Deep Stops Increases DCS

[Freewillow]

@adjuplessis: I read it, but don't remember. Not worth it for me: I made up my mind a year ago: no deep stops when air - and not trimix - is used . Bühlmann and GF +/- 80/80 for small deco dives. I do not dive deep and long. Not worth it and a non sense @ my age - 67 .

 

[leadduck]

DCS diagnosis is pretty safe; if the symptoms go away in the chamber, then it was very likely DCS.
Post-dive exercise will increase DCS rate. But that doesn't matter as long as there's no difference between the compared groups (such as if subjects of group A1 would exercise a lot more than subjects of group A2). The DCS cases are described in the NEDU report appendix. If you think there was a problem, please point to the data that shows it.

 

[Macan]

@leadduck, you posted a link on BRW's paper that both deep and shallow stops are ok but deep stop is more efficient. Any thoughts on their findings?

 

[leadduck]

This is not correct. A critical part of decompression occurs on the surface. Therefore it's part of the integral supersaturation calculation.

(...)

I have used thresholds (e.g. 20fsw) to look at integral supersaturation for different profiles, for example VPM-B vs GF. Reflecting a threshold only helps the shallower GF profile. The reason is that all the faster compartments exceed the threshold in both profiles, but due to the lower on gassing in the slower compartments in the shallower model it is sometimes true that more of the slow compartments never reach the threshold in the GF profile and therefore never contribute to integral supersaturation.

Good that the integral runs to infinity, makes perfect sense to me. But then I have a question about the threshold:

In the other post you linked, Dr.Doolette wrote "intRSS is the time integral of the relative supersaturation (RSS) (...)
RSS=(SS-THR)/Pamb, where SS is the supersaturation (Pamb-Ptis), Pamb is ambient pressure , and THR is a threshold."

Now I wonder, because with THR other than 0, the integrand RSS does not vanish and the integral intRSS to infinity diverges. You cannot integrate a constant such as THR to infinity. Isn't it that you integrate SS/Pamb, and then subtract the threshold from the integral?

If you try to compare integral supersaturation to vastly different dives, then you have to bring in a lot of other factors to use it properly. One obvious example would be a dive to 15ft for 2 days. That dive would load up the compartments and when you returned to the surface you could calculate ISS. But studies have shown that divers can return directly to the surface fully saturated from about 20fsw without developing DCS. So there seems to be a threshold value that must be exceeded for DCS risk to start developing. Dr. Doolette discussed the idea of a threshold in my link that you referenced.

Sure. I expect one threshold per compartment makes sense when estimating DCS risk, because experiments showed that compartments differ in sensitivity to supersaturation.

There should be also a time constant for healing effects because a dive that I did one year ago won't affect my DCS risk today. But repetitive diving over 2 weeks does, so I expect this time constant to be about 10 days and to be irrelevant when comparing single dives.

However I still don't see how a threshold and long term healing effects make it necessary to compare only dives of equal runtime. Say you integrate RSS for one deco profile over 100min total runtime and 200min surface time, and for another profile for 200min runtime and 100min surface time. It's like both had 300min runtime, the first profile had a 200min stop at 0fsw, and the second profile had a 100min stop at 0fsw. You compare integral supersaturations for each compartment after 300min. What's the problem with that?

I've declined to post charts incorporating a threshold just because the problem with VPM and benefit of GF is demonstrated without that adjustment and we'd spend another 2 years chasing more meaningless posts, charts, and conspiracy theories by Ross if I did.

I think the choice of threshold values would indeed cause discussion, because this is like choosing M-values, or deciding whether slow or fast compartments are more important to protect. Depending on how you pick the threshold per compartment, one or the other profile will show the better DCS risk. So, just comparing intRSS makes sense to me, because it shows nicely (a) how VPM+x or ZHL+GF put different priorities on slow vs fast compartments, and (b) it shows how redistributing stop time from shallow to deep shifts the supersaturation stress to the post-dive phase because the released gas volume during the deco phase is smaller.

