Decompression Sickness, or the bends to use its common name, is a condition that arises in divers when the nitrogen dissolved in the tissues of the body comes out of solution, ie, it bubbles or fizzes. This happens when the partial pressure of nitrogen dissolved in the tissues is approximately two bars above ambient pressure. Minute bubbles are thus formed in the blood, very much like what happens when you take the top off a fizzy drink bottle1.
Atmospheric pressure is measured in millibars, 1000 millibars being the standard pressure at sea level, or one bar. The actual pressure at sea level is 1013 millibars. Partial pressure is the total pressure exerted by a gas in a mixture of gases and is equal to the sum of the partial pressure that each member gas has and would alone have if the other gases were absent. The air we breathe is a mixture of 79% nitrogen, 20% oxygen and 1% other gases - consisting mainly of argon but small but important traces of carbon dioxide and water vapour. Therefore, the partial pressure of nitrogen in the air at sea level is 790 millibars.
As a diver descends in the water, the ambient pressure increases at a rate of one bar for every ten metres. So at a depth of ten metres, the partial pressure of nitrogen is 1.6 bars, at 20 metres 2.4 bar and at 30 metres 3.2 bars.
In the descent, the body starts to absorb nitrogen to match the partial pressure of air being breathed. The pressure increases as the diver goes deeper and it takes a while for the absorbed nitrogen to reach the same partial pressure as the nitrogen in the air being breathed, so what you breathe in has a different pressure to what you have already working in your body. At a depth of 20 metres this takes approximately 40 minutes, at 30 metres the time is reduced to 20 minutes and at 45 metres it is reduced to eight minutes. These times are the known as the bottom times and is the time a diver can spend on the bottom without having to do decompression stops. At a depth of 10 metres, the diver can stay down as long as they like and come straight to the surface as the partial pressure of nitrogen never exceeds 1.6 bars.
As a diver begins to ascend, the ambient pressure begins to go down, but the partial pressure of absorbed nitrogen reduces at a far slower rate. If a diver spends 40 minutes at 30 metres, the partial pressure of absorbed nitrogen will have reached approximately 3.2 bars. As he passes through 20 metres, the ambient pressure will have reduced to two bars, thus allowing the nitrogen in the tissues to come out of solution. In practice, the critical depth is nearer 15 metres as some nitrogen would have begun to dissipate as the diver ascends. This assumes that the diver ascends at the recommended rate of 15 metres per minute. The ascent speed can be judged by following the small bubbles from the exhaust gases from the breathing apparatus (see also Scuba Equipment). The smaller bubbles ascend slower than the larger ones and if you have ascended the right way, you should reach the surface in a cloud of little bubbles.
The effects of decompression sickness can range from minor discomfort to death. If a diver has only slightly exceeded his bottom time, he may experience tingling in the extremities and on sensitive areas of skin. This is sometimes called the niggles. This mild form of decompression sickness nearly always clears up on its own and without treatment. However, even if the diver has followed the decompression tables, it can still produce the mild symptoms as described above. This can serve as a warning that the diver is prone to decompression sickness and should reduce his bottom times on subsequent dives accordingly. Different divers have different tolerances to decompression sickness; those with large amounts of body fat are more predisposed than the skinnier sort. This is because body fat is slow to absorb nitrogen, however once it has absorbed it, it relinquishes it to the respiratory system equally slowly.
Divers suffering these minor symptoms should be kept under medical observation as they can develop into more severe decompression sickness. The symptoms can develop into 'pins and needles' in the legs, inability to urinate and vertigo. Because of the vertigo, it is also referred to the staggers.
A more severe form of decompression sickness is when the bubbles reach the joints: this causes severe pain. In extreme cases the diver will contract every muscle in their body, lying on the deck bent double - hence the bends. Immediate treatment is essential in these circumstances as delay can lead to permanent disability.
The most severe form is when the bubbles reach the heart, lungs or brain and this can lead to death. This form of decompression sickness is rare as these organs fortunately absorb nitrogen faster than other tissue and they also surrender it to the respiratory system faster. It only occurs when the diver has grossly exceeded their decompression limits. Symptoms are a severe chest pain. Deep breathing and smoking aggravate the pain. This is often called the chokes.
The symptoms of decompression sickness can appear from mere seconds after surfacing to an hour later. If a diver complains of chest pains immediately after surfacing, the possibility of pulmonary barotrauma (burst lung) should be considered. This is caused by the diver ascending far too fast, causing the air in his lungs to expand faster than it can be exhaled, thus rupturing the tissue of the lungs.
The only effective treatment for decompression sickness is recompressing in a decompression chamber. Oxygen can be administered on-site in an emergency but this should only be administered by trained personnel. It won't cure the problem, but can go some way to ensuring that it doesn't get any worse. Recompression increases the ambient pressure and forces the offending nitrogen bubbles back in to solution, ie, makes them stop bubbling. Decompression chambers are usually cylindrical in shape and come in many sizes. They range from small portable ones, just big enough for one person lying on their back, to large ones capable of holding two casualties lying down and an attendant, or six persons sitting. The larger ones also incorporate an air lock that allows food and water and medical staff to enter the chamber while it is under pressure.
