Modern Ratio Deco usage?

[Michael Guerrero]

You are writing the probability numeric value (a fraction where 1 = certain) as a percentage (where 100 = certainty), i.e. 100 times smaller than it should be. Really the expectation is one bend in 50 dives to one bend in 25 dives. I do not think either is something I want to be experiencing or should be termed very, very safe.

Factors other than the profile that differ bettween these test dives (which had to be aggressive to give useful bend rates in a practical number of dives) and the dives people generally do dive mean that we don't get bent as often as expected by these results.

Yep, all true and my mistake on looking at the chart.

For anyone who wants to see this presentation it is David Doolette's presentation at Rebreather Forum 3.0 called Decompression Methods.


Simon Mitchell also gives a presentation in 2015 called Decompression Controversies that you can watch on Youtube.

 

[boulderjohn]

He may have said 50/80 in the decompression controversies video.

". . .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 ...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).

Thanks for this. It is indeed where I got the 50/80 message, and his reasoning is why I use those GFs now.

 

[Remy B.]

Thanks for the Link Michael.

Now I understand better, Mitchell used a very good analogy about popo/kaka and internet, where the people with less experience can get is wrong, since I don't know nothing I was combining VPM- and padding the shallows like GF.

Even that they show that the fast tissues seem not to be affected or the ones causing the big problems and are rather the slow tissues, it was not conclusive from which point the Deep stop had to start.

I hope that NEDU or other research body get to conduct that missing link to interconnect the Models and protect both fast and slow tissues, I guess there is not enough data as well to determine if pushing your fast tissues for years can cause negative effects at long term.

Even that I do baby deco's and I have for now set my own limit to 60m max, where apparently the VPM still Ok, I like to incline my self to GF 40/70 just hypothetically it seems balanced or in between VPM and GF 50/80

 

[Kevrumbo]

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.

...
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.

More Simon Mitchell snippets (sifting through the Deep Stops Increases DCS thread again):

...I am saying (based largely on the results of the NEDU study) that transient high / peak supersaturation in fast tissues does not seem to matter as much as we thought it might, and therefore that protecting fast tissues from supersaturation early in the ascent by using deep stops does not seem as effective as assumed by bubble models. This is especially so when it comes at the cost of increased supersaturation (both in terms of peak levels and duration) in slower tissues later in the ascent. The NEDU study is telling us that this is where the problems seem to come from.

@Kevrumbo asks:
. . .Explain from a physiological basis why allowing supersaturation of Fast Tissues in this instance is less risky to those tissues and results in a lower overall incidence of DCS in the shallow stops trials of the NEDU Study. . . ?

Hello Kev,

The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum I attended last week. Hopefully this will find its way into the literature at some point soon. In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).

Simon M

Deep Stops Increases DCS

 

[Remy B.]

If I'm not mistsken Kev, they had more DCS type 1, with the NEDU non deep stops, of course they are of the lesser concern than type 2, but IMO there have to be a balance, where they can minimize the type 1 as well.

It put me think if a diver like me uses the standard setting of 30/70 GF and get a type 1, and blame the algorithm and create distrust, and then try the VPM and get a type 2, that will be very fuc..up.

For now I will try with GF 40/70, and wait if hopefully that further studies and test are done to fill in that gap of the type 1 DCS, if I understood right.

 

[Michael Guerrero]

Remy,

They had 3 Type 1 DCS hits in the traditional profile and 10 Type 1/2 hits in the deep stops profile. It was not more in the traditional profile.

I think you're kind of asking for the impossible. Trying to prevent critical supersaturation in the fast tissues means you need to stop deeper to allow them to offgas their inert gasses. That necessarily means that slower tissues will continue to ongas. This is exactly what the NEDU study is showing. And what Kev's quotes are saying is that the NEDU data seems to indicate that either the fast tissues are more tolerant of critical supersaturation than the slow tissues. Either way, it would appear that critical supersaturation in the slow tissues (like fat) is more problematic and more likely to cause DCS.

I think when you combine this data with the research that shows that He and N2 washout at roughly the same rates in the aqueous tissues, there really is no argument you can make that is backed up by data for conducting deep stops. A big part of people's affinity for deep stops was because He is a "fast gas," and they would point to the studies that show that He is washed out after 6 hrs whereas N2 was normalizing after around 12. The rationale was that because He comes out of your tissues so quickly, you need those deep stops to prevent critical supersaturation and bubbling. However, studies have shown that He and N2 washout at similar rates in aqueous tissues. Also, they often neglect to factor in that N2 is 5 times more soluble than He in lipid tissues (like fat), and your body has a lot of this tissue. Not only that, but the lipid tissues are slow tissues.

So, He moves in and out of the aqueous tissues at about the same rate as N2, but is much less soluble in the lipid tissues, and so there will not be very much of it at the end of a diver as compared to N2. It seems pretty clear to me why N2 would still be offgassing after He is washed out. You've absorbed a lot more of it than you have He, and it is washing out of slow tissues.

 

[rjack321]

Does Ratio Deco emulate a VPM schedule?

No its got even more time deeper and "in theory" should be even more discredited - at least in terms of deco shape. The total time is quite close to other deco calculation strategies it just overemphasizes deep stops, which the 75% of depth piece is part.

 

[rjack321]

I voted that I use RD as a backup. For many years it was my primary mode on OC (with some shallow stop padding), down to 200ft. But since switching to a CCR I now use a shearwater with 40/75 or 40/80 settings. 75hi when doing repetitive dives, 80hi if its just a single dive. I know that RD is possible on a CCR I just chose not to do that as I haven't seen the efficacy of it.

 

[huwporter]

No its got even more time deeper and "in theory" should be even more discredited - at least in terms of deco shape. The total time is quite close to other deco calculation strategies it just overemphasizes deep stops, which the 75% of depth piece is part.

Worth mentioning that GUE have recently updated their ascent procedures to remove the 75% recommendation, in favour of planning deep stops more closely aligned with Buhlmann theory.

 

[rjack321]

Worth mentioning that GUE have recently updated their ascent procedures to remove the 75% recommendation, in favour of planning deep stops more closely aligned with Buhlmann theory.

yes it used to be something like -5/110 as a GF setting. Now its more like 15/100
Still nothing like 40/80