Shearwater's take on the helium penalty

[rossh]

Divers can test and experiment with some 1/2 time changes now, using V-Planner. You can see how the affects the current plans.

Under the Config -> Advanced -> Set my own -> Cell times, one can set and change half times for N2 and He as they please. Be careful please.

This features is also coming to MultiDeco soon.

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[Superlyte27]

Hello,

I get why you might say this, but I think you have misunderstood the purpose and methodology of the study.

The context for the USN was that several other Navies are developing trimix tables for use with CCRs, with one of the putative advantages being shorter decompression (than for a heliox diluent). Based on work by their own scientists (David Doolette), the USN suspected that this advantage was more assumed than real so they set out to investigate this in the study.

They reported an experiment designed to answer the question of whether decompression from a trimix dive is more efficient than a heliox dive. Identical dives (under tightly controlled conditions) to 200' (60m) for 40 minutes bottom time with 119 minutes of decompression were performed by rebreather divers using a PO2 set point of 1.3 and either heliox (oxygen 12% helium 88%) or trimix (oxygen 12%, helium 44%, nitrogen 44%) diluent. To be clear, the dives were identical in every way, other than the diluent used. The primary outcome measure was decompression sickness.

It is crucial to understand that this was what is referred to as a non-inferiority trial with pre-defined stopping criteria to be invoked if it became apparent, to the chosen level of statistical certainty, that heliox was not inferior as a decompression gas to trimix. They were not interested in proving that one of the gases was better than the other (which as Superlyte27 implies would require a much larger trial).... indeed, if you think about it, given Doolette's animal work which showed no difference in helium and nitrogen kinetics in two relevant tissues, why would they hypothesize that one or the other would be superior? What they were interested in determining was whether the gas they currently use (heliox) was no worse in terms of decompression efficiency than the one they could potentially change to (trimix).

After 50 heliox dives were completed with no DCS and 46 trimix dives were completed with 2 cases of DCS....

Simon M

My point is, that with such a small group and small number of dives, it's possible that everything was right and two people could have gotten bent anyway. Just dumb luck plays in all the time. Perhaps it was dumb luck the got bent, or dumb luck that someone in the other group didn't get bent. But at this point t and this size, you just don't know.

 

[Dr Simon Mitchell]

My point is, that with such a small group and small number of dives, it's possible that everything was right and two people could have gotten bent anyway. Just dumb luck plays in all the time. Perhaps it was dumb luck the got bent, or dumb luck that someone in the other group didn't get bent. But at this point t and this size, you just don't know.

Hello again,

I understand that, but the fact remains that after there were 2 cases of DCS in the trimix group and none in the heliox group in their study of approx. of 100 dives, the probability of heliox proving inferior to trimix (even if they continued on with a much larger trial), fell below their statistical criteria for stopping. They have proven their point to an accepted level of statistical certainty. You can get to this point more quickly in a smaller study if all you are trying to prove is non-inferiority. If they were trying to claim that heliox was superior on the basis of those results you would be entirely correct. A difference of two cases would not be enough, and a much larger study and a bigger difference in numbers of cases would be required.

Simon M

 

[RayfromTX]

You are soooo patient.

 

[rossh]

My point is, that with such a small group and small number of dives, it's possible that everything was right and two people could have gotten bent anyway. Just dumb luck plays in all the time. Perhaps it was dumb luck the got bent, or dumb luck that someone in the other group didn't get bent. But at this point t and this size, you just don't know.

That is a valid concern and I share it with you. A test of one result surely needs some supplementary testing to accept the concept. Particularly so as this test is being used as the lever to usher in such a significant shift in theory.


We can add some context to this test and its meaning, from our own tech world experiences:


************


How common is the profile in the real world? 200ft/66m 40mins, 120 mins deco, CCR

Fortunately we have a public database that contains 140,000 records of real dive logs that can be searched. These come straight out of tech diver computer logs, in the real world. The outcome reports on some of these, are with DAN in its PDE data collections.


A dive of 200ft/66m and 40 mins on CCR is common enough and there are hundreds of examples in the database. But really square bottomed dives in the real world are less common, as most divers will wobble around as they look at the dive site and slowly ascend a little. I did find enough similar examples below, with the same depth and time range. They are all CCR examples, but very few OC examples exist (too much OC gas needed).

some examples ( many are a little deeper than 200ft )

VPM-B:
09-02951 10-16117 10-16116 14-10731 11-09849 12-17958 09-08863 11-09849 10-14006 12-17954 09-02951 10-00989 12-17954 10-00989 12-00840 10-14011 09-08671 12-20235

VPM-GFS:
13-20971 13-03954 13-03265 15-09325 14-20374 12-04099 11-21673 12-07764 15-11751 14-12418 13-20968 13-20970 11-15341

