Ross, the figure you object to in that video is a simple depiction of an unarguable concept. If you have a fixed amount of time to spend decompressing and you distribute more of that time to deeper stops, then you are going to take up more gas into slower tissues during those deep stops, and those slower tissues are going to become more supersaturated later in the ascent. This is a simple, inescapable physical truth.
In the early 2000s we believed that this was an acceptable compromise based on the hypothesis that deep stops and reduced supersaturations in faster tissues early would prevent / control bubble formation in decompression diving, but every piece of evidence that has emerged since that time suggests that stops as deep as prescribed by bubble models are too deep, and that they result in greater bubble formation / decompression stress (multiple studies) and greater risk of DCS (NEDU study) than would occur if we distributed our decompression time shallower. The only plausible explanation is that protecting the faster tissues early to the extent that deep stop approaches do, is not worth the extra decompression stress that accrues in slower tissues later.
THAT is what the `science` says, and if necessary, I will lay it out, human study by human study here, as I have done on other boards. Can you show me ONE human study that supports your claim that deep stop approaches are superior in decompression diving?
In the interests of open disclosure, no one would claim that the data are definitive, and in particular, what we are not sure of is how much shallower our deepest stops should be. But the overwhelming weight of evidence as of this date is that deep stops are over-emphasized by bubble models. This does not mean that that bubble models do not work or that you should not use them... but it does imply they are not the most efficient way to decompress.
Once again, the flaws in this argument need to be illuminated for several reasons, not least because they are a window into the level of your 'science'.
There is one kernel of truth in it all. Many hyperbaric units around the world (including my own) have witnessed a decline in numbers of DCS cases over the last 20 years. But the idea that this is in some way linked to improvements in technical diving decompression is ludicrous. Here is why.
First, DCS case numbers on their own are meaningless in the absence of a corresponding denominator that would allow calculation of a rate (eg cases per number of dives or divers). For example, the decline in case numbers could simply represent a decline in diving activity. There is some evidence that this is actually the case. We published a clear decline in case numbers from 1996 to 2012, but there was a very similar decline in new diver certifications over the same period. [1] See this figure:
Second, DCS case numbers come from the entire diving world. Nobody has systematically reported technical divers separately, and technical divers represent a tiny proportion of the total case load. Thus, it would be entirely possible for case numbers to be declining overall because of fewer cases among the vast group of scuba air divers who don’t do decompression diving, but for technical diving cases to still be increasing. Ross has no way of knowing whether this is so but it is entirely plausible.
Third, in 2004 (right in the middle of the decade he refers to) the publication of the Mild and Marginal DCS Workshop Proceedings by DAN and the UHMS resulted in a major paradigm shift in the way DCS cases are managed globally. [2] Essentially the workshop defined “mild DCS” and then legitimized the option of managing such cases without evacuation and recompression particularly where recompression is difficult to access. This resulted in many cases of DCS that would have been recompressed before the workshop, not being recompressed over the subsequent years. This is almost certainly a contributor to declining case numbers and it has nothing to do with decompression efficacy.
Finally, as I have alluded to, there are very few data reporting technical diving DCS case numbers or rates, and the data that exist suffer from all the known confounders associated with the means by which they were collected (usually voluntary reporting or surveys). Retrospective survey data typically underestimate the true incidence of medical problems – often markedly so. If you look for problems prospectively you always find more. I know from personal experience that the incidence of DCS (albeit mainly mild) among technical divers is at least an order of magnitude higher than Ross thinks it is. I have been medical officer on at least 10 major technical diving expeditions and we see DCS frequently. In the last one involving 45 divers at Truk 5 divers (>10%) had a DCS event (and those were the ones I found out about). I am certain that most DCS symptoms in technical diving, specifically at the mild end of the spectrum, are self-treated and are never reported.
In summary, Ross has essentially no basis whatsoever for claiming that technical diving has become safer over the last 15 years, and even less for suggesting that safety improvements are due or partly due to use of bubble models.
Ross is fond of portraying me as the only dissenting voice. In fact, the opposite is true... its just that none of my colleagues can be bothered. Ross is conveniently ignoring the fact that he has had Neal Pollock and David Doolette on these forums telling him he is wrong. Despite the fact that multiple experts in the diving science community have told him he is wrong, he continues to insist he is right.
Simon M
References:
1. HAAS RM, HANNAM JA, SAMES C, SCHMIDT R, TYSON A, FRANCOMBE M, RICHARDSON D, MITCHELL SJ. Decompression illness in divers treated in Auckland, New Zealand 1996-2013. Diving Hyperbaric Med, 44, 20-25, 2014
2. MITCHELL SJ, DOOLETTE DJ, WACHOLZ C, VANN RD (eds). Management of Mild or Marginal Decompression Illness in Remote Locations – Workshop Proceedings. Washington DC, Undersea and Hyperbaric Medical Society, 240pp (ISBN 0 9673066 6 3), 2005
