UTD Ratio deco discussion

[Kevrumbo]

Extremely enlightening and thought provoking. Ever since Ross joined this thread I was really hoping we would be able to get your perspective on it as well.

Just wanted to clarify your position on one thing.

Am I correct in understanding that you are not opposed to deep stops at 66% depth if these are sufficiently padded at the shallowest part of the dive? In other words, UTDs present stop at 66% may not have science behind it at this point but if it is retained on the theoretical premise of bubble control or reducing the gradient, as long as it is followed by an extended shallow stop, it is not dangerous than a straight ascent to GF-lo of 50?

Thanks for showing up Doc.

Well, while waiting for @Dr Simon Mitchell to come back online, I kind of proposed the same strategy during a Truk trip two-and-half years ago:

@Kevrumbo:
The best most prudent compromise to practically apply from the NEDU Study & discussion, is to do the Deep Stops, and extend out the O2 profile at 6m such that you have a surfacing Gradient Factor of 60% or less (per the readout of a Petrel Computer upon surfacing from your O2 deco stop) --to ensure inert gas elimination from those Slow Tissues. This is especially warranted if you're doing multiple deco dives per day for a week or more -and I would also recommend taking a day-off/break after three consecutive days of multiple deep deco dives per day. . .

Simon replies:
I would agree that this is a workable compromise. Deep stops can be safely incorporated into a dive profile if you want to use them. However, there is no evidence that you gain anything by doing so, and the available evidence suggests that their use is not the most efficient use of deco time. Thus, if you have a fixed amount of decompression time, the decompression will become less safe if you over-emphasize deep stops.

@Kevrumbo:
Simon, I'm simply not willing to risk bubble nucleation & formation in my Fast Tissues for the sake of not loading/supersaturating my Slow Tissues later on in the deco profile (per indication of those "heat maps" by UW Sojourner); essentially a "Robbing Peter to pay Paul" dilemma. And yes, I have plenty of time and an 11L Cylinder (AL80) full of O2 to clean-up those Slow Tissues. . .

Simon Mitchell replies:
Hello Kev,

I guess this is the sticking point. It is an article of faith for you that allowing fast tissues to supersaturate early in a profile that places less deep stops in your ascent is harmful, and there is probably nothing I can do to change your mind on that. However, I must point out that you only believe that because someone has told you it is so. It is an attractive theoretical assumption that many people believe(d) in the absence of any confirmatory data. The point is, that there is now data that challenge the idea. As UWSojourner's heat maps have illustrated the NEDU deep stops profile did reduce fast tissue supersaturation compared to the shallow stops profile, but this did not result in better outcomes. If tight control of fast tissue supersaturation early in the ascent is as important as you believe, why did the profile with the best control of fast tissue supersaturation early in the ascent produce the highest DCS rate?

Anyway bud, if you do what you say you are going to do and significantly pad your shallow oxygen decompression it may not matter too much what you do earlier. Just don't have a seizure please! . . .

Simon

Deep Stops Increases DCS

 

[Dr Simon Mitchell]

Just wanted to clarify your position on one thing.

Am I correct in understanding that you are not opposed to deep stops at 66% depth if these are sufficiently padded at the shallowest part of the dive? In other words, UTDs present stop at 66% may not have science behind it at this point but if it is retained on the theoretical premise of bubble control or reducing the gradient, as long as it is followed by an extended shallow stop, it is not dangerous than a straight ascent to GF-lo of 50?

Thanks for showing up Doc.

Hello,

In moving from 75% to 66% UTD seem to be acknowledging that some degree of backing away from deep stops is appropriate based on current evidence. In all fairness, that's progress. If I was asked I could not be specific about how far to back away (as I have previously stated) so it is hard to be critical of other people's interpretations. Having said that, my intuition is that 66% is likely to still be too deep for the decompression to be optimal. But getting to the real point of your question (and Kev is right, it is similar to a conversation we had in the past); if deep stopping is still part of one's decompression philosophy, then yes, you can do it successfully if you adequately compensate at the shallow end. Knowing what that compensation should be may be challenging, but in theory the answer is yes. David and I have always said that. It may not be the most efficient use of decompression time, but you could certainly do it.