ZHL-GF:
13-16259 15-03820 14-12762 12-25040 14-12762 15-06136 12-25039 12-25033 12-25035



We can make some generalizations about those examples:

• The samples all use deep stop type profiles,
• The samples all have about 80 to 100 mins of total deco,
• The samples all have about 45 mins in the 20ft / 6m / last stop portion,

**************

Looking that the profile in the nedu test, it used a shallow stop styled profile. The nedu trimix profile is similar to a raw ZHL-C (no GF), for the first part, but the nedu profile then adds an excessive amount of last stop 20ft time: it has 90 mins at 20ft on an inspired mix of 81% to 87% O2 (1.3 to 1.4 pO2 at 20ft). In GF terms, the nedu profile is approximately equal to a trimix ZHL-C with GF of 100/25 (yes backwards: 100 lo, 25 hi). You cannot normally make plans this far off scale in deco programs. The nedu ascent is approx equal to say: a raw ZHL-C shallow ascent, but then extend the shallow portions by 3x normal.

That last stop used in the nedu test is 90 mins of inspired 81% to 87% O2, which is quite excessive. If you use an RD formula, its comes to more than a 2:1 for the last stop alone (plus the rest of the ascent).



The nedu test profile differences (compared to real world tech divers practices), has

• nedu test profile is a shallow stop profile,
• nedu test profile has double the last stop O2 deco time,
• nedu test profile is about 20 to 40 mins more deco in total,
• nedu test profile has an extra 45 mins of high O2 deco.
• nedu test profile has a 30 minute pre-breath at 0.7 that reduces deco further.

It would seem the nedu test profiles are very long compared to real world tech dives.

With such excess deco time added, should we be surprised that the nedu were not able to properly predict the pDCS in the testing? This elongated test profile was not "probing for the edge" of deco by any stretch of the imagination.

The exercise level during the nedu test dive is classed as Easy to Moderate in the "Navy Seal Physical Fitness Guide".

And it also raises the question - how did two divers get injured when the profile is so far in excess of normal? Or why did this aberration in the nedu test results occur?

************

If you would like to compute these dives in MultiDeco, then use these legs and settings:


0.0001, 30, 12/44, 0.7 <<< 30 min prebreath
32, 0, 12/44, 1.3 <<< auto swap to 1.3
200, 40, 12/44, 1.3 <<< bottom

descent at 40fpm, ascent at 30fpm, 20 ft last stop


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[Dr Simon Mitchell]

Ross,

It seems that NEDU cannot publish anything that meets with your approval.

That is a valid concern and I share it with you.

It is not a valid concern. The study is sound and measured in its conclusions. I can understand why someone might suspect the numbers are too small, but I don't think you understand the non-inferiority methodology.

A test of one result surely needs some supplementary testing to accept the concept.

Well, allow me to point out that you not only "accepted" but also (in the face of contrary evidence) fervently defended a major decompression "concept" (the utility of deep stops) that had absolutely no testing at all.

But to the point, there is more than one relevant result. The data reported in the NEDU human study is entirely confluent with the finding of no difference between helium and nitrogen kinetics in relevant tissues in a carefully conducted animal experiment, published here:

D. J. Doolette, R. N. Upton, and C. Grant, "Altering Blood Flow Does Not Reveal a Difference Between Nitrogen and Helium Kinetics in Brain or in Skeletal Muscle in Sheep," Journal of Applied Physiology, Vol. 118 (2015), pp. 586-594.

I will certainly concede that in science we like to see important work replicated in other labs. Given that NEDU is the only group who have been able to conduct diving research using DCS as an outcome measure in humans over the last 50 years I would not hold your breath waiting.

We can add some context to this test and its meaning, from our own tech world experiences....snip....It would seem the nedu test profiles are very long compared to real world tech dives.

All this proves is that the divers on your database mainly use VPM and do not use the US Navy LEM-he8n25 probabilistic model to plan their decompressions. Hardly a revelation.

With such excess deco time added.....

Who says the decompression time is excessive? You impute that from the shorter length of decompression prescribed by your algorithm, but where has VPM ever been properly tested for outcome? There is no database of dives of known outcome for VPM or GFs for that matter. You / we have no objective idea of the true DCS rate associated with use of your program (or GFs - I am not trying to single out VPM here). And "true" in this context does not mean the outcome of retrospective voluntary surveys or some comfortable notion we might develop because not too many people have told us they got bent. It means prospectively observed divers exposed to relevant risk factors (such as underwater work) in an environment with a positive reporting culture (such as a study) conducting dives to the limits of the algorithm. When you gather data under those circumstances (as they did in this study) the measured incidence of DCS for any algorithm will be far higher than commonly perceived.

This elongated test profile was not "probing for the edge" of deco by any stretch of the imagination.

