Deep Stops Increases DCS

[UWSojourner]

Leadduck
You are absolutely correct that any useful model must have a higher (less negative likelihood) than the null models such as the 1-step or 6-step sets. This same point was made earlier in this thread (back in post 424 on page 43) by Daniel Mewes. It is not clear what Wienke means by no fit value will be better than the 6-step set.

I made the same point in a lengthy commentary on this Deep stops model correlation manuscript (before it was published) and an earlier paper on another forum 2 years ago (still waiting to hear back from the author).

Correlation of popular diving models with computer profile data and outcomes

I will repost that commentary here, although it makes much the same point as you do, I have a bit of commentary on risk functions etc. that you might find interesting. I will have to split it up to stay under the 100000 character limit.

Any interpretation can only be based on what is written - and most of what is written is unclear - but from what is written it appears that the four models (USN, ZHL-6, VPM, RGBM) fit to the data set in the paper 1) are not actually the four algorithms of those names that are used to produce decompression schedules; 2) they differ from each other only by having different half-time compartments; and 3) none of them fit the data better than simply assigning identical risk to all dives in certain depth ranges, irrespective of how long the bottom time, how long the decompression, and what breathing gases are used (as you point out). This paper contributes nothing to the debate about whether deep stops or shallow stop schedules are more efficient.

David Doolette

Thanks for posting this. It certainly makes sense of some things for me.

 

[EFX]

In the complex world of science, financial conflicts of interest are recognised as a critically important influence on the way data are gathered, analysed and presented. Such conflicts do not come any more obvious and potentially important than in commentary about safety / efficacy of deep stops by someone who sells a deep stop algorithm. This is a discussion about that issue and so it should be declared.

No! This is not a discussion about the safety / efficacy of deep stops by someone who sells a deep stop algorithm. This is a discussion about an NEDU study that is passing off [un]safe extended multilevel deco stops as deep stops and then trying to prove that has any relevance to true deep stop algorithms, despite the heat maps and the obsession with supersaturation, and without any discussion of bubble mechanics.

Every time I give a lecture or submit a manuscript for publication I am subject to intense scrutiny (and rightly so) on this issue. Simon M

Mr. Mitchell, it's the timing of it. Within 2 pages of Igor's postings of BRW's summary findings we're all given this ominous warning. This is not a lecture hall. We are not reviewing anyone's manuscript for publication. This is SB and as Boulderjohn said, there are lay people coming in here who do not need to be swayed by such heavy-handed tactics. Enough said. Let's move on.

 

[boulderjohn]

his is SB and as Boulderjohn said, there are lay people coming in here who do not need to be swayed by such heavy-handed tactics.

Please be careful about invoking my name for this. As an academic, I taught logic and the art of writing effective research using sound protocols. If you even go back to Greek rhetoric, the idea of ethos--the credibility of the speaker--was considered a key component of persuasion. As I struggle to read through this material and evaluate it, the credibility of the speakers is very much an issue. If there is a financial aspect to someone's participation, that is very relevant. (You cannot just claim there is a financial aspect--it has to be demonstrated.) If the speaker has a financial stake in an argument, it is up to the speaker to rise above that argument and demonstrate that the argument holds water regardless of any apparent conflict of interest.

 

[UWSojourner]

No! This is not a discussion about the safety / efficacy of deep stops by someone who sells a deep stop algorithm. This is a discussion about an NEDU study that is passing off [un]safe extended multilevel deco stops as deep stops and then trying to prove the results of that study has any relevance to true deep stop algorithms, despite the heat maps and the obsession with supersaturation, and without any discussion of bubble mechanics.

Mr. Mitchell, it's the timing of it. Within 2 pages of Igor's postings of BRW's summary findings we're all given this ominous warning. This is not a lecture hall. We are not reviewing anyone's manuscript for publication. This is SB and as Boulderjohn said, there are lay people coming in here who do not need to be swayed by such heavy-handed tactics. Enough said. Let's move on.

I don't agree EFX. Not everyone reading now (or in the future) would know without it being part of the thread that 2 detractors of the NEDU study posting here both have financial interest in products that could be directly impacted by it.

It's not heavy-handed to state it. Heck, I have absolutely NO financial interest in any dive product/position and Ross untruthfully "disclosed" that I did. So Dr. Mitchell pointing out actual conflicts of interest (that were NOT disclosed in one of the papers posted) is entirely appropriate.

I agree such conflicts do not automatically invalidate arguments, but they should be known as people absorb the information and listen to the arguments.

 

[Shotmaster]

Running around calling everyone names and generally being nasty doesn't usually work well if you are trying to get people to see things from your point of view. Having a hidden agenda is not good either. It is better to be up front and put it out there. I definitely have new opinions about some of the posters that I didn't have before.

