UTD Ratio deco discussion

[Kevrumbo]

I'm not sure I understand why you feel decompression is affected by gas density. Can you explain that please? In any event, in the key comparative studies it was only the decompression strategy that was different. The same density gases were breathed at depth in divers using each decompression strategy.

My view on the possibility of gas density impacting decompression, is based mainly in the potential for CO2-retention due to gas density, and following impact on decompression due to any such increase in CO2.
I came across an interesting article about the matter, linking to it here:
Relationship between CO2 levels and decompression sickness: implications for disease prevention. - PubMed - NCBI

Mentioned briefly in this sadly unreferenced article by DAN: Carbon Dioxide Level | Decompression Sickness - DAN Health & Diving

If gas density and CO2-retention are related, and increased gas density is higher on air, surely the results of two dives with similar dive profile - using deep stops - on air and a lighter gas respectively, would have differing results not solely as a result of different gas contents, but also gas densities, and the gas contents cannot be said to be an isolated factor?

Following, if two dives on the same dense gas (air) use respectively a shallow profile (low density) and deep profile (high density), a change in decompression effect cannot be said the be isolated to the profile? The gas densities would be different, too.
Hence my concern that using air on a typical trimix dive (high density instead of lower), would skewer decompression results in favor of a shallower stop every time, regardless of how such profiles might work if using a more appropriate mixture (in terms of gas density).

@Dr Simon Mitchell , is @Dan_P inferring that a lower and easier Work-of-Breathing (WOB) of a dense gas like Air at shallower depths is skewing the NEDU Study results to favoring shallow decompression stops?

Nevertheless @Dan_P , the same disadvantageous pattern of slow tissue supersaturation remains within the RD strategy and has to be compensated for in practice with extended time at the shallower stops.

 

[Dr Simon Mitchell]

Hello Dan,

Hi Simon,

My view on the possibility of gas density impacting decompression, is based mainly in the potential for CO2-retention due to gas density, and following impact on decompression due to any such increase in CO2.

I think you are over-extrapolating here. Even if we assume that the association between CO2 and DCS postulated in that paper is real (which is not certain) the CO2 "conditions" in that study compared to the various studies that have investigated deep stops are very different indeed.

In the Mano study the "high CO2" caisson workers were inspiring CO2 at dangerously high levels for substantial periods. This will have ensured a either a degree of CO2 retention or, at the very least, a physiological response (such as an increase in ventilation and cardiac output) to keep the CO2 levels normal in every worker. Under these circumstances it is certainly plausible that the CO2 exposure could affect inert gas kinetics and DCS risk. The situation in the various deep stop studies, with one group of divers (deep stops) breathing a slightly more dense gas than another group (shallower stops), is completely different. In none of the deep stop studies did the density of the gas breathed at the deep stops exceed the threshold (6g/L) that Gavin Anthony's data demonstrated resulted in an elevated risk of CO2 retention (and that was in exercising divers - not divers resting on decompression).[1] We have shown that divers resting on decompression do not tend to retain CO2 (albeit at the shallowest stop) [2] - but I reiterate that gas density did not exceed the risk threshold at the deepest stops in any of the deep stop studies. In addition, the differences in density between deep and shallower stops in the relevant studies was actually quite small.

What all this adds up to is that whereas every worker in the Mano study was either retaining CO2 or working harder to eliminate it, you cannot assume that any divers were doing the same at deeper stops (relative to shallower stops) in the deep stops studies.

Good on you for thinking laterally, but you are over-extrapolating. The gas kinetic differences in deep an shallower stops profiles (eg in the NEDU study, but also other examples) have been carefully analysed, and the deep stops profiles exhibit greater integral supersaturation (an integral of supersaturation and time). Supersaturation is what drives bubble formation. The principle of Occam's Razor dictates that this is the most likely explanation for the results of the deep stops studies. We don't need to look for more esoteric explanations when there is a very obvious one staring us in the face.

Happy to discuss further. Reference 1 can be downloaded if you google "rebreathers in scientific diving proceedings".

Simon M

1. ANTHONY TG, MITCHELL SJ. Respiratory physiology of rebreather diving. In: Pollock NW, Sellers SH, Godfrey JM (Editors). Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015 Workshop. Wrigley Marine Science Center, Catalina Island, CA, 66-79, 2016

2. MITCHELL SJ, MESLEY P, HANNAM JA. An observational field study of end tidal CO2 in recreational rebreather divers on surfacing after decompression dives. Aerospace Med Hum Perform 86, 41-45, 2015

 

[Storker]

Hi Storker,

Bruce has never been short of equations! But its all just theory

"Theory" as in the scientific meaning of the term (like "Darwin's theory of evolution" or "Newton's theory of gravity" or "Einstein's theory of relativity"), or "theory" as it's used in tv crime shows ("I have a theory"), i.e. pure speculation?

Sorry, I just couldn't resist the bait...

 

[Dan_P]

@Dr Simon Mitchell thanks for the kind words!

I'll need to give this a serious look in regards CO2 and it's relevance to DCS;
D.E. Busby's "Space Clinical Medicine" seems like an interesting read for myself, particularly as it states "...CO2 markedly increases an individual's susceptability to decompression sickness.", but whether the application can possibly be said to be direct in the context we're discussing, will require some scrutiny (after all, it's called "Space Clinical Medicine" and I'll need to look at levels as well as conditions).