Simon M

 

[mikeny9]

Hi Dr. Mitchell,

Thanks for taking time to respond to me. I think it's quite rare a recognized expert with your background actively participates in these often pedantic online discussions, and for that you deserve some sort of an award!

The reasons for this were articulated at length in the original deep stops threads on rebreather world.

I've read through these two:
Deep Stops (rebreather dive charts)
Deep stops debate (split from ascent rate thread)

The only mention of oxygen-accelerated decompression was that it would be harder to detect differences between deep and shallow stop profiles, which you already mentioned, and that oxygen-accelerated decompression may further mitigate DCS risks. If these are the only threads you're referring to, I wouldn't describe the statements in them as "articulated at length;" are there other threads you are referring to?

The recent Spisni study comparing RD and GF 30/80. This study is particularly interesting because it contained a design flaw in that the RD decompression was longer than the GF one. If you want to evaluate the efficacy of the deeper stops and the RD S curve in comparison to another approach which involves shallower stops and no S curve, then you at the very least need to make the profiles the same length (because, all other factors being equal, a longer decompression should always be safer).

Where was the longer time distributed in the RD schedule when compared to the GF one? Was it in the shallower section, mid section, deeper section, or equally spread out?

“Debunks” is probably too strong a word, but this study provides a signal that the RD approach (as it stood at the time) was inferior to another commonly used approach with shallower stops and no S curve.

The RD approach I've been asking people to "debunk" is one which involves calculating the first stops starting at 66% of average depth rather than the old way of 75%. The calculated depth is typically rounded shallower as well. While it's definitely not similar to something like GF 50/80, it is, as you put it in another thread, an indirect nod to the NEDU study. You mentioned at one point that you can't suggest what gradient factors people should use, but that 66% is probably still too deep. What is the deepest percentage or GF-low that you think isn't too deep? If a GF-low of 50 is probably not too deep, and a GF-low of 30 is, we should be able to do a binary search through your opinion of what is and isn't too deep. I'm not trying to criticize, but simply just curious where you would draw the line.

Also, thanks for enumerating and briefly summarizing the studies involving deep stops. I am not familiar with all of them, but don't they generally stop stop much deeper sooner, with the exception of the Swedish Navy study, and much longer (inclusive of th Swedish Navy study) than the 66% 1 minute stop would in RD 2.0?

...if deep stopping is still part of one's decompression philosophy, then yes, you can do it successfully if you adequately compensate at the shallow end.

If deep stopping is still part of one's decompression philosophy, and as long as "deep" is as deep or deeper than you think is necessary, and the amount of deep stop time is abritrary, then if "you could do it successfully if adequately compensating at the shallow end," couldn't you also eliminate or significantly reduce the additional time performed in the shallow end by using oxygen-based decompression?

...but RD emphasizes (or at least emphasized) deep stops as a central part of its approach

I think "central" is probably a bit too much, it's one part of a few different parts that make up the overall ascent strategy as it was taught to me.

Also, regarding the Helium penalty, in the quote from the Shearwater article here: Eliminating The Helium Penalty - Shearwater Research, you mention "...we are doing the right amount of deco but probably for the wrong reason." Why do you think the right amount of deco calculated from a Buhlmann ZHL-16 algorithm involving any Helium-based mixture is the right amount of deco? If people are doing it for the wrong reason, what is the right reason?

According to RD methodology, the criticism of Buhllmann ZHL-16 is that it does not take into account the higher diffusivity along with the lower solubility of Helium versus Nitrogen and so will "penalize" by giving you longer decompression total times with high Helium mixes.

I'm glad you brought up the Helium penalty Kev. That was my guess too as to why the O2 time would be significantly longer as well.