Initially the pressure in the chamber will be increased to a 'depth' depending on the severity of the symptoms. Once the casualty is stabilised decompression starts. The decompression methods used vary depending on the type and severity of the decompression sickness. During this time, the casualty may be given oxygen to breathe that can aid in the dissipation of the nitrogen in their system. Drugs like digitalis can sometimes be administered, along with steroids and plasma. The methods and techniques used are beyond the scope of this entry, but suffice to say that the Royal Navy manual on decompression treatment is about four inches thick.
Re-entry decompression is when the diver re-enters the water and goes down to a depth that relieves the symptoms and then continues with the missed decompression stops. It is hardly ever effective as once the symptoms have appeared, effective decompression can take hours. However, it is permissible to re-enter the water to complete missed decompression stops if it is done before symptoms appear and this may be necessary if the diver is running low on air. Surfacing, getting a fresh cylinder and submerging again should only take about five minutes and should not be a problem. Re-entry as decompression should otherwise not be contemplated, except when there is no other alternative and death or severe disability is the only other possible outcome. If diving in a place where there are no decompression facilities available within a reasonable space of time, diving should be restricted to no-stop times.
The best way to prevent decompression sickness is to not dive below ten metres, but that is obviously impractical. The second best method is not to dive longer than the bottom time for the particular depth at which you are diving. This is where most sport diving is done as the next method is not a bag of laughs. Sometimes diving beyond the bottom time will be desirable, in which case doing decompression stops on the way up is necessary. These are stops at varying depths where the diver just hangs about, going neither up nor down, breathing normally, giving the absorbed nitrogen time to disperse. Hanging about on the end of a rope at ten metres in the English Channel in the middle of March is not much fun. It is at times like this that one begins to question one's own sanity.
Where to stop and for how long is worked out from decompression tables and every diving organisation has its own tables. They are all similar, but with modifications to suit the types of diving that the organisation does. Diving tables determine how long the diver can stay on the bottom without having to do decompression stops. If these times are exceeded, the tables will tell the diver at what depth to stop and for how long. Dive computers are also available. These are electronic devices that measure pressure over time to determine if decompression stops are required. They should only be used if there are at least two of them on a dive. In that case, the times can be compared, and the most conservative one used. To remain effective, the computers have to be recalibrated at regular intervals and in any case, waterproof diving tables should be carried in the event of computer failure.
Even if decompression tables are followed strictly, it will leave the body with a surfeit of dissolved nitrogen. Consequently, repeat dives should be treated with caution and flying after diving can be a problem (see below). If two dives are planned in any 24-hour period, the deeper dive should always be done first. The important thing is not to mix and match information from different tables, especially when doing repeat dives.
Decompression stops should always be planned before divers enter the water. To go down and stay on the bottom until you have done what you want to do, or your air is running low, then ascending to do decompression stops if and when they are required without any pre-planning is a recipe for disaster. Spare air cylinders are often hung down from the surface tender to hang at appropriate depths. This is to help any divers who may be getting short on air while doing all that hanging about.
This method of in-water decompression is only practical at depths of up to 50 metres2. At greater depths, the bottom time is so small and the decompression times so great as to make it impractical. Deep-sea commercial divers therefore use a different method. They descend to depths in a diving bell. They then exit the bell through a hatch in the floor to do whatever it is they have to do. The water is kept out of the bell since the air inside it has the same pressure as the water outside. They can then spend 24 hours or more on the bottom, returning to the diving bell to eat and rest. This is known as saturation diving as the body becomes saturated with helium. Helium replaces the nitrogen in the air because at depth, nitrogen becomes narcotic, a condition known as Nitrogen Narcosis.
When it is time to return to the surface, the hatch in the floor is closed and the bell is winched up while the pressure in the bell is maintained. On reaching the surface, the diving bell is locked onto a decompression chamber. The pressure in the decompression chambers is then equalised to the pressure in the bell and the divers transfer between the two. They then go through a prolonged decompression routine that can take days. A deep-sea commercial diver can spend more of his working life in a decompression chamber than in the water doing his job. Note that commercial divers are usually trained engineers, photographers or surveyors, etc, whose work takes them under water3.
Diving At High Altitudes
Decompression tables are designed on the assumption that the surface of the water is at or near sea level. When diving at high altitudes, such as in mountain lakes, extreme caution should be used. Because of the reduction in ambient air pressure, the possibility of decompression sickness is increased. Altitude decompression tables do exist but they should be treated with caution as they have not been as extensively tested as sea level tables have.
Flying After Diving
For the same reasons as listed above, flying after diving can be hazardous. Cabin pressure inside a pressurised commercial aircraft is usually maintained at a pressure equal to 6000-8000ft and this could be enough to induce decompression sickness after diving. It is recommended that you do not fly for at least two hours after any dive. If the dive involved decompression stops, the recommended delay is 24 hours. Flying in un-pressurised aircraft should not be a problem provided the altitude is kept below 5000 feet and the above rules are followed.
1 Although not to the same degree as when you shake the bottle first.
2 Well, down to 50 metres.
3 The bell, being winched up and down, could therefore really be called a commute.