 

[scubadada]

The NEDU study showed that divers decompressing according to the probabilistic BVM(3) bubble model had statistically more DCS than divers decompressing according to the deterministic, gas content VVAL18 Thalmann algorithm. The Navy decided not to adopt the bubble model.

Since 2011, people have been attempting to explain the results. People have also attempted to generalize the results to more commonly used, non-Navy decompression algorithms such as VPM and Buhlmann ZHL-16C. This has led to considerable, often acrimonious, debate.

So, here we are. Each of us must weigh the evidence and make our own decision, I have

 

[shoredivr]

I was hoping Simon Mitchell would chime in, then I found this post from earlier in this very thread, where he answers several of Rossh's statements.

The problem, Ross, is that David agrees with my interpretation of what you refer to as "junk science". I'm just the one who is stupid enough / motivated enough by diver education to be bothered engaging with you on these forums. As David has pointed out, your involvement in writing software to implement VPM (coupled with your authoritative internet behaviour) will inevitably cause some people to assume you have training and expertise in the relevant physiology and pathophysiology (which you don't). Couple that with the fact that you habitually peddle misinformation about important work conducted by true experts and which carries important safety implications, and it creates a collection of circumstances which I find difficult to ignore.

We can, of course, argue about what is "junk science" all day, but if people want to conduct their own appraisal of how much "science" the respective participants in this discussion have validated before their peers, all they need do is type Doolette DJ, or Hemingway R, or Mitchell SJ, into the pubmed search engine (www.pubmed.com). I freely admit that this does not resolve the debate taking place here, but as a scientist who is being accused of "junk science" I do contend that this might provide some sense of who is most (or least) likely to be promoting "junk science" here on scubaboard.



This is complete and utter nonsense. The discussions of integral supersaturation by myself and others have only ever occurred in the same context in which the concept was first applied by David.



David has answered this already.



I can see that I'm simply going to have to include this in every post (because this is about the 4th time I have said it). Did you not see the bit where I said:

I am well aware that extremely high tissue supersaturations can produce bubbling and symptoms with almost no delay. I am also aware that very low levels of supersaturation may be sustained for very long periods without the development of problems. However, these facts do not constitute a valid argument against time being important at levels of supersaturation between these extremes.

Simon M

 

[211ratsbud]

The NEDU study showed that divers decompressing according to the probabilistic BVM(3) bubble model had statistically more DCS than divers decompressing according to the deterministic, gas content VVAL18 Thalmann algorithm. The Navy decided not to adopt the bubble model.

Since 2011, people have been attempting to explain the results. People have also attempted to generalize the results to more commonly used, non-Navy decompression algorithms such as VPM and Buhlmann ZHL-16C. This has led to considerable, often acrimonious, debate.

So, here we are

So often rather than experiment a bunch of models: the navy hypothesized a probable competitor to existing formulas and did in depth analysis. It's not relative to anything but their specific work and task. That's how I interpret the datum. Is that more correct?

 

[EFX]

So often rather than experiment a bunch of models: the navy hypothesized a probable competitor to existing formulas and did in depth analysis. It's not relative to anything but their specific work and task. That's how I interpret the datum. Is that more correct?

Yes! You are exactly correct. As others have pointed out the Navy is testing profiles that no sane sport diver (includes both rec and tec) would do let alone even plan. Let's look at the profiles from the NEDU study which includes two dives to 170 ft for 30 minutes on air. Both profile's were adjusted to have the same decompression time but the second profile inserted deep stops.

Shallow deco stops (depth/time): 40/9, 30/20, 20/52, 10/93. DT = 174.
Deep deco stops (depth/time): 70/12, 60/17, 50/15, 40/18, 30/23, 20/17, 10/72. DT = 174.

DT is total decompression time neglecting the ascent time. DT is calculated by adding the times for all the stops. The significant thing about these profiles is the DT - it is the same. The Navy had to shorten the DT for the deep stop profile to make it a constant between the profiles. But in doing so they added a variability to the test in the off-gassing rate.

