Modern Ratio Deco usage?

[stuartv]

based on current research, it appears that the fast tissues are more tolerant of supersaturation than the slow compartments in terms of DCS outcomes.

Tangent: Does this statement suggest that what really should happen is to adjust the M-values used, to bring the fast and slow compartments more in line with each other (with respect to incidence of DCS)?

I mean, wouldn't the ideal set of M-values produce the same rate of DCS whether the leading compartment was slow or fast?

 

[Kevrumbo]

I think the important metric is how much supersaturation the fast tissues can sustain/tolerate before it actually matters. Or are you saying you're not happy with the critical supersaturation M-values previously derived by decompression scientists?

In furtherance of your second point, if we know extending the shallow stops (conditions permitting) increases conservatism by allowing for more intert gas washout under increased pressure, then why can't I have a GF Lo setting of 80 or 90, and a GF Hi setting of 70 or 60? All I'm really doing is shaping my ascent curve. And if I'm willing to get closer to the Buhlmann numbers early in the dive where the fast tissues control the ceiling, because they appear to be able to tolerate more supersaturation than the slow tissues, and then walk those numbers back to a more conservative level as the slower tissues begin to control the ceiling, shouldn't I be able to do that?

Is the answer "no" because it wasn't part of Erik Baker's original paper?

I've asked Shearwater to add this feature to their dive computers a couple years ago, but haven't seen it happen yet .

Deep Stop/Dual Phase Bubble Model Theory also postulates about inert gas molecules affinity to diffuse into and occupy free phase micronuclei/pre-cursor "bubble seeds", caused by tribonucleation. Hence the motivation for holding a high ambient pressure deep stop to keep the surface tension & internal pressure of these random proto-bubble seeds high enough to prevent inert He and/or N2 gas from diffusing into them, as well as keeping these inert gas molecules in dissolved state longer in tissue and venous blood for return to the lungs for more effective inert washout. In other words, you don't want early bubble formation in fast neural tissue growing pathogenically larger in a Boyle's Law expansion as you ascend (type II DCS), or bubbles in blood returning to the lungs (type II pulmonary "chokes"). . .

So how hard you want to drive the pressure gradient closer to Buhlmann's M-value in these fast tissue compartments early in the deco profile depends whether you subscribe to the above theory, and/or are confident in the "robustness" of your fast tissues to handle the inert gas supersaturation load. The NEDU study seems to imply that the fast tissues are capable of sustaining this load without DeepStops all the way to a shallow depth decompression stop.

 

[PfcAJ]

Don't forget that "compartments" don't correlate to a real biological structure. They're part of a model.

 

[Kevrumbo]

Don't forget that "compartments" don't correlate to a real biological structure. They're part of a model.

I agree. How would you model or factor in deco stress markers of the immune/inflammatory system response of different individual divers as well?

Tangent: Does this statement suggest that what really should happen is to adjust the M-values used, to bring the fast and slow compartments more in line with each other (with respect to incidence of DCS)?
I mean, wouldn't the ideal set of M-values produce the same rate of DCS whether the leading compartment was slow or fast?

[Off-topic reply to tangent]: Ideally (and ethically), you would want to bloodtest for some scalable biochem-physiological protein marker or indicator of decompression stress and develop a corresponding deco algorithm or "M-values" from that data -something more definitive, correlative and diagnostic than Doppler VGE scores- rather than using overt & patent signs of acute DCI as incidence rate criteria. Something as easy as a post-dive pin prick blood test analogous to what a diabetic would do to measure his blood sugar.

Buhlmann's Model is based on mathematical half-lives of representative tissue compartments -you can't just de facto "adjust" the M-values. Do you mean adjusting the Gradient Factors? And what kind of DCI rate are you trying to correlate (type I, II or AGE)?

 

[Michael Guerrero]

In other words, you don't want early bubble formation in fast neural tissue (type II DCS), or bubbles in blood returning to the lungs (type II pulmonary "chokes"). . .

Unless of course type II DCS is not caused by in situ bubble formation within neurological tissues, but instead is the result of a shower of bubbles exceeding the capacity of your lungs to purge them from your body, or alternatively a shunting of bubbles from the venous to arterial side of the heart (PFO). Either way, this would result in a potentially large number of bubbles traveling from venous to arterial blood and reaching your neurological tissues right after leaving the heart. Off-gassing from neurological tissues may only exacerbate the issue as the inert gas would have an affinity for the newly arrived bubbles and the potential to make them larger and more destructive to the surrounding neurological tissues through bloodflow occlusion. We do know that bubbles do traverse the venous blood and return to the lungs after every dive, and your lungs are capable of coping with and eliminating them up to some point.

One of the interesting and counterintuitive points in Doolette's presentation of the data was that the VGE scores for the divers running a dual-phase model were much higher than those for the dissolved gas dives. So the model that was supposed to control for bubble formation produced a significantly larger number of grade 3 and 4 bubbles in divers than did the dissolved gas model.

