DCS is unpredictable. In general, anything that increases blood flow to an organ will increase the rate of N2 loading. Anything that interferes with blood flow from an organ will reduce the capacity to off-load N2. These alterations may explain the possible predisposing factors that increase the likelihood of DCS.
Any dive deeper than 33 feet (10 Meters) can produce DCS although in general, the deeper the dive, the greater the risk. The longer the dive, the more gas absorbed (until saturation). The more the DCS.
Some people appear to be more susceptible to DCS than others. Even an individual may vary in susceptibility at different times, and DCS can develop after a dive profile which has been safely followed on many previous occasions. Others frequently get DCS despite conservative diving.
Repeated dives to similar depths over a period of time reduce the incidence of DCS. This may be due to the elimination of bubble nuclei. A diver returning to these dives after a 2 week break loses the benefits of this adaptation or acclimatisation.
Older divers tend to be more predisposed to DCS. This age factor probably comes into play after the 3rd decade.
This appears to be a predisposing factor probably due to increased N2 solubility (4-5 : 1) in fat compared to water. This may be relevant for those with a BMI of > 25.
Factors causing the diver to be unwell such as dehydration, hangover or exhaustion tend to predispose to DCS.
DCS, particularly involving the musculo-skeletal system and joints, is more likely with recent bruises, strains or chronic injuries.
A previous episode of DCS, especially if it was unexpected from the dive profile, or if it damaged tissue (as in neurological DCS), makes the diver predisposed to similar subsequent episodes.
Patent Foramen Ovale.
One reason for some people to have an increased susceptibility to DCS is that they have a small hole in their heart. All of us had a hole in our heart when we were a fetus. In about a third of the population some remnant of this hole remains, it is called a patent foramen ovale, or PFO. These people have an increased susceptibility to DCS, the likely reason is that bubbles that would normally be trapped in the lungs without causing symptoms pass through the hole, by-passing the lung filter, and on to other parts of the body, where they cause noticeable symptoms. However, most results suggest the risk from a PFO is not great enough for it to be appropriate to test all divers for this.
Diving in cold conditions makes DCS more likely, especially when the diver is inadequately insulated. More precisely, coldness during the dive inhibits inert gas uptake (because of restricted circulation) but allows more N2 to dissolve in body fluids — while coldness during decompression inhibits inert gas release. Theoretically, it may be better to be cold during the dive and warm on decompression, unless bubble formation occurs. Warming will then reduce gas solubility and increase bubble growth and DCS.
The association between cold exposure and DCS is complex and contentious. During decompression and post-diving the cold environment may cause peripheral constriction of blood vessels and more bubble formation. Alternately, taking hot showers also tends to cause increased bubble formation and DCS.
Alcohol and other drugs
It has been observed that divers who over-indulge in alcohol, or perhaps take other drugs or medications, may be especially susceptible to DCS. In the case of alcohol, especially taken the night before, the effects may be due to the associated dehydration or the vascular dilatation (remember the throbbing headache and “hangover”), increasing N2 take-up.
Strenuous exercise during a dive is likely to increase the N2 uptake by increasing blood flow to muscles, increasing gas uptake and favoring DCS development. Gentle exercise during decompression, by promoting circulation from the tissues probably aids in N2 elimination. Strenuous exercise after the diver has returned to the surface makes the development of DCS, particularly in the musculo-skeletal system, more likely by promoting bubble formation. Strenuously exercising (shaking) a beer can, before opening it, aptly illustrates this phenomenon. During the first hour after a dive, particularly when there has been a large N2 uptake, it is best to rest quietly as this is the period of maximal N2 elimination.
The less physically fit the diver, the more likelihood of DCS, probably because more energy is used and more blood flow is required for the same outcome – transporting more N2.
There is some evidence that women have a higher incidence of DCS for certain dive profiles. There are subtle differences in physiology and body composition which could explain this. The decompression tables in current use only evolved after extensive testing on men alone.
Deep dives (greater than 18 meters), long dives, decompression dives and any dives exceeding the limiting line (in RN based tables) all have a higher incidence of DCS.
These allow insufficient time for N2 elimination from fast tissues, thus encouraging bubble
Multiple ascents during a dive imply multiple decompression’s and often involves rapid ascents. Bubbles in the blood (fast tissue bubbles) are likely to form during these ascents. The bubbles may not be adequately filtered by the lungs, passing along into the tissues, or may be reduced in size during the second or subsequent descent, allowing them to escape through the pulmonary filter into the tissues. DCS is then more likely.
Each repetitive dive begins with a N2 load of some degree from the previous dive. Since bubble formation even after routine dives is common, a repetitive dive will often start with the diver carrying N2 bubbles from the previous dive. These bubbles will be supplemented by N2 taken up during subsequent dives, and make DCS more likely.
Reverse Dive Profiles.
Divers are advised to do their deep dive first, in repetitive dives, and to dive to progressively shallower depths when multi-level diving. If this order is not followed, DCS is more likely.
Flying after diving.
The jet age often finds divers flying home after a dive holiday within hours of their last (sometimes literally) dive. International airliners are pressurized to an altitude of about 2000 meters (6500ft.) above sea level. This means a pressure reduction on the diver of about 25% with a corresponding increase in the degree of N2 supersaturation as well as a corresponding increase in the size of any bubbles he may be carrying. The increase in size of critical bubbles may be sufficient to provoke symptoms.
Often there is more than one factor increasing the likelihood of DCS. Thus in one large Australian series over half the cases engaged in multiple dives, deep dives (greater than 30metres) and/or had ingested alcohol within 8 hours. Another 20% were precipitated by
aviation exposure. Thus many of these divers would have had at least 2 factors increasing their likelihood of DCS.