. In a diving context, the two are often called decompression illness. Another term, dysbarism, encompasses decompression sickness, arterial gas embolism, and barotrauma.
Medical treatment
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% 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 100% oxygen
One of the most significant breakthroughs in altitude DCS research was the discovery that breathing 100% oxygen before exposure to a low barometric pressure (oxygen prebreathing), decreases the risk of developing altitude DCS. Oxygen prebreathing promotes the elimination (washout) of nitrogen from body tissues. Prebreathing 100% oxygen for 30 minutes before starting ascent to altitude reduces the risk of altitude DCS for short exposures (10-30 min. only) to altitudes between 18,000 and 43,000 feet. However, oxygen prebreathing has to be continued. without interruption with inflight 100% oxygen breathing to provide effective protection against altitude DCS. Furthermore, it is very important to understand that breathing 100% oxygen only during flight (ascent, en route, descent) does not decrease the risk of altitude DCS, and should not be used instead of oxygen prebreathing.
Although 100% oxygen prebreathing is an effective method to protect against altitude DCS, it is logistically complicated and expensive for the protection of civil aviation flyers (commercial or private). Therefore, it is only used now by military flight crews and astronauts for their protection during high altitude and space operations.
Predisposing factors
- Altitude: There is no specific altitude that can be considered an absolute altitude exposure threshold, below which it can be assured that no one will develop altitude DCS. However, there is very little evidence of altitude DCS occurring among healthy individuals at altitudes below 18,000 feet who have not been SCUBA (Self Contained Underwater Breathing Apparatus) diving. Individual exposures to altitudes between 18,000 feet and 25,000 feet have shown a low occurrence of altitude DCS. Most cases of altitude DCS occur among individuals exposed to altitudes of 25,000 feet or higher. A US Air Force study of altitude DCS cases reported that only 13% 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).
- Repetitive Exposures: Repetitive exposures to altitudes above 18,000 feet within a short period of time (a few hours) also increase the risk of developing altitude DCS.
- Rate of Ascent: The faster the rate of ascent to altitude, the greater the risk of developing altitude DCS. An individual exposed to a rapid decompression (high rate of ascent) above 18,000 feet has a greater risk of altitude DCS than being exposed to the same altitude but at a lower rate of ascent.
- Time at Altitude: The longer the duration of the exposure to altitudes of 18,000 feet and above, the greater the risk of altitude DCS.
- Age: There are some reports indicating a higher risk of altitude DCS with increasing age.
- Previous Injury: There is some indication that recent joint or limb injuries may predispose individuals to developing "the bends."
- Ambient Temperature: There is some evidence suggesting that individual exposure to very cold ambient temperatures may increase the risk of altitude DCS.
- Body Type: Typically, a person who has a high body fat content is at greater risk of altitude DCS. Due to poor blood supply, nitrogen is stored in greater amounts in fat tissues. Although fat represents only 15% of an adult normal body, it stores over half of the total amount of nitrogen (about 1 liter) normally dissolved in the body.
- Exercise: When a person is physically active while flying at altitudes above 18,000 ft., there is greater risk of altitude DCS.
- Alcohol Consumption: The after-effects of alcohol consumption increase the susceptibility to DCS (as well as to the usual results of operating equipment or machinery while intoxicated).
Scuba diving before flying
A SCUBA diver breathes air under pressure higher than sea level atmospheric. This makes more nitrogen dissolve in the body (body nitrogen saturation). The deeper the SCUBA dive, the greater the rate of body nitrogen saturation. SCUBA diving in high elevations (mountain lakes), at any given depth, results in greater body nitrogen saturation than in SCUBA diving at the same depth in water whose surface is at sea-level. After SCUBA diving, if not enough time is allowed to eliminate the excess nitrogen stored in the body, altitude DCS can occur during exposure to altitudes as low as 5,000 feet or less.
What to do if altitude DCS occurs
- Put on your oxygen mask immediately and switch the regulator to 100% oxygen.
- Begin an emergency descent and land as soon as possible. Even if the symptoms disappear during descent, you should still land and seek medical evaluation while continuing to breathe oxygen.
- If one of your symptoms is joint pain, keep the affected area still; do not try to work pain out by moving the joint around.
