Risk Factors for Decompression Illness

Every diver intuitively understands that the depth, duration and ascent protocol of a dive are perhaps the most important determinants of the risk of decompression illness (DCI). In general, we assume that the longer and deeper the dive, the more nitrogen is absorbed, and the greater the risk; especially if the longer duration and deeper depth are not compensated for with a suitable ascent (decompression) protocol.

On top of this, there is a group of widely cited “factors” which are consideredPhoto to increase Brett Forbes risk, either by increasing the likelihood of nitrogen bubble formation, or by predisposing to the adverse effects of any bubbles that do form. The fact that such factors exist is suggested by studies which have used Doppler ultrasound to detect markedly variable numbers of venous bubbles after identical depth / time exposures, and studies which show that symptoms may occur even when relatively few bubbles are detected. Problem is, we really don’t know for sure what these factors are. Brett Forbes Photo There are a number of ideas. For example, we all know that divers should lose weight, do the deepest dive first, and be more conservative if they are female in order to reduce risk. Right?? Well, maybe. Unfortunately, some of these “sacred cows” are rich in assumption and poor in supportive science. Indeed, in some cases, divers have been deceived into decades of bizarre practice on the basis of someone’s opinion alone.

Perhaps the biggest problem we face in assessing the significance of a “risk factor” is a lack of demographic data describing the diving habits of the recreational diving population. For example, we can look up treatment statistics that might tell us that 65% of all divers treated for DCI were repetitive diving. However, this figure of itself tells us nothing about the significance of repetitive diving as a risk factor for DCI. If 65% of all dives performed were repetitive dives, then, all other factors being equal, we would expect that 65% of DCI cases would come from repetitive dives. However, if only 40% of all dives were repetitive dives, but these dives produced 65% of our DCI cases, then we might begin to suspect that repetitive diving was a risk factor for DCI. Unfortunately, we usually do not have that critical denominator (the data that describe the activity of the general diving population) because there is no easy way of obtaining it.

With that in mind, this article reviews many of the so-called risk factors for DCI, and point out which are truly supported by sound science, and which should be treated with skepticism; and perhaps even ignored. The “risk factors” are most usefully divided into 3 categories: those relevant to the diver; those relevant to the dive; and those relevant to the post-dive period.

Part 3 of a 3-part article by Dr. Simon Mitchell

Alert Diver, SEAP 6 THE DIVER Previous DCI men. Results of surveys comparing the relative rates of DCI between Body fat Previous DCI may predispose to men and women have been another episode by reducing the inconclusive, and the results of It has been suggested that since body’s ability to compensate for any actual experiments comparing the nitrogen is more soluble in fatty subsequent bubble formation, or risk of DCI between the sexes have (lipid) than watery (aqueous) tissues, some unknown mechanism. This often been contradictory. There has a greater percentage body fat results theory is supported by caisson work been some interesting recent work, in greater nitrogen loading, more data, and there are limited supportive as yet unpublished, that supports the bubble formation, and higher risk of data from contemporary diving. notion that the risk of DCI for DCI. This sounds plausible, and Divers previously treated for DCI do women may change in different some old studies of compressed air appear over-represented in the phases of the menstrual cycle. workers are supportive of the Diver’s Alert Network database of However, there is no conclusive notion. But two more recent studies divers treated for DCI. However, answer on the question of how their have failed to show a relationship these divers may just dive more than risk compares to men. Most diving between body fat or body mass index everyone else, or they may physicians agree that if there is a and DCI. It is probably fair to call repeatedly indulgence in risk-taking difference, it is likely to be trivial. this one “controversial” and behaviour. In general, we consider conclude that divers are better off the risk of another episode after Dehydration with a normal body mass index. But successful treatment of mild DCI to be purging dive clubs or even the ranks only trivially greater than baseline. Pre-dive dehydration may exacerbate of commercial divers of fatties on However, if recovery from either DCI because blood will tend to be the basis of the available evidence mild or more serious DCI is “thicker” and flow less easily through is not justified. incomplete, then the risk of another any small blood vessels damaged by episode is considered to be bubbles. Despite the conviction with Age unacceptably high and cessation of which this theory is taught, there are diving is usually recommended. no supportive data from actual Advancing age may increase the risk diving, and the two best animal of DCI, perhaps because of a general Being female studies drew opposite conclusions. decline in physical fitness and ability Nevertheless, in view of the strong to compensate for damage, or the For many years it has been proposed theoretical basis for this risk factor presence of other possible risk that women might be at higher risk and the known potentiation of factors such as previous DCI and of DCI than men. This notion has dehydration during immersion, it is higher percentage body fat. Several been based on a variety of probably appropriate to encourage studies appear to confirm increased hypotheses. Most popular has been divers to actively maintain good risk in older subjects and a recent the concept that their higher mean hydration. Since alcohol has a study showed a positive correlation body fat percentage might put them diuretic effect, concerns over between advancing age and numbers at risk (see above). It has also been dehydration constitute reason of venous bubbles after suggested that hormonal changes enough to advise against moderate decompression. Although one study during menstruation can result in to heavy use of alcohol on an in the late 1980s found that fluid retention (oedema) and fluid evening prior to diving. increasing age of itself was not a shifts, and that some oral significant risk, it must be concluded contraceptive pills can cause a that the weight of evidence supports microsludging or slowing of the age as a risk factor. Not surprisingly, circulation. All these factors no clear threshold age at which risk theoretically suggest that women increases has been identified. might be more prone to DCI than