Perhaps sacrificing the slow tissues for the benefit of the fast ones, with the necessary additional on-gassing of the slow tissues that these profiles produce, means that later in the dive as you are bubbling, the bubbling in the slow tissues is more substantial and aggressive (this is what the heat maps show), increasing your decompression stress and pushing you toward a tipping point where deco stress becomes deco sickness. Bubbling in the slow tissues would certainly last longer given their long halftimes. And of course the amount of N2 in the breathing mix matters given its high solubility in lipid tissues as compared to He.

I have no idea what proportion of fast, middle, and slow tissues exist in the body, so I don't know if this is the case or not. I'm not sure that decompression researchers know the answers to all these things either. And again, people dive both kinds of profiles, and a huge number of variants in between, every day without getting bent.

Insert something about an academic argument here.

 

[Kevrumbo]

. . .I'm not sure that decompression researchers know the answers to all these things either. And again, people dive both kinds of profiles, and a huge number of variants in between, every day without getting bent.

Insert something about an academic argument here.

No academic argument. . .

Just restating anecdotal common sense wisdom for now in light of all these paradoxical results & observations above and especially regarding implications drawn from the NEDU Deep Stops Study:

". . .As I have pointed out before, it is difficult to recommend a definitive practical application of this conclusion to divers for all sorts of reasons. However, the use of GFs does give the diver an opportunity to "de-emphasise" deep stops to an extent. In my own experience, this means that whereas I might have used a GF-Lo of 10 or 20 at the height of the belief in deep stops, I would now use 40 or 50. Given I am frequently the only diving physician in remote locations I usually also dive a conservative GF-Hi (70 - 75) but that is a personal choice. There is nothing definitive in these recommendations. It just seems like a sensible response to the data that we have as of February 2014. . ."

Simon M

Deep Stops (rebreather dive charts) - Page 22


". . .This brings us to the fact that we have seen the recent emergence of data that suggest the deep stop approach might not be the best one. In particular, these data suggest that the logic of protecting fast tissues from supersaturation early in the ascent at the expense of increased supersaturation in slow tissues later in the ascent may be flawed. Whether one likes the relevant studies or not, they are all we have at the present time to illuminate this issue.

So, was the bubble model fashion a bad idea, and where does this leave the majority who have been using some degree of "deep stopping" with apparently good outcomes? Do we abandon deep stops entirely eg go to 90/90 as you hypothesised, do nothing, or do something in between?

I guess the first thing to acknowledge is that the studies indicating a disadvantage for deep stops are not the diving equivalent of multiple large multicentre randomized trials all showing that the risks of a widely used drug are greater than its benefits. On the basis of that sort of data you would probably stop using the drug overnight. The diving world data concerning deep stops are not as definitive as that. As more data emerge the situation may become clearer (or more blurred)! So, at the present time, as an informed commentator, I would not go so far as to recommend that the entire world dives 90/90 or 80/80 from tomorrow onward even though I personally would not be surprised if the outcomes were as good as (or better than) what we are getting now (if we were able to measure them!). Partly, this hesitancy to advocate substantial wholesale change arises from the certainty that every diver who subsequently got bent would inevitably blame it on the change in their decompression practice! The deep stop trend evolved over a substantial period, and if the data continue to be supportive, it may have to "de-evolve" over a substantial period.

Equally, I do believe the data are strong enough (and bear in mind they are the only data) to consider a change in practice if you are a strong "emphasizer" of deep stops. In practical terms, "de-emphasizing" deep stops (or lessening any potential disadvantage) would mean using bubble models on very high conservatism settings, and with gradient factors, avoiding very low GF-lo values. I have been evolving my own use of GFs and am currently around 50/80 [@Macan, @Beau640, @boulderjohn ] ..sometimes as low as 70 for the high value when we are at places like Bikini and I am the only diving physician. Pre-NEDU study I was GF lo of 20. This is my personal perception of a sensible graduated response to the way the evidence is currently evolving. I may well go further in future (guided by the evidence).

Sorry about the long post. Hope it makes sense."

Simon M

Deep stops debate (split from ascent rate thread) - Page 13

 

[kensuf]

Don't forget that "compartments" don't correlate to a real biological structure. They're part of a model.

And the second word in the phrase "Decompression Theory" is, uh, Theory.

BTW, no one could pay me enough to use VPM. I think it's been sufficiently discredited.

 

[Bennno]

And the second word in the phrase "Decompression Theory" is, uh, Theory.

I think decompression theory is a scientific theory.
A scientific theory is a system or a model based on proven fact which allows you to make predictions.

 

[PfcAJ]

I think decompression theory is a scientific theory.
A scientific theory is a system or a model based on proven fact which allows you to make predictions.

That is correct.

But there's a lot of talk in this thread now about tissues. Phrases like "sacrificing slow tissues" like they're real things that the models are tracking.

 

[ofg-1]

Decompression is more of a scientific model than a theory. It is a collection of mathematical expressions that appear to be true in some of the population on some days under some conditions. Most of the current models have some error built in to them, or we would not spend 8 pages of conversation quibbling about gradient factors and slow tissues. And AJ, when I hit 60, all of my tissues slowed down, some more than others.