Running the same dive plan on my Perdix with VPM-B +2 gave this profile:
Deep deco stops (depth/time): 100/1, 90/2, 80/3, 70/4, 60/5, 50/5, 40/7, 30/14, 20/20, 10/34. DT = 95.

Looking at the difference between both deep stop profiles DT time for the first four stops yields 12,17,15,18 and 1,2,3,4. The NEDU's definition of deep stops is very different from what sport divers use. The stops are shallower and longer. Rossh has repeatedly pointed this out in previous posts. In order for the NEDU study to compare their so-called deep stop profile with VPM-B they had to create a non-existent conservative setting of +7 to achieve the same DT!

What did I learn from the study? I learned that increasing the length of deep stops and for the same dive shortening the shallow stops (against what DC's will tell you) will get you bent. Gee, no surprise there!

So, here is a question I posted in Decompression Controversies:

Why would you be so quick to agree that bubble models in general and VPM in particular are inferior to dissolved gas models in general and Buhlmann GF's in particular, when the NEDU study is testing something quite different from what [non-Navy] bubble models are doing?

Note: [non-Navy] was not part of the original post. It was added here for clarity.

 

[UWSojourner]

Good that the integral runs to infinity, makes perfect sense to me. But then I have a question about the threshold:

In the other post you linked, Dr.Doolette wrote "intRSS is the time integral of the relative supersaturation (RSS) (...)
RSS=(SS-THR)/Pamb, where SS is the supersaturation (Pamb-Ptis), Pamb is ambient pressure , and THR is a threshold."

Now I wonder, because with THR other than 0, the integrand RSS does not vanish and the integral intRSS to infinity diverges. You cannot integrate a constant such as THR to infinity. Isn't it that you integrate SS/Pamb, and then subtract the threshold from the integral?

I think RSS only contributes to risk if positive, else 0. Once SS above threshold is below zero no further risk is added.

However I still don't see how a threshold and long term healing effects make it necessary to compare only dives of equal runtime. Say you integrate RSS for one deco profile over 100min total runtime and 200min surface time, and for another profile for 200min runtime and 100min surface time. It's like both had 300min runtime, the first profile had a 200min stop at 0fsw, and the second profile had a 100min stop at 0fsw. You compare integral supersaturations for each compartment after 300min. What's the problem with that?

If the dives are both to the same depth and vary only in bottom time, then ISS will be larger for the 200min RT. So the simply ISS would not be misleading. But we all know that dive has more deco stress.

But what about a 200min runtime at 15fsw and a 100min runtime at some deeper depth X that results in a slightly lower ISS? In that case you'd say the 200min RT dive has higher deco stress, but we already know you can be fully saturated at that depth and come directly to the surface. So it would mislead. I think it's just safe to restrict "raw ISS" to comparisons of dives that vary only by distribution of stop time. If used that way, then it can be a reasonable measure of decompression stress. If you use raw ISS beyond that I think you could get false signals, and the more you diverge from comparing similar dives the more chance that will occur.

 

[KevinNM]

So often rather than experiment a bunch of models: the navy hypothesized a probable competitor to existing formulas and did in depth analysis. It's not relative to anything but their specific work and task. That's how I interpret the datum. Is that more correct?

Pretty much all the research on decompression was done on people doing work at depth. Because these sorts of experiments are extremely expensive and the people who have the money and interest to fund these studies are governments or commercial concerns who need to have people do work at depth.

They are commonly extended to recreational diving (diving where you are not being paid to dive) but were certainly not designed for this. That's just how it works. The PADI tables are the only ones I can think of where they were deliberately working at creating tables for recreational diving, and they don't cover deco diving.

The reduction in funding for deco research is apparently largely due to migration of work at depth to ROVs and one atmosphere suits by commercial organizations, so it isn't likely to get better.