Does anyone know if GUE is changing anything in light of the NEDU deep stop study? I was under the impression they are using ZHL-16 20/85 in DecoPlanner as the standard.

 

[Kevrumbo]

Does anyone know if GUE is changing anything in light of the NEDU deep stop study? I was under the impression they are using ZHL-16 20/85 in DecoPlanner as the standard.

From talking to them at the Long Beach Scuba Show here in LA over three weeks ago, their version of RD sounds like a simple straightforward 9m/min (30ft/min) ascent rate from operational bottom to the first deco switch depth (GUE Tech 1 depths around 45m/150ft with the first deco stop on Eanx50 at 21m/70ft), along with any adjustments needed through Decoplanner.

 

[mikeny9]

I also made a plot that illustrates the profiles we've been discussing for a dive to 170' for 20 minutes using 18/45 and 50% O2 for deco:

 

[Dr Simon Mitchell]

Thanks for taking time to respond to me. I think it's quite rare a recognized expert with your background actively participates in these often pedantic online discussions, and for that you deserve some sort of an award!

Thank you for the positive words. Actually, mostly it is a pleasure, and what would be the point of being interested in diving science if you were not engaged in trying to translate it through to application in the community? In addition, I learn a lot from these forums too.

I've read through these two:
Deep Stops (rebreather dive charts)
Deep stops debate (split from ascent rate thread)

The only mention of oxygen-accelerated decompression was that it would be harder to detect differences between deep and shallow stop profiles, which you already mentioned, and that oxygen-accelerated decompression may further mitigate DCS risks. If these are the only threads you're referring to, I wouldn't describe the statements in them as "articulated at length;" are there other threads you are referring to?

I think the bit you are under-appreciating in those threads is the evaluation of supersaturation patterns across the range of fast and slow tissues for typical technical dives using typical technical diving gases, but decompressed according to a deep stops or shallower stops approach. We believe that the NEDU study result is explained by greater slower tissue supersaturation late in the dive, and greater total integral supersaturation overall. If we are right about that (and there is no more plausible explanation), then it is likely that typical technical dives (using constant PO2 and oxygen decompression) which emphasise deep stops will still be disadvantaged to some extent because the same disadvantageous patterns of supersaturation are seen. The disadvantage may be smaller and more difficult to measure in a small trial, but there is no reason to believe that the direction of advantage / disadvantage will change.

Where was the longer time distributed in the RD schedule when compared to the GF one? Was it in the shallower section, mid section, deeper section, or equally spread out?

I think you have answered your own question with your excellent diagram. I would contend that if the 30/80 profile had been adjusted to whatever GF-hi was required to make it the same length as the RD profile (and thus a truly fair comparison for decompression efficiency), and if that time had been spent breathing oxygen shallow, the outcome difference between the profiles could only have been greater.

The RD approach I've been asking people to "debunk" is one which involves calculating the first stops starting at 66% of average depth rather than the old way of 75%. The calculated depth is typically rounded shallower as well. While it's definitely not similar to something like GF 50/80, it is, as you put it in another thread, an indirect nod to the NEDU study. You mentioned at one point that you can't suggest what gradient factors people should use, but that 66% is probably still too deep. What is the deepest percentage or GF-low that you think isn't too deep? If a GF-low of 50 is probably not too deep, and a GF-low of 30 is, we should be able to do a binary search through your opinion of what is and isn't too deep. I'm not trying to criticize, but simply just curious where you would draw the line.

We simply do not have enough data for such precision. My choice of GF-low of 50 is based on not much more than instinct, with some basis in fact. For example, when the Swedish study compared GF-low of 30 to DCAP (a more traditional gas content model developed by Bill Hamilton and which produced shallower stops) the DCAP profiles produced less bubbles. It is weak evidence, but I had initially backed away from 10 or 20 in the deep stops days to 30, and these data suggested to me that 30 is still too deep. But for someone like me to give a solid recommendation requires harder data which we don't have, so I won't provide a recommendation.