7 Physical fitness THE DIVE once daily dives on sequential days There is an intriguing and growing Repetitive diving and multi-day have repeatedly been shown to body of recent evidence that animals diving progressively lower the risk of DCI subjected to exercise conditioning as the sequence proceeds; a regimens over days prior to diving are Repetitive recreational dives, phenomenon referred to substantially more resistant to multiple ascents to the surface within “acclimatization”. The mechanism is decompression illness than a single “dive”, and multi-day diving uncertain but it may relate to unconditioned animals. Even more are considered to increase risk of consumption of gaseous micronuclei intriguing is the finding that a single DCI. The suggested basis for by bubble formation on each single episode of heavy exercise about a increased risk in closely spaced daily dive, but with sufficient time day prior to diving appears to reduce sequential dives is bubble formation for most bubbles to decay and bubble formation (in humans) and from dissolved inert gas after the first disappear before the next dive, thus decompression illness (in animals). ascent to the surface. During any leaving less nuclei available for The mechanism responsible for this subsequent dive, these bubbles may bubble initiation. The protective protection is not established, and nor affect inert gas kinetics (thus making effect seems to disappear after about is the relevance of these data to the diver’s tables or computers less 5 days without diving, and this is human divers completely clear. accurate) and may act as seeds for interpreted as the time it takes for Nevertheless, it will be an interesting larger bubbles. In addition, it has been micronuclei to be replaced. area of research to watch. In the demonstrated in animals that venous meantime, remaining physically fit bubbles trapped in lung capillaries Interestingly, this phenomenon has for diving makes sufficient intuitive after diving will redistribute into the not been recorded in recreational sense to warrant it being a goal for arterial circulation during another divers. This may be because all divers. dive if the surface interval is short. recreational multi-day diving This sounds disadvantageous, but it frequently involves multiple dives on Fatty food is of unknown relevance in human each day of the multi-day program. DCI. This may not allow sufficient time for bubbles to resolve prior to the It has long been suggested that the next day’s first dive. Just to high concentration of lipo-protein These concerns warrant attention to complicate matters further, recent complexes in the blood after a fatty conservatism during repetitive and unpublished studies from the meal may be disadvantageous in multi-day programs of diving, but DCIEM in Canada suggest that diving because interactions between they do not mandate avoidance of closely spaced repetitive dives may bubbles and these complexes might repetitive or multi-day diving. Indeed, actually be a good thing result in the release of insoluble fatty because the subsequent emboli. All that can be said is that dive resolves bubbling this mechanism is of highly from the previous one uncertain relevance in human DCI. before the process reaches a peak. Not Patent foramen ovale surprisingly however, their work suggests that For completeness, it must be the final decompression mentioned that there are compelling the day is the one that human data suggesting that a patent needs to be particularly foramen ovale (PFO) is a risk factor conservative because for serious neurological DCI. This that is when the day’s communication between the two “sins” will catch up with upper chamber of the heart (the atria) you. was discussed in the earlier two articles of this series.