- Upon landing seek medical assistance from an FAA medical officer, aviation medical examiner (AME) military flight surgeon, or a hyperbaric medicine specialist. Be aware that a physician not specialized in aviation or hypobaric medicine may not be familiar with this type of medical problem. Therefore, be your own advocate.
- Definitive medical treatment may involve the use of a hyperbaric chamber operated by specially trained personnel.
- Delayed signs and symptoms of altitude DCS can occur after return to ground level whether or not they were present during flight.
Things to remember
- Altitude DCS is a risk every time you fly in an unpressurized aircraft above 18,000 feet (or at lower altitude if you SCUBA dive prior to the flight).
- Be familiar with the signs and symptoms of altitude DCS (see Table 1). Monitor all aircraft occupants, including yourself, any time you fly an unpressurized aircraft above 18,000 feet.
- Avoid unnecessary strenuous physical activity prior to flying an unpressurized aircraft above 18,000 feet. and for 24 hours after the flight.
- Even if you are flying a pressurized aircraft, altitude DCS can occur as a result of sudden loss of cabin pressure (inflight rapid decompression).
- After exposure to an inflight rapid decompressions, do not fly for at least 24 hrs. In the meantime, stay vigilant for the possible onset of delayed symptoms or signs of altitude DCS. If you present delayed symptoms or signs of altitude DCS, seek medical attention at once.
- Keep in mind that breathing 100% oxygen during flight (ascent, en route, descent) without oxygen prebreathing before take off does not prevent altitude DCS.
- Do not ignore any symptoms or signs that go away during the descent. This could confirm that you are suffering altitude DCS. You should be medically evaluated as soon as possible.
- If there is any indication that you may have experienced altitude DCS, do not fly again until you are cleared to do so by an FAA medical officer, an aviation medical examiner, a military flight surgeon, or a hyperbaric medicine specialist.
- Allow at least 24 hrs. to elapse between SCUBA diving and flying.
- Be prepared for a future emergency by finding where hyperbaric chambers are available in your area of operations. However, keep in mind that not all of the available hyperbaric treatment facilities have personnel qualified to handle altitude DCS emergencies. To obtain information on the locations of hyperbaric treatment facilities capable of handling altitude DCS emergencies, call the Diver's Alert Network at (USA phone number) (919) 684-8111.
History
An alternative name is 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. This was a major factor during construction of Eads Bridge, when 13 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. The first documented cases of DCS were reported in 1841 by a mining engineer who observed the occurrence of pain and muscle cramps among coal miners exposed to air-pressurized mine shafts designed to keep water out. The first description of a case resulting from diving activities while wearing a pressurized hard hat was reported in 1869.
Treatment
Recompression is the only effective treatment for severe DCS, although rest and oxygen (increasing the percentage of oxygen in the air being breathed via an oxygen mask) applied to lighter cases can be effective. Normally this is carried out in a recompression chamber. 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 circuit oxygen resuscitators.
Signs and symptoms
Bubbles can form anywhere in the body, but most frequently in the shoulders, elbows, knees, and ankles.
This table lists the different DCS types with where the bubbles form and their most common symptoms. "The bends" (joint pain) accounts for about 60 to 70% of all altitude DCS cases, with the shoulder being the most common site. Neurologic symptoms are present in about 10% to 15% of all DCS cases with headache and visual disturbances being the most commons. "The chokes" are very infrequent and occur in less than 2% of all DCS cases. Skin manifestations are present in about 10 to 15% of all DCS cases.
Signs and symptoms of DCS can be skin rashes, extreme fatigue, joint pain, visual disturbances, balance disturbances, breathing difficulties, lack of strength, numbness, paralysis, unconsciousness and death. Symptoms indicating impairment of the central nervous system point to a serious injury. There can be joint pain, typically in the elbow or knee. There are other terms describing other symptoms, such as the "chokes", the "niggles" and the "staggers".
| DCS Type | Bubble Location | Signs & Symptoms (Clinical Manifestations) |
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| BENDS | Mostly large joints of the body (elbows, shoulders, hip, wrists, knees, ankles) |
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| NEUROLOGIC | Brain |
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| Spinal Cord |
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| Peripheral Nerves |
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| CHOKES | Lungs |
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| SKIN BENDS | Skin |
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