Have you read the methods? They used a probabilistic model to choose a trimix decompression profile that was assessed to carry an acceptable risk by navy standards (~2%), and which had maximal estimated risk separation against the same profile when conducted using heliox (estimated risk 5.6%). It was, by definition, "probing for the edge of deco". That the model over-estimated the risk for the profile when performed with heliox and under-estimated the risk for the same profile when performed with trimix is acknowledged in the report. The models are not precise tools, but the incidence in both cases fell within the 95% confidence intervals for the estimate.

In any event, the imprecise risk prediction by the model used to select the profiles does not invalidate the results of the study. Identical decompression profiles were performed with either heliox or trimix. The trimix dives resulted in two cases of DCS and the heliox dives resulted in none. This allowed the authors to conclude that the use of heliox was not inferior in terms of decompression efficiency to use of trimix.

Simon M

 

[rossh]

Ross,

It seems that NEDU cannot publish anything that meets with your approval.

Simon M

No... wrong again Simon.

I'm sure the Nedu's work is all above board. Its how the test is interpreted, its relevance, meaning and context to tech world, that is important to me and us all.

My objection is with the people, who deliberately over exaggerate the meanings of the nedu work (and other papers), and blow it out of context. All too often some individuals try to over hype the implications of the nedu work, in order to force an agenda for change onto the public.

The ideas presented in this thread, are not Sheawater ones... they get their ideas directly from you and David, and you collaborate with Shearwater on various projects, and with his funding too.

In essence, your participation and comments in this thread, is to rubber stamp your own ideas and theories.

What's needed here is a strong authoritative voice to keep the discussion centered, and stop the invalid ideas from shooting off at tangents. Your not doing that Simon.

Instead you seem to want to rant out loud about anything I write. You continue to write nasty and implied accusations about me, like the one above.

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[Freewillow]

No... wrong again Simon.

Instead you seem to want to rant out loud about anything I write. You continue to write nasty and implied accusations about me, like the one above.

.

Well, I do not share your opinion. I do not see nasty and implied accusations here.
On the contrary, from what I read from you two, the ratio Sciences/BS. is not weighted in your favor


Dr Freewillow, Ph.D.

 

[Dr Simon Mitchell]

I'm sure the Nedu's work is all above board.

Sorry, you will have to forgive me for failing to appreciate that when you referred to "off the scale decompression programs", "excessive decompression", "not probing the edge of decompression by any stretch of the imagination", " aberration in the results", "not able to properly predict the pDCS" etc you were really saying the that the study was all above board.

Its how the test is interpreted, its relevance, meaning and context to tech world, that is important to me and us all. My objection is with the people, who deliberately over exaggerate the meanings of the nedu work (and other papers), and blow it out of context.

Can you please point out where you think that is happening in relation to the heliox vs trimix study?

The ideas presented in this thread, are not Sheawater ones... they get their ideas directly from you and David

I don't think I have ever discussed this particular issue with anyone from Shearwater. To the best of my knowledge the link between this subject and shearwater is that they hosted a Michael Menduno article about the study on their website.

and you collaborate with Shearwater on various projects, and with his funding too.

Shearwater have provided us with research funding to study the behaviour of carbon dioxide scrubbers in rebreathers. That work has nothing to do with any of their products, or any of the debates I have with you. And their funding of work of no relevance to their business is a huge philanthropic service to the diving community. I don't think you will get much traction trying to make it out to be something inappropriate.

In essence, your participation and comments in this thread, is to rubber stamp your own ideas and theories.

Well, I am hardly going to rubber stamp your ideas and theories if I don't agree with them! Mostly I am defending investigations published by my colleagues (so not actually my "ideas and theories" at all), but when you stray into areas that I personally have investigated and published, then I will defend those too. The key point is that the "ideas and theories" that I defend are generally either investigated and published or at the very least widely endorsed among my colleagues. In contrast, you are frequently guilty of promoting unsupported notions held only by you.

Instead you seem to want to rant out loud about anything I write. You continue to write nasty and implied accusations about me, like the one above.

I probably should not grace this with an answer, but it is pretty funny that you complain that I am being "nasty" (for suggesting that you don't like things NEDU publish) given the things you wrote on the decostop, here (bottom of post 114 - or just about any of Ross's subsequent posts in the thread for that matter):

The Deco Stop

Simon M

 

[rossh]

I probably should not grace this with an answer, but it is pretty funny that you complain that I am being "nasty" (for suggesting that you don't like things NEDU publish) given the things you wrote on the decostop, here (bottom of post 114 - or just about any of Ross's subsequent posts in the thread for that matter):

The Deco Stop

Simon M

On the deco stop, we can write the truth, and expose all the facts... without interference or fear of having your post deleted. Such freedoms are not permitted here.

Feel free to join in the discussion over there......

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