Alert Diver, SEAP 8 Deep diving good idea that has become both data to support this contention, and enshrined in dive training dogma, the explanation is not so much the There is some reasonable data from and adhered to with almost religious cold per se, but the change in the carefully controlled military fervour. The conference concluded diver’s temperature during the dive. operations that suggests deeper dives there was no need for such a rule If uptake of nitrogen occurs while are associated with more risk. U.S. provided: the dives are conducted without the diver is warm and blood flow to Navy experience, between 1968 and the need for decompression stops, the dives the peripheries is good, and 1981 was that the accident rate for are within the recreational diving depth elimination occurs while the diver is dives of 15m (50ft)or less was range (0 – 40m); and the reverse profile cool and peripheral blood flow has 0.06%. The accident rate for dives gradient does not exceed 12 metres.. reduced, then nitrogen elimination to between 15.5-30m (51-100 ft) will be slower (than uptake) and the was 0.23%, nearly four times the rate Heavy exercise risk of DCI increased. Thus, divers for shallow fives. The accident rate are best advised to keep their for dives between 30-61 m (101-200 Exercise during diving accelerates temperature as stable as possible ft ) was 0.54%, more than double the absorption of inert gas in those through the dive. In cold water, this 15.5-30m rate and nine times the rate tissues whose perfusion is increased is best achieved by wearing a dry for shallower dives. This is probably during work. Not surprisingly, suit. also true for recreational diving. moderate to heavy exercise during a dive has been clearly shown to POST-DIVE Reverse profile diving increase decompression require- ments and the risk of DCI in both Exercise The performance of several dives in animals and humans. This is one of shallow to deep sequence, the few unequivocal risk factors. Human data suggest that exercise commonly referred to as “reverse Interestingly, blood flow to the legs after diving may hasten onset, and profile” diving, is a particular form and inert gas elimination is enhanced, increase both severity and incidence of repetitive diving that has been and the risk of DCI reduced, if mild of DCI. The mechanism is not considered to increase the risk of exercise is performed during certain but may involve the DCI. This is perhaps the classical decompression. generation of momentary areas of (almost) baseless sacred cow. When depressurisation in working tissues, the distinguished participants at a Cold thus promoting bubble formation. recent conference on the issue Some forms of exercise, particularly searched the literature to find the Exposure to cold is often cited as a lifting heavy weights have a tendency basis for the rule, guess what; they risk factor for DCI. There is human to raise right heart pressures and may found nothing. It was someone’s

9 promote the passage of venous We can see from the above that some blood (and any bubbles) across a of the so-called risk factors for DCI latent patent ovale. (such as heavy underwater work) are undisputed and should be carefully Rewarming considered in our diving planning strategies. Others (such as As has been mentioned previously, dehydration) are less well the solubility of dissolved nitrogen established, but given the falls as the temperature of a tissue uncertainty we should probably rises. The main implication of this assume they are significant and act is that if rewarming of cold accordingly. Finally, one or two (such peripheral tissues is conducted too as conservative reverse profile dives) quickly (for example, by jumping have been discredited. Not everything straight into a hot shower), then it is always as it seems. may precipitate bubble formation We can see from the above that some and DCI. There are at least several of the so-called risk factors for DCI published case reports of DCI (such as heavy underwater work) are arising under such circumstances. undisputed and should be carefully considered in our diving planning Altitude exposure strategies. Others (such as dehydration) are less well Ascent to altitude following diving established, but given the favours nitrogen bubble formation uncertainty we should probably and growth, and is an undisputed assume they are significant and act precipitant for DCI. Ascent to as accordingly. Finally, one or two (such little as 300m soon after diving has as conservative reverse profile dives) precipitated DCI in sport divers have been discredited. Not everything leaving dive sites by road. However, is always as it seems. the most troublesome altitude issue is the prescription of a safe interval between diving and flying in passenger aircraft, most of which are pressurised to the equivalent of approximately 2400m altitude. Limited human data suggest that a 12 hour delay is sufficient following a single no-decompression dive, and a longer but undetermined period is required after multiple dives. Unfortunately, no pragmatic recommendation will prevent all cases. Unequivocal DCI has occurred in sport divers with no pre- flight symptoms, who delayed flying more than 48 hours after diving.

Alert Diver, SEAP 10