Deep Stops Increases DCS

[lowviz]

Ha!

Take it slowly, it is pretty straightforward. Below is the full graph from 40' to 170' that includes USN "Exceptional Exposures".

Just for you:

 

[Patoux01]

@lowviz About the "bigness" of a dive, IANTD France uses "E = Phydro*sqrt(duration)" as a measure of "engagement" (not sure how that word would translate to English, but that's pretty much the bigness you're mentionning), see here on page 1. (edit: it's in french, no english version available to my knowledge)
Not sure it'd do anything different than what you just did but well...

 

[rossh]

You don't stop taking on gas until you reduce the pressure by surfacing. You can't cheat physics. It is impossible to stay longer and not continue to build saturation while holding deep stops. Dissolved gas in a tissue at a higher surface tension will try to equalize as ascent (pressure is lowered) begins in those tissues. Tissues that are at a lower tension will continue uptake until they reach saturation or the pressure is reduce below their tension pressure. If you have two dives of equal length where one moves to shallower depth faster, it will have less saturation than the one which holds a deep stops. Therefore the only way to account for this additional saturation is to add more decompression time, whether that is at a shallow water stops or on the surface. Holding a deep stop is still subsurface diving. If I hold a deep stop for a few minutes, it is no different than diving to that depth. I am becoming saturated at that pressure. The debate really hinges upon where the most efficient stops occur for stops below the surface and that minimize the incidence of DCS. I have become weary of certain "Theorists" acting as if things are fact. I can accept observed cause and effect with a high degree of reliability, but misrepresentation of "opinion" or "theory" as fact is unforgivable. I can write computer code and make things come out they way I want them to, however this does not mean that reality lines up with the computer model. The model must be verified with actual test data, not something pulled from one's head or other oriface.

This basic description is OK from a generalized wide angle point of view. But once the specific of the dive situation is added, some extra description is needed. The above described the on gassing, but missed the off gassing side of the equation during the dive and surface.


"Tissues that are at a lower tension will continue uptake until they reach saturation or the pressure is reduce below their tension pressure.


In ascent, this also applies in the opposite direction. Tissues on/off gas towards the inspired pressure line, which is always less than the divers ambient pressure. Hence while holding a stop, any tissue that is faster than the current ceiling limiting one, will be off gassing. A tissue that drops its pressure below the ambient line, contributes no further stress to the dive. This occurs in all dive profile patterns - shallow and deep. Any tissue that on gassed from the deeper stop, but also has a faster time than the current limiting one, will no longer have any influence on the ascent.


"If you have two dives of equal length where one moves to shallower depth faster, it will have less saturation than the one which holds a deep stops"


Whilst true, it is a hypothetical situation. Real dive models all get longer for the same dive and deeper stops, because of internal gas tracking changes. All models have this ability built in now.

But, this extra on gas, may not change the actual decompression limit or required time by any significant amount. A deco model sets it limits based on the maximum value of any one discrete cell limit values. If during the course of the dive, no significant change in cell pressure is observed, at the time that cell becomes limiting then deco times may change only slightly.

In the case of deep vs shallow stops the difference in gas load is actually quite minor. The cells have been subjected to typically 30 mins of full depth (7?) ATA differences. When it comes to the ascent, and the different in stop types, the extra pressure changes are small in the tissue, when compared to the depth exposure just carried out. i.e deep stop add very little extra to the overall gas volume.


This graph compares two profiles: with / without deep stops, and examines the resulting stress changes.

It hard to see much difference at all. That's because the deep stop changes are tiny compared to the effects of the bottom time segment. In this sample, it adds about 12% to the deco time.


You can see in the pure deep stop comparison:

• Fast cells have half the decompression stress in the deep stop dive.
• Fast cell time surface pressure not affected by deep stops.
• The most affected slow cell in deep stop (#9), at surface time, only rose 4kPa .
• The surface stress / off gas has risen by an average of 4kPa across whole ascent.

The real world affect of a deep stop approach, is to significantly decrease in-dive stresses, but only increase surface stress and required deco time by a small amount.

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[rossh]

Read the paper. Some animals had high VGE, some had low VGE. ALL were autopsied (including the living ones who were sacrificed after an hour). The ones with high VGE had high tissue bubbles. The ones with low VGE had low tissue bubbles. Therefore VGE and tissue bubbles were correlated.

Correlated to what??

The rabbits died because they skipped 4 hours of human level deco (8 hours "Simon Mitchell" deco).

All rabbits absorbed to same volume of gas.... All rabbits were decompressed in the same deathly (no deco) manner. All rabbits tried to off gas the same volume of bottom gas, for all the same reasons.


