Suunto Tech RGBM dive profiles...

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The simplified model used in that video, is not real and it is impossible to even create. It has no oxygen, and that missing most important aspect, changes how the off gas aspect / slow tissue on gas actually works. If the video actually used real math and real model gas formula, then the argument and conclusion diminishes to frivolous.

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.

For a decade, the standard of dive planning was VPM-B / RGBM or a GF plan that emulated those. During that same period, tech training and diving grew significantly, using that planning as a basis. Recreational diving even adopted some of the bubble model attributes.

Also during that same period, the number of DCS treatments (both tech and rec) reduced to the lowest on record.

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:

Haas - DCI treated in Auckland-2.jpg


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.

verified by nothing more that the unsubstantiated opinion of one person.

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
 
Running the dive plan on my Perdix with VPM-B +2 gave this profile for the same depth and BT:
Deco stops (depth/time): 100/1, 90/2, 80/3, 70/4, 60/5, 50/5, 40/7, 30/14, 20/20, 10/34. DT = 95.

The NEDU deep stop profile doesn't come close to the VPM-B profile. The deepest stops for VPM-B are very short compared to the Navy's deep stop schedule (1,2,3,4 min vs. 12,17,15,18 min.). Also note that VPB-B gives much less DT.

Yes, of course. The reasons for the longer profiles were clearly stated in the NEDU study methods (see also the link below).

But the question you need to ask yourself, is given your 95 minutes of decompression, what is the best way of distributing the stops? As I said in my post above the deepish distribution you describe above will result in greater slow tissue loading and greater slow tissue supersaturation later in the ascent. That pattern was what was found to be disadvantageous in the NEDU study. You claim it is all about magnitude / duration of the stops, but every other study that has looked at deep stops (some of which involve profiles that are much more `real world`), has found they result in more decompression stress.

The conclusions drawn by Mitchell et al, are not based on actual scientific evidence (because no VPM-like profiles were tested) but were based on assumptions carried over to general deep stop dives. If you're going to test deep stop profiles based on bubble models than use a profile realistic to those used by actual PDC's that generate deep stop profiles. Duh!

There is now a moderate amount of human evidence that is corroborative of NEDU from studies using deep stop approaches applied in recreational diving. It is not perfect, but notably it is all pointing in the same direction. In contrast, you do not have a single human study that supports your apparent advocacy for deep stops in decompression diving. Not one.

The overriding conclusion was that the decompression strategy producing the least bubbles on surfacing was the correct one. However, this conclusion neglects the major science behind deep stops in that a lower gradient at the shallow stops may generate DCS due to larger size bubbles in the tissues. Deep stops as the theory goes provide smaller radius bubbles, an hence higher internal pressures, to reduce the gas flow into those bubbles. As bubbles grow in size their internal pressures drop causing lower gradient inert gas to enter those bubbles and enlarge them even more.

What "major science"? Every human study that has examined the issue has demonstrated greater bubbling after surfacing from dives that place emphasis on deep stops. Are you seriously suggesting that despite this, somewhere in the body where it really matters the absolute opposite process is magically occurring?

I understand the motive for conducting the study as it was done. The Navy wants its divers to spend more time doing deco in shallower rather than deeper waters. If the divers get in trouble it's much easier to provide support in shallower than deeper seas.

That was not the motive for the study. If you have not read the excellent explanatory post by David Doolette on RBW then I seriously suggest that you do:

Deep stops debate (split from ascent rate thread) - Page 14

Simon M
 
******MOD POST******
This thread is being closed to further replies. The original question that started the thread was completely unrelated to the NEDU study discussion
MilosDJ:
For some (yet) unknown reason, I am unable to install Suunto DM5 software on my computer. So I need little help.
and the OP subsequently indicated in Post # 15 that the question originally posed had been answered.
MilosDJ:
Thanks Ken, I solved the installation problem.

Unfortunately, the thread was derailed by an entirely off-topic post (#4), but that problem wasn’t caught until additional off-topic discussion had continued. Nonetheless, the NEDU study design and results / bubble model issues/ etc. are being discussed in at least three other threads.

NEDU Study

In-Water Recompression, Revisited

UTD Ratio deco discussion

If you wish to comment on the NEDU study, various decompression models, etc., feel free to use one of the current threads to do so. The discussion is useful, but in the interest of efficiency, and consolidation of information, please focus it in other existing threads, rather than continue it here.
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https://www.shearwater.com/products/teric/
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