405_April_24

bubbles must be passing across the lungs in some other way. Significant post-dive passage of bubbles between the venous and arterial circulation has also been observed via shunts called intra-pulmonary arterio-venous anastomoses (IPAVA), which, like a PFO, allow blood and bubbles to bypass the small capillaries in the lungs that usually trap bubbles. The IPAVA are usually closed but even moderate levels of exercise will open them and allow bubbles to pass into the arterial system. The amount of exercise required differs from person to person, but it can be very little. Once the exercise stops, the IPAVA quickly close again. This might explain why divers still suffer cerebral DCS although they do not have a PFO. Bubbles can still be present in a diver for up to two hours after surfacing. Usually, there will be more bubbles after deeper and longer dives. Some divers are more prone to producing bubbles than others, but we do not know why. Arterial Bubbles and DCS risk Cerebral DCS is rare. But 25% of divers probably have a PFO, and a PFO is not the only factor involved. This begs the question: why does cerebral DCS not occur more often?

This is one possible explanation. Typically, cerebral DCS occurs about 20-30 minutes after surfacing, which is when the volume of bubbles in a diver’s body is at its peak. After a dive, all body tissues desaturate at different rates. The brain is a very fast tissue, and the nitrogen saturation of brain tissue halves every 12.5 minutes. Therefore, 20 to 30 minutes after the dive, the brain will normally be desaturated to such an extent that bubbles passing into and lodging in brain tissue will shrink and disappear rapidly, causing no harm. Key Points 1 . Post-dive bubbles are normally eliminated by the lungs, but, in certain conditions, they may cross over to the arterial side. 2. Arterial bubbles occur more frequently than previously thought, but often without acute negative effects. 3. Post-dive exercise, especially exercise involving holding your breath briefly, may increase DCS risk. All divers, not only those who know they have a PFO, should avoid it.

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DIVE LOG Australasia #405 - April ‘24