Bubble model vs. Gradient Factors redux

[Dr Simon Mitchell]

Thanks for your response in this thread. Given no comparitive human study has demonstrated benefit in deep stops, I'm curious why you believe the ideal strategy "is almost certainly not as far as raw Buhlmann". Buhlmann's work was based on real dives, and had some rational method to determine the parameters he did. Why do you suspect he could have been neglecting the importance of protecting the fast tissues.

I ask that because I'm interested, not stating I disagree with your suspicion. When I can dive again I can assure you mu gf lo will be lower than my gf hi.

Hi Elmo,

Good question. "Almost certainly" may be too strong, and I was relying on the following phrase "but that information is still to come" to provide important context around the uncertainty. Having said that, Buhlmann's testing was not exhaustive, and focused on depth ranges that were mainly shallower or (in a few cases) deeper than than the 60 - 90m depth range typically of interest to technical divers. David and I mention this in the paper I have uploaded with reference to Buhlmann's original work [1]. There is a slightly fuller discussion in a recent book chapter [2]. I am mindful that although the available evidence is signalling that we should avoid over-emphasizing deep stops, the fundamental principle behind them is not stupid, and every decompression has to have a deepest stop; we just have not figured out quite yet "how deep is optimal". Call it intuition as much as anything else, but I suspect that GF 100:- is probably too much supersaturation in faster tissues early in an ascent from a deepish technical dive. But I don't know for sure.

Hi @Dr Simon Mitchell

How do you feel about other markers for decompression stress other than bubbles, the chemokines obtained in the Spisni study for example? Are there other markers that would prove valuable?

Hi scubadada,

Unfortunately there is no well validated biomarker for decompression stress. The best characterized index is venous gas emboli (VGE), but as you point out there are other markers such as chemokines / inflammatory markers (including [possibly] microparticles), and certain physiological parameters like flow mediated dilation of blood vessels. But they are all fairly "soft". Notwithstanding the acknowledged poor positive predictive value for VGE in 'diagnosing' DCS, VGE remain the strongest surrogate for decompression stress we currently have to work with.

Simon

1. Doolette DJ, Mitchell SJ. Recreational technical diving part 2. Decompression from deep technical dives. Diving Hyperb Med 43, 96-104, 2013
2. Mitchell SJ, Doolette DJ. Extreme scuba diving medicine. In: Felletti F (ed). Extreme Sports Medicine. New York, Springer Publishers, 313-333, 2017

 

[EFX]

So somebody needs to test a "conservative" VPM-B profile with an "aggressive" Buhlmann profile with the same run time Bubble model vs. Gradient Factors redux As an example that could the NEDU profile with VPM-B+4 vs GF 66/66. Something like that would maximize the differences between deep stops with VPM vs. shallow stops with Buhlmann. Somehow, I don't see this happening in the current environment.

Yes, without thinking a lot about comparing those profiles I think that could be a good place to start. You still need to make both profiles more aggressive to enable a certain number of bent divers (the NEDU study did this) or, if the product of bubble size and bubble quantity (or some other similar metric) correlates with increasing symptoms.

What do you mean by environment; Because of covid-19 or deco method bias?

 

[scubadada]

Yes, without thinking a lot about comparing those profiles I think that could be a good place to start. You still need to make both profiles more aggressive to enable a certain number of bent divers (the NEDU study did this) or, if the product of bubble size and bubble quantity (or some other similar metric) correlates with increasing symptoms
What do you mean by environment; Because of covid-19 or deco method bias?

I disagree, You can't purposely bend divers with current informed consent. That was he Navy. I think that bubble detection, and other surrogate markers will have to do with regard to data collection.

 

[dmaziuk]

I posted this a couple of times already:

While you'll probably never have a first deco stop at 60 msw, the difference in allowed supersaturation for fast TCs (vs. the rest of them) at that depth is quite striking.

 

[Dr Simon Mitchell]

See post #68 and the attached file. Magically!? I guess without any scientific evidence it would seem like magic wouldn't it. I completely agree with you that if supersaturation was all that mattered then stopping shallower rather than deeper is far better. However, I want to know if bubble size (BS) matters also. The only way we're going to know is if we test a deco model that actually calculates BS. It would be nice if we can measure BS.

Hi again EFX,

The powerpoint is emblematic of the combination of confident statements and complicated looking maths that seduced our community (including me) into embracing bubble models 20 years ago. But I'm afraid I have little remaining faith in much of what is portrayed. The best bit is this:

But because they make similar suggestions, any of these bubble models is likely to be superior to standard “Haldanean” tables.