 

[David Doolette]

No! This is not a discussion about the safety / efficacy of deep stops by someone who sells a deep stop algorithm. This is a discussion about an NEDU study that is passing off [un]safe extended multilevel deco stops as deep stops and then trying to prove that has any relevance to true deep stop algorithms, despite the heat maps and the obsession with supersaturation, and without any discussion of bubble mechanics.

The "obsession" with supersaturation versus bubble mechanics is well founded. The putative cause of DCS is injury as a result of bubble formation. Supersaturation is a required condition for bubble formation and growth. There is broad (universal?) agreement that modelling tissue gas uptake with a range of exchange rates - as we do for instance with a collection of compartments with mono-exponential gas exchange - captures the essential processes, at least crudely. However there is no such broad agreement on what models of bubble mechanics are salient and how to parameterize those models, which depends on unknown (and since DCS-sites are unknown, essentially unknowable) values for gas diffusivity, solubility, nuclei radii, bubble number density, diffusion region dimensions, perfusion heterogeneity in that region, shear modulus, surface tension etc. Different collections of values for these unknowns will give different bubble dynamics - so the result is a "my model" - "your model" argument.

A good example of this is the NEDU study, the A2 deep stops schedule was generated by a probabilistic bubble model called BVM(3). The BVM(3)-estimated risk of DCS and the observed incidence of DCS of the Deep stops schedule were statistically indistinguishable, so it worked quite well in that respect. However BVM(3) failed to predict how much lower the incidence of DCS on the A1 shallow stops schedule turned out to be. In one of the recently posted papers Wienke provided an alternative deep stops air decompression schedule of comparable duration to the A1 and A2 schedules. "His" model predicts "his" schedule would be lower risk than either the A1 or A2 schedules. The essential difference between "his" schedule and the A2 schedule is that "his" schedule has not been tested.

 

[The Chairman]

The "obsession" with supersaturation versus bubble mechanics is well founded. The putative cause DCS is injury as a result of bubble formation.

This actually makes a lot of sense.

FWIW, I'm a padder. I'm almost always going to pad my last stop by five minutes. I've cut tables before I dive mostly to determine gas requirements, but not often and I always let my PDC du jour take over. Most of my deco is on cave dives and usually amounts to less than 45 minutes, including the padding. Furthermore, I won't do deco dives for more than two days without a break and then usually only one a day.

I simply don't get the push it, push it, push it mentality. I certainly don't intend to derail this lively discussion, but I think there's a number of people like myself that are wondering if this discussion is relevant to us at all? So what kinds of run times are going to become problematic? If I'm below 80, I'll do a 2 minute half stop. I hardly ever see 200 anymore, but that will change as I am hoping to do trimix on my SF2 in the next couple of months and I'll be using my Petrel IIs for that. So, what's my take-a-way here? Is it this is way beyond what I need to worry about? If not, what should I be changing.

Caveat, probably against most sage advice, I usually dive only in a bathing suit and t-shirt. The cave dives are getting a bit chilly towards the end now, so I have a wet suit on order. I'm also obese by anyone's standard. Kind of like a deco Buddha.

 

[Shotmaster]

Hahahaha-"deco Budda", that's funny!

 

[rossh]

I don't agree EFX. Not everyone reading now (or in the future) would know without it being part of the thread that 2 detractors of the NEDU study posting here both have financial interest in products that could be directly impacted by it.

It's not heavy-handed to state it. Heck, I have absolutely NO financial interest in any dive product/position and Ross untruthfully "disclosed" that I did. So Dr. Mitchell pointing out actual conflicts of interest (that were NOT disclosed in one of the papers posted) is entirely appropriate.

I agree such conflicts do not automatically invalidate arguments, but they should be known as people absorb the information and listen to the arguments.

I don't agree with that.

Simon Mitchell is heavily vested in these arguments, in a different kind of currency - community stature and lasting scientific recognition.

He is a working Dr., and hyperbaric specilaist who has chosen to publish papers and make public opinions. He travels the world giving talks on these and other ideas, based on the strength of his name and reputation. Like others in science, he embarked on a direction and a set of ideas early in his career, and is pursuing that now.

The currency of Simon Mitchell is community stature and lasting scientific recognition. Those are the valuable commodities to academics and take life time to achieve. A career and future prospects hinges on these matters too.

Simon Mitchell is not a neutral or unbiased in any way. He is more heavily invested in the direction this takes, than you or me.



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[ajduplessis]

Wienke said it best IMO. The NEDU Study show us exactly what NOT to do!