Also, thanks for enumerating and briefly summarizing the studies involving deep stops. I am not familiar with all of them, but don't they generally stop stop much deeper sooner, with the exception of the Swedish Navy study, and much longer (inclusive of th Swedish Navy study) than the 66% 1 minute stop would in RD 2.0?

No, I don't think so. NEDU didn't, Blatteau didn't, Spisni obviously did because they were using original RD, Sweden didn't as you have mentioned. Ljubkovic would have been close on a quick glance (it was based on VPM profiles).

If deep stopping is still part of one's decompression philosophy, and as long as "deep" is as deep or deeper than you think is necessary, and the amount of deep stop time is abritrary, then if "you could do it successfully if adequately compensating at the shallow end," couldn't you also eliminate or significantly reduce the additional time performed in the shallow end by using oxygen-based decompression?

Yes of course, oxygen decompression will always shorten required decompression time. But that of itself does not mean that concerns about appropriateness of deep stops go away. Whatever length of decompression you do, you obviously would like to perform it in such a way as to minimise risk. So, for example, a key question would be: "in committing to x minutes of decompression, could my risk in the dive have been lower if I had performed the same length of decompression, but distributed my stops differently (less deep more shallow)?"

Also, regarding the Helium penalty, in the quote from the Shearwater article here: Eliminating The Helium Penalty - Shearwater Research, you mention "...we are doing the right amount of deco but probably for the wrong reason." Why do you think the right amount of deco calculated from a Buhlmann ZHL-16 algorithm involving any Helium-based mixture is the right amount of deco? If people are doing it for the wrong reason, what is the right reason?

I think you are taking the term "right" a little too literally. I don't mean to imply that Buhlmann represents the truth in the universe about decompression, but rather that by assuming a helium penalty in use of Buhlmann we have accidentally (for want of a better word) done the "right" thing. The "right reason" you ask about pertains to the fact that dives performed up to a no decompression limit or decompression ceiling are generally not iso-risk as the dives get deeper (even following the ceilings prescribed by the same model). Put another way, a dive performed perfectly to the Buhlmann ceiling in decompressing from 50m is not as risky as a dive performed perfectly to the Buhlmann ceiling in decompressing from 100m. Thus, it is probably fortuitous that added conservatism based on the higher helium fractions used as we go deeper have helped compensate for this increase in risk. That is why, despite David's demonstration that helium and nitrogen kinetics in relevant tissues are actually similar, we are not willing to endorse the idea that we should all go out and tell our computers we are using nitrogen on deep dives and therefore have shorter decompressions.

Simon M

 

[Diver0001]

I also made a plot that illustrates the profiles we've been discussing for a dive to 170' for 20 minutes using 18/45 and 50% O2 for deco:
View attachment 412330

You can see that the RD profiles both make a deeper ascent line and slightly longer ascents than the Buhlmann profiles. What would be really telling would be to plot those profiles in the forum a heat maps showing the tissue loading as opposed to showing it in terms of ascent lines. I think if you saw the heat maps even with the longer ascents it would really make it clear what is happening there.

R..

 

[mikeny9]

We simply do not have enough data for such precision. My choice of GF-low of 50 is based on not much more than instinct, with some basis in fact. For example, when the Swedish study compared GF-low of 30 to DCAP (a more traditional gas content model developed by Bill Hamilton and which produced shallower stops) the DCAP profiles produced less bubbles. It is weak evidence, but I had initially backed away from 10 or 20 in the deep stops days to 30, and these data suggested to me that 30 is still too deep. But for someone like me to give a solid recommendation requires harder data which we don't have, so I won't provide a recommendation.

So it's somewhere above 30 and equal to or lower than 99, since 50 itself is sort of a shot in the dark as well? I know you are being careful to not recommend anything, but based on the logic if you think 30 is too low then the true answer lies somewhere above but not equal to 30 and equal to or beneath 99 right?