But the observed difference is:

• half converted the dissolved venous gas into lots of VGE gaseous state, and died as a result (pulmonary DCS),
  
• while the other half off gassed mostly in a dissolved state, and avoided near instant death.

They both off gassed the same volumes of gas, but in different gaseous states.


That is the question that needs to be solved..... what causes some subjects to transport / convert the same volume of gas into a different states, where, why and how?

Why did rabbit A convert a lot of his gas into VGE, while rabbit B kept his gas in a dissolved state ??? And where exactly did it form in his veins?

Find the answer to that question. Only then will we know the real value of VGE. What makes us a rabbit A, or a rabbit B?



In the test they killed some rabbits by skipping a bunch of deco, and found venous bubbles. Big deal. Doesn't add anything to the knowledge. I have to think the researchers were hoping to see a lot more than they did.


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Simon, please stop trying to tell the public that bubble models manage VGE - they do not. Please stop trying to tell the public that tissue bubbles are venous bubbles - they are not.


No deco model in use today, bothers with tracking VGE... there is no point - it adds nothing to the equation.

VGE are everywhere in almost all tech dives, and half of all recreation dives, yet they remain harmless, for at least 99.95% of the time. Obviously they are not a source of wide spread trouble. Yet you seem to want to over emphasize the value of VGE based on no science that supports that conclusion.


VGE are secondary indicator at best, unreliable, poor predictors of any useful data, cannot be used in an absolute measure (deco model). That is the expert opinion, and I suggest everyone start paying attention to it.


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

I hope this does not become yet another attempt, to do an end run around the peer review process, and play voodoo scary hyperbole games with VGE on the public That would be a disgraceful abuse of privileged.



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[Storker]

That is the question that needs to be solved..... what causes some subjects to transport / convert the same volume of gas into a different states, where, why and how?

WAG: this thing called variability between subjects, perhaps? Maybe the same reason that if you expose many enough subjects to conditions that carry a 5% risk of DCS, then about 5% of the subjects will be bent? In this experiment about 50% of the subjects died, so ipso facto this was a profile with a 50% DCS fatality risk for rabbits.

I hope this does not become yet another attempt, to do an end run around the peer review process,

I'm not quite certain what this is supposed to mean, but if only verbatim quotes from peer reviewed studies can be used as arguments in the ongoing discussion, I expect Ross to give us a list of his peer reviewed papers accepted in reputable scientific journals. We can then judge the two sides' merits impartially. I would expect Ross to follow the same strict guidelines he is demanding from Simon, so I assume he won't post anything he can't document with citations from these papers.

 

[boulderjohn]

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[David Doolette]

I hope this does not become yet another attempt, to do an end run around the peer review process, and play voodoo scary hyperbole games with VGE on the public That would be a disgraceful abuse of privileged.
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You have tried this “peer-review” angle a few times lately. Do you really want to go down that path Ross?

Peer-review is the scientific convention whereby new pieces are added to the body of scientific knowledge only after passing a filter of independent subject matter experts who determine the work is technically sound and the interpretation of the results is valid. As has been pointed out already, no such convention exists for internet forums. A feature of internet forums is that anybody can post with a voice of authority, even if they have no true subject matter expertise – a feature you take full advantage of Ross. This fact alone makes it laughable that you should call for such a filter for people with opinions contrary to your own. Furthermore, two of the targets of your call for peer review, Simon and I, whose opinions concerning VGE are similar, are careful to base our opinions on peer-reviewed science.

But, if you really want to appeal to peer-review, here are the references of my three peer-reviewed publications concerning the uses (and limitations) of VGE measurements as an indicator of the probability of DCS:

Møllerløkken A, Blogg SL, Doolette DJ, Nishi RY, Pollock NW. Consensus guidelines for the use of ultrasound for diving research. Diving Hyperb Med. 2016 Mar;46(1):26-32.

Doolette DJ. Venous gas emboli detected by two-dimensional echocardiography are an imperfect surrogate endpoint for decompression sickness. Diving Hyperb Med. 2016 Mar;46(1):4-10.

Doolette DJ, Gault KA, Gutvik CR. Sample size requirement for comparison of decompression outcomes using ultrasonically detected venous gas emboli (VGE): power calculations using Monte Carlo resampling from real data. Diving Hyperb Med. 2014 Mar;44(1):14-9.


These are long and technical reports, but their peer-reviewed conclusions can be distilled as follows:

“Ross, you message concerning VGE is wrong.”