A logically fallacious conclusion that because all the bubble models are saying the same thing they must therefore constitute the superior approach to decompression.

The invocation of "magic" in my previous comment was entirely intentional, because that is what it would take for the claim to be true. In a slower tissue that has become more supersaturated (and for a longer duration) at the surface after a bubble model decompression than after a gas content model decompression from the same dive, explain to me how the supersaturated gas would "know" that because the diver is following a bubble model decompression it should form a greater number of smaller bubbles rather than fewer larger bubbles, and (for that matter) how it would go about doing that?

Simon M

 

[rjack321]

I posted this a couple of times already:
View attachment 583867
While you'll probably never have a first deco stop at 60 msw, the difference in allowed supersaturation for fast TCs (vs. the rest of them) at that depth is quite striking.

Its not hard to have a first stop around 60m even diving something like 50/75 (which is what I dive). 25mins at 100m would have first stop at 60m (10/70 diluent, ppO2 = 1.2)

 

[dmaziuk]

Its not hard to have a first stop around 60m even diving something like 50/75 (which is what I dive). 25mins at 100m would have first stop at 60m (10/70 diluent, ppO2 = 1.2)

Fair enough. I just made the line go to 7 because it sounded like a round number at the time.

For comparison, here's what it looks like for ZH-L12:

(note different half-times)

 

[rjack321]

Hello,
Yes, of course it is possible, but it is not particularly useful - not if you are seeking the 'truth in the universe' about optimal decompression strategy.

Lets say you believe in deep stops, and that's why you are interested in 10/85. And lets say that the 10/85 profile for a hypothetical dive prescribes 85 minutes decompression compared to 70 minutes for the 50/85 profile. What would a comparative study of these two profiles tell you?

Could it tell you that one profile is associated with better outcomes than the other? - YES it could.

And lets say your 10/85 profile has better outcomes, does that prove that deep stops are a good idea? - NO it doesn't

It doesn't because the 10/85 profile prescribed a longer decompression and you can't separate that from the effect of the stop depth distribution.

Well it is possible to do a more complicated study design varying between 10/85 and 50/85 while also varying the total deco time. A 2x2 latin square. Its 2x as expensive but does (potentially) address this persistent question of:

Is the shape of the deco more or less important than the time?

(I am pretty sure I know the answer to that. I am only suggesting an alternative design because the "debate" about shape vs time has been going on for many years.)

 

[Dr Simon Mitchell]

Is the shape of the deco more or less important than the time

Hi rjack.

I still have not done a good enough job of explaining this.

The above is not really a "persistent question" - not among decompression physiologists at least. Pretty much anyone can design a safer decompression - just make longer. Put another way, the role of decompression time is undisputed - there is no argument about it - so long as you don't invest the time in the wrong shape, longer is better in terms of allowing inert gas to be eliminated safely. So why perform a study to see if longer decompressions are better?

Let me try to come at it from another angle. One of the related arguments I hear is something like:

"I think deep stops are OK so long as you do enough shallow stops to compensate (a fair hypothesis), so I would like to see that sort of profile tested. So why don't we test 5:60"?

Yes, you could test this (very long decompression), and yes, you could get a result. But you are still faced with the inevitable question: what if I did the same long duration of decompression but distributed my stops differently - could I be even safer still?

Thus, the principle debated issue in technical bounce diving decompressions is the shape. How much emphasis should be placed on deeper stops? Where is the sweet spot for first stop depth? Does the optimal shape change with different depths? Once we definitively understand these things I can pretty much guarantee you that a longer decompression (particularly shallow oxygen stops) will be safer from a DCS point of view.

To be fair, testing lots of combinations of shape and time would not be entirely without value if we could do it easily. But this sort of work is time consuming, expensive, and difficult to perform.

Simon M

 

[rjack321]

To be fair, testing lots of combinations of shape and time would not be entirely without value if we could do it easily. But this sort of work is time consuming, expensive, and difficult to perform.

Simon M

No I get it. I wasnt actually suggesting spending double the money and doubling the hassle to do 2 variables at once. I guess this issue seems to come up over and over amongst divers/forums and I was throwing out a concept that, at least statistically, its not a either match time or don't kind of question. You could given time and money do 2 variables at once. The reason its not done is because longer is almost universally safer. I am personally quite satisfied that longer is "safer" in general, unless the extra time is added to intermediate stops in which case the added on-gassing may offset the longer duration for no change in risk.

Richard