I shall also ask DAN for a reference to the statement about the matter, as previously linked to.

I admit I am preoccupied with a notion that CO2-retention from the bottom portion of a deep air dive, as well as reduced ventilation capacity from dynamic airway compression related to depth/density (even so it's relating to exposures of lessor levels than gas density=6g/L at work), may have an interenforcing effect - and that I don't understand how this can be accounted for in isolation of possible factors in a study utilizing air on dives to such depths.
Have you insights on the process that can help me understand that last portion, please?

DAN has also been quoted for a position that "deep stops" (again, the vexing terminology ambiguity) are benefitial on shallower recreational dives - that's of course a different can of worms, as the exposures are fundamentally different in relation to slow tissue saturation.
For that latter statement, I have already asked DAN for a reference, and am eagerly awaiting reply.

All at the same time, I feel it's in place to make clear that I'm not conflicted by a notion of shallower stops compared to RD1.0 as tested, nor that bubble models haven't yielded the once widely expected results. I appreciate that slow tissue supersaturation is an issue here.
But I remain very cautious of getting (too) up in arms about this matter on the basis currently available, Occam's Razor notwithstanding.

 

[Kevrumbo]

@Dr Simon Mitchell , is @Dan_P also inferring that localised tissues are being further inert gas loaded by increased blood perfusion as a result of CO2 caused vasodilation? Especially the "double whammy" of a dense gas at deep depth AND maximal FN2 like Air. . .

 

[Dan_P]

@Kevrumbo I don't see a basis for certainty to prompt a claim towards it, but I certainly see how CO2, if it has an impact on deco, might cause reasonable concern of skewed results in a deep air trial unless mechanisms of impact are fully understood and accounted for.
I'm not confident the latter is the case as I lack understanding of how such an adjustment would/could be calculated and taken into account in practice, both method and analysis.

But it seems obvious to me that any effect from vasodilative property, if present, would be diminishing with approximation to the surface, thus favoring shallow stops to an increasing level due to gas density rather than whichever other metric one might wish to isolate (even irrespective of macroscopic properties and general gas kinetics across helium and nitrogen).

Further, CO2's specific permeation coefficient is mentioned by D.E. Busby's "Space Clinical Medicine" as reason for CO2's major early constituency in bubble formation at altitude (space).

I note that this is in addition to insecurity as to the nature of relation across VGE and DCS, with VGE being a cardinal measure of differentiation.
Surely, for all we know, VGE could technically speaking be alinear in nature, irrelevant or even a positive thing in terms of decompression.

At least in principle, the same could be said to hold true for inflammation markers.

Also, I'm not sure whether or not the difference in results prompts a shift in emphasis on magnitudes of strictly monumental proportions in the first place.

However;
As our scientific understanding evolves, we continue to have an improved basis of understanding to build our decompression upon - regardless if one wishes to one day use an algorithm near-perfect, or a planning system to closely approximate it (such as RD).
I feel it's important to reiterate the difference between the two.

 

[Kevrumbo]

note that this is in addition to insecurity as to the nature of relation across VGE and DCS, with VGE being a cardinal measure of differentiation.
Surely, for all we know, VGE could technically speaking be alinear in nature, irrelevant or even a positive thing in terms of deco

NOT a "positive thing" or "irrelevant" per se @Dan_P, but benign with regard to fast tissue supersaturation and the results of the NEDU study, hence obviating or eliminating the need for deepstops.

 

[Dan_P]

Well, technically, for what we know, maybe.
Alinear, more probable, I think.
It's place not fully understood, definitely.

But I think it's fair to say we'll need a different trial, to be certain of anything truly monumental;

For instance, isolate any (if any) effect of CO2 by using the same (relevant) gas in two similar dives with similar deco time but with significantly differing deep stop emphasis - say, GF-lo=30 and GF-lo=100 - while controlling results across trimix dives in a technical setting and air/nitrox in a recreational one;

Now scale to a number of dives at varying depths and times.

And even then, we couldn't be certain that we've isolated factors as intended and that results can be extrapolated.

In light of my concerns, I feel it's premature to say all that much with certainty, or to perceive the material available as a "debunking" of deep stops in general.
To illustrate, I feel the discussion of "99% or 95%" is sometimes misinterpreted as "100% or 0%".

In either case, certainly Ratio Deco is no algorithm, but I see significant benefits to it's use, irrespective of bubble mechanics and tissue supersaturation.

 

[Patoux01]

*says deep stops work*
*gets various studies and explanations of why they don't seem to*
*yes but I think it works, so do more work. What if it was gas density, even if we're far from any known threshold ?! Oh and what if VGE, a deco stress indicator is positive ?*


So either scientists have been wrong all along, or maybe they're somewhat right...

 

[stuartv]

In either case, certainly Ratio Deco is no algorithm

How can you say this?

You take a data set (e.g. depth, time, gas mixes) and you apply a process to that data which produces a result (an ascent plan).

I write software for a living. That is EXACTLY an algorithm, in my book.

If it's not an algorithm, and you teach it to a student, then assign them a "problem" to solve - i.e. give them some dive data and assign them to produce a suitable ascent plan from it - how can you say if their answer is right or wrong?

Isn't the process of checking their answer one where you take the same input data and go through steps (what one might call an "algorithm") to produce your own answer and compare it to theirs? The algorithm is what defines the steps you go through such that the result is repeatable (i.e. you get the same answer as the student).