Yes of course, oxygen decompression will always shorten required decompression time. But that of itself does not mean that concerns about appropriateness of deep stops go away. Whatever length of decompression you do, you obviously would like to perform it in such a way as to minimise risk. So, for example, a key question would be: "in committing to x minutes of decompression, could my risk in the dive have been lower if I had performed the same length of decompression, but distributed my stops differently (less deep more shallow)?"

Personally, I'd rather take more care to decompress the fast tissues and cleanup any additional saturation incurred in the slow tissues due to a supposedly less efficient ascent with oxygen decompression. In other words, I'd rather risk a Type I hit, attempt to mitigate said Type I hit with oxygen decompression, than risk a Type II hit, even though the studies suggest there is no additional risk to the fast tissues. The evidence is not convincing enough for me to risk a Type II hit.

I think you are taking the term "right" a little too literally. I don't mean to imply that Buhlmann represents the truth in the universe about decompression, but rather that by assuming a helium penalty in use of Buhlmann we have accidentally (for want of a better word) done the "right" thing. The "right reason" you ask about pertains to the fact that dives performed up to a no decompression limit or decompression ceiling are generally not iso-risk as the dives get deeper (even following the ceilings prescribed by the same model). Put another way, a dive performed perfectly to the Buhlmann ceiling in decompressing from 50m is not as risky as a dive performed perfectly to the Buhlmann ceiling in decompressing from 100m. Thus, it is probably fortuitous that added conservatism based on the higher helium fractions used as we go deeper have helped compensate for this increase in risk. That is why, despite David's demonstration that helium and nitrogen kinetics in relevant tissues are actually similar, we are not willing to endorse the idea that we should all go out and tell our computers we are using nitrogen on deep dives and therefore have shorter decompressions.

Is the reason in this explanation akin to "more decompression in the shallow stops is more conservative anyway?" In other words, even though the Helium penalty is recognized, it's better and safer to do more decompression in the shallows anyway even if we're not using Helium?

You can see that the RD profiles both make a deeper ascent line and slightly longer ascents than the Buhlmann profiles. What would be really telling would be to plot those profiles in the forum a heat maps showing the tissue loading as opposed to showing it in terms of ascent lines. I think if you saw the heat maps even with the longer ascents it would really make it clear what is happening there.
R..

Of course, the slow tissues would show additional on-gassing. However, theoretically, the fast tissues would off-gas more with the longer ascent profiles than the ones with a shorter ascent. The ability to measure fast tissue saturation is another hot topic of debate, I'd rather not open up that can of worms. As I mentioned above, personally I'd rather be damn sure my fast tissues have off-gassed sufficiently to avoid a Type II hit, incur any additional saturation of slow tissues because of it and risk a Type I hit, and attempt to mitigate the additional slow tissue saturation and risk of a Type I hit with oxygen decompression, or additional shallow stop padding, as @Kevrumbo suggested.

 

[Diver0001]

Of course, the slow tissues would show additional on-gassing. However, theoretically, the fast tissues would off-gas more with the longer ascent profiles than the ones with a shorter ascent. The ability to measure fast tissue saturation is another hot topic of debate, I'd rather not open up that can of worms. As I mentioned above, personally I'd rather be damn sure my fast tissues have off-gassed sufficiently to avoid a Type II hit, incur any additional saturation of slow tissues because of it and risk a Type I hit, and attempt to mitigate the additional slow tissue saturation and risk of a Type I hit with oxygen decompression, or additional shallow stop padding, as @Kevrumbo suggested.

I think you've got the basic idea. What the NEDU study shows, as Dr. Mitchell will surely point out, is that while bubble models or deep ascent profiles do protect the fast tissues, this protecting of the fast tissues is not what is needed to lower your overall risk of DCS.

This idea that fast tissues need more protection than slow tissues is one of the things I was talking about before that has been proven to be incorrect.

R..

 

[Patoux01]

A while ago I read something about people stepping back from the "type 1 and type 2" thing. Somewhere on here, good luck finding that.