DIVE LOG AUSTRALASIA FEB 2025 ISSUE 410

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numbers of gas micronuclei or other places where bubbles can easily form in blood or tissues. The difficulty in confirming a finding like this in humans is our inability to intentionally subject humans to such dangerously provocative decompressions in experiments. Indeed, although using actual DCS as an outcome measure in human experiments can be justified in some circumstances, the risk of the experimental dives cannot be too high and the studies need huge numbers of subjects if we are seeking meaningful differences in outcome when testing a protective strategy or a different decompression strategy etc. It follows that such trials almost never get done for ethical and economic reasons. It is much more common to use ultrasound counts of the venous bubbles that form after many safe dives as an indicator of “decompression stress”. This involves a presumption that reducing these bubble counts indicates a safer dive. Pre-dive exercise has undergone this type of testing in humans, and the results are confluent with the animal studies. Intense exercise at any time between 24 hours and immediately prior to diving has been shown to reduce the formation of bubbles in the venous blood of human subjects after diving. There are 5 or 6 papers that have looked at this and there were no contrary results (although one did show benefit of exercise at 2 hours but not 24 hours prior to diving). This level of confluence strongly suggests that the effect is real and something that we should take notice of. It does seem likely that exercise prior to diving will reduce the risk of DCS. Translating that finding into real-world practice is challenging and given the risk of DCS is very low in typical recreational scuba air diving, it is not necessary to aggressively advocate the “exercise before diving” strategy. However, it is an option for risk reduction that technical divers performing more risky dives might bear in mind. “Intense exercise” means something like a 30 – 45 minute run or cycle ride as fast as you are comfortable going, and this level of EXERCISE AND DIVING: DOES IT AFFECT RISK OF DECOMPRESSION SICKNESS Professor Simon Mitchell, University of Auckland.

`xw|v|Çx I wrote in my last article that one of the subjects of consistent interest to divers is the issue of how to minimise their risk of suffering decompression sickness (DCS). I pointed out that everybody appreciates that the depth-time profile of a dive is of paramount importance in determining this risk. But what divers are frequently referring to when they ask questions about risk minimisation are those factors other than depth-time profile that may be relevant. In the last article I discussed the potential effect of water temperature on risk, but pointed out that there are other important factors such as exercise. It is exercise that I will discuss in this article. The relationship between exercise and risk of DCS is one of the most complex issues in the pathophysiology of this fascinating disorder. It turns out that exercise can be your friend or enemy, depending on when you perform it, and to some extent on its intensity. Specifically, exercise can have very different implications depending on whether it occurs before a dive, during bottom time, during decompression, or after the dive. For my money, one of the most intriguing discoveries in diving medicine in the span of my career has been the finding that pre-dive exercise may reduce the risk of DCS, perhaps dramatically so. This started in the early 2000s with an experiment by a Norwegian group which aimed to determine whether being fit reduced the risk of DCS. The work was performed using a rat model of DCS (yes, you can train rats to be fit – or not). To cut a long story short, they showed that even a single bout of exercise about 20 hours before a decompression that resulted in death of most rats seemed to prevent serious (and sometimes any) manifestations of DCS. It did not seem to relate to being fit so much as the timing as the last bout of exercise prior to diving, and even a single properly timed exercise session was protective. There has been much discussion of the possible mechanism, and this is beyond the scope of this article. It probably has something to do with transiently reducing 15 Dive Log Australasia #410 February 25

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