Whats the benefit of commercially available devices for the detection of venous gas bubbles after dives?
In addition to the direct effect of gas bubbles in the tissue, arterial gas emboli (AGE) are the cause of decompression illness (DCI). The latter occurs when venous gas bubbles (usually called venous gas emboli, VGE) enter the arterial circulation via cardiac (at the level of the heart, e.g., PFO = patent foramen ovale) or pulmonary (at the level of the lungs, e.g., via intrapulmonary arteriovenous anastomoses, IPAVA) right-to-left shunts, bypassing the pulmonary filter, where they reliably get stuck in the pulmonary capillaries and are exhaled. VGE can be easily detected with ultrasound. Today, commercially available tools that can detect venous gas bubbles after a dive promise to optimize dive profiles and thus minimize the risk of DCI.
Poor correlation of venous gas embolism with decompression sickness.
It has been known for some time that VGE has only a very limited correlation with the occurrence of DCI. At this year's Rebreather Forum 4 (RF 4), David Doolette, one of the leading decompression physiologists from NEDU, confirmed that this is still true. In his state-of-the-art presentation, using unpublished data extracted from published studies, he showed that even the highest-grade VGE, such as I showed in my last article, do not lead to DCI in 90% to 95% of cases. It is true that DCI tended to occur at higher levels of VGE, but only in isolated cases.
"For an individual, measuring VGE ist not a useful tool."- David Doolette, Rebreather Forum 4
Even more remarkable was the analysis of individual risk. Doolette compared intraindividual identical dives, i.e. congruent dives, which one and the same diver completed several times. A large spread of VGE was found. If no or only very few VGE could be detected on one day, many VGE occurred after an identical dive with the same diver on another day and vice versa. Again, there was only a very weak correlation between the extent of VGE and the occurrence of DCI. These results are impressive because of their very large intraindividual variation. Apparently, one tends to have many venous gas bubbles on one day and few on another. The good news is that the risk of DCI is small even with many VGE.
The reasons for this are certainly complex. As mentioned, most gas bubbles are filtered out in the pulmonary capillaries, which explains the vast majority of the discrepancy between VGE and DCI. However, as Doolette pointed out, IPAVA are also likely to be involved in this variance. Under which circumstances they shunt gas bubbles past the lung, however, is still poorly understood.
The benefit of tools for self-detection of VGE after dives is questionable.
These results cast considerable doubt on the usefulness of commercially available bubble detection tools. Certainly, if you always detect a high VGE load after dives, you should alter your diving profiles or habits.
Why even arterial gas emboli (AGE) usually do not cause DCI, is explained in another article in this blog.