Very quick question to the décompression science experts on ScubaBoard:
Where can I find a thorough and current (2018) scientific discussion about why the bubble theoretical models may have apparently less appeal, following recent scientific studies ?
Hello Roger,
The answer to this question is simple. It is because the only evidence from comparative human studies suggests that bubble models over-emphasise deep stops and that the decompressions they prescribe are therefore not optimally efficient.
A more nuanced answer is as follows:
Bubble model / deep stop approaches to ascent from bounce dives came to the fore in the early 2000s. The field was ripe for their introduction because (not surprisingly) there was an incidence of decompression sickness (DCS) in technical dives (which there always will be no matter how we decompress), Doppler technology had shown that divers were forming bubbles even when sticking to their prescribed decompression protocols (most of which were Buhlmann based), and bubble models were claimed to prevent or at least control bubble formation. To add to this a very influential diver (Rich Pyle) reported his anecdotal experience of feeling better after decompressions that included one or two pauses at deeper depths to decompress fish swim bladders (he is a marine biologist who captures fish specimens at depth). These so-called "Pyle stops" became conflated with (and considered supportive of) deep stops prescribed by bubble models even though they are quite different. In any event, you get the picture. There was a perfect storm of circumstances that bestowed a powerful theoretical attraction on a bubble model approach that promised to control bubble formation better than the alternatives.
It is important to understand, however, that there was not a shred of evidence that bubble models achieved either a reduction in bubbles or DCS. The simple fact is that bubble models (or use of GFs to make Buhlmann look like bubble models) were widely adopted on the basis of theoretical attraction alone and absolutely no testing. I was one of those who, somewhat unobjectively, believed in the theory.
Such was the strength of this theoretical attraction that the first comparative human study which showed greater bubble formation after a deep stop decompression (published in 2005 by an expert French Navy group) was largely ignored.
Then came the NEDU study.
The NEDU study is widely misunderstood, though everything anyone needs to understand it appeared in the famous Rebreather World threads. Many divers simply say that it was air diving and air decompression and therefore it is not relevant. However, the study demonstrated that protecting the fast tissues from greater supersaturation early in the ascent did not seem to confer any benefit, and that the most likely explanation for the worse outcome in the deep stops profile was the greater supersaturation in the slow tissues later in the ascent (because slow tissues had continued to take up gas during the deep stops).
An important concept that was introduced to the public during discussion of the NEDU study was that of comparing different approaches to decompression from the same dive using integral supersaturation. Supersaturation exists when the dissolved tissue gas pressure exceeds the ambient pressure, and it is the primary driver for bubble formation. Integral supersaturation is the integral of supersaturation and time - which is important because the longer you are supersaturated, the longer bubbles have to form. There is a strong biological plausibility to using integral supersaturation in comparing the likely efficiency of two decompression approaches from the same profile. Evaluation of integral supersaturation correctly predicted the outcome of the NEDU study.
To answer your question about whether bubble model designers were involved in planning the NEDU study, the answer is absolutely yes. The report's authors are the world's foremost decompression modellers and they were testing their own bubble model (with an expectation that it would be superior). Non NEDU bubble modellers were also involved in pre-study meeting and, for example, a RGBM profile was proposed that was similar to the NEDU bubble model profile, but it had some even deeper stops. This is shown on pages 180-183 of the pdf I have uploaded to this post, along with a cogent argument against the notion that the even deeper stops would have made any difference to the outcome of the study. You should read these pages. Even though Ross tries to deny it (effectively arguing that VPM works on low conservatism but not high conservatism), you can generate a VPM profile that also looks very similar using a conservatism factor of about +7. This is well within the parameters of the VPM model and is a perfectly legitimate application. Notably, the integral supersaturation produced during these bubble model decompressions from the NEDU dive are higher than the NEDU shallow stops profile. There is little reason to believe that they would have produced a better result.
Leaving NEDU aside, there have been other relevant comparative human studies, commentary about which has been linked to elsewhere in this thread. These include the Spisni study of ratio deco, an unpublished Swedish Navy study, and a non comparative human study that demonstrated consistently high bubble grades after VPM decompressions. It is notable that all of these studies involved dives and gases much more like those commonly employed by technical divers and none were favourable to bubble models or deep stop approaches.
These studies have demonstrated that bubble models or other approaches to decompression that emphasise deep stops may not be as efficient as approaches with less emphasis on deep stops. By "efficient", I mean that if you are going to spend a particular amount of time decompressing from a dive, the risk of DCS is probably lower (maybe by a small amount – but nevertheless lower) if you follow a path with less emphasis on deep stops than a bubble model / deep stops approach. The simple incontrovertible fact is that the ONLY relevant evidence available as of this date suggests that widely adopted but untested “deep stop” approaches to decompression over-emphasise the deep stops. This does not mean you can’t use bubble models or other deep stop approaches, or that you will die or suffer DCS if you do. Far from it. Indeed, the real risk differences between the various approaches may be small, and as I have said before; if bubble models appear to work for you, then keep using them. But if you want the truth in the universe about optimal decompression then there is a very strong signal in the current literature that bubble models (or similar approaches that emphasise deep stops) are not it.
My problem is that I cannot tell you exactly how far to back away from bubble model prescribed deep stops if you are inclined to do so. Every decompression will have an optimal deepest stop. We just need to figure out a better way to define it.
Simon M