For those who want a longer version, I have copied the abstracts below and I would direct you to my post #973 http://www.scubaboard.com/community/threads/deep-stops-increases-dcs.497187/page-98 which is my summary of the scientific consensus on the utility of VGE.

Møllerløkken A, Blogg SL, Doolette DJ, Nishi RY, Pollock NW. Consensus guidelines for the use of ultrasound for diving research. Diving Hyperb Med. 2016 Mar;46(1):26-32.
The International Meeting on Ultrasound for Diving Research produced expert consensus recommendations for ultrasound detection of vascular gas bubbles and the analysis, interpretation and reporting of such data. Recommendations for standardization of techniques to allow comparison between studies included bubble monitoring site selection, frequency and duration of monitoring, and use of the Spencer, Kisman-Masurel or Eftedal-Brubakk scales. Recommendations for reporting of results included description of subject posture and provocation manoeuvres during monitoring, reporting of untransformed data and the appropriate use of statistics. These guidelines are available from www.dhmjournal.com.


Doolette DJ. Venous gas emboli detected by two-dimensional echocardiography are an imperfect surrogate endpoint for decompression sickness. Diving Hyperb Med. 2016 Mar;46(1):4-10.
INTRODUCTION: In studies of decompression procedures, ultrasonically detected venous gas emboli (VGE) are commonly used as a surrogate endpoint for decompression sickness (DCS). However, VGE have not been rigorously validated as a surrogate endpoint for DCS.
METHODS: A data set for validation of VGE as a surrogate endpoint for DCS was retrospectively assembled comprising maximum VGE grades measured using two-dimensional echocardiography and DCS outcome following 868 laboratory man-dives. Dives were conducted according to only ten different experimental interventions such that the ten cumulative incidences of DCS (0-22%) provide relatively precise point estimates of the probability of DCS, P(DCS). Logistic models relating the P(DCS) to VGE grade and intervention were fitted to these validation data. Assessment of the models was used to evaluate the Prentice criteria for validating a surrogate endpoint.
RESULTS: The P(DCS) increased with increasing VGE grade. However, the difference in the P(DCS) between interventions was larger than explained by differences in VGE grades. Therefore, VGE grades did not largely capture the intervention effect on the true endpoint (DCS) in accord with the Prentice definition of a surrogate endpoint.
CONCLUSIONS: VGE can be used for comparisons of decompression procedures in samples of subjects but must be interpreted cautiously. A significant difference in VGE grade probably indicates a difference in the P(DCS). However, failure to find a significant difference in VGE grades does not necessarily indicate no difference in P(DCS).


Doolette DJ, Gault KA, Gutvik CR. Sample size requirement for comparison of decompression outcomes using ultrasonically detected venous gas emboli (VGE): power calculations using Monte Carlo resampling from real data. Diving Hyperb Med. 2014 Mar;44(1):14-9.
INTRODUCTION: In studies of decompression procedures, ultrasonically detected venous gas emboli (VGE) are commonly used as a surrogate outcome if decompression sickness (DCS) is unlikely to be observed. There is substantial variability in observed VGE grades, and studies should be designed with sufficient power to detect an important effect.
METHODS: Data for estimating sample size requirements for studies using VGE as an outcome is provided by a comparison of two decompression schedules that found corresponding differences in DCS incidence (3/192 [DCS/dives] vs. 10/198) and median maximum VGE grade (2 vs. 3, P < 0.0001, Wilcoxon test). Sixty-two subjects dived each schedule at least once, accounting for 183 and 180 man-dives on each schedule. From these data, the frequency with which 10,000 randomly resampled, paired samples of maximum VGE grade were significantly different (paired Wilcoxon test, one-sided P ⋜ 0.05 or 0.025) in the same direction as the VGE grades of the full data set were counted (estimated power). Resampling was also used to estimate power of a Bayesian method that ranks two samples based on DCS risks estimated from the VGE grades.
RESULTS: Paired sample sizes of 50 subjects yielded about 80% power, but the power dropped to less than 50% with fewer than 30 subjects.
CONCLUSIONS: Comparisons of VGE grades that fail to find a difference between paired sample sizes of 30 or fewer must be interpreted cautiously. Studies can be considered well powered if the sample size is 50 even if only a one-grade difference in median VGE grade is of interest.

 

[lowviz]

@lowviz About the "bigness" of a dive, IANTD France uses "E = Phydro*sqrt(duration)" as a measure of "engagement" (not sure how that word would translate to English, but that's pretty much the bigness you're mentionning), see here on page 1. (edit: it's in french, no english version available to my knowledge)
Not sure it'd do anything different than what you just did but well...

Patoux01,

I really appreciate your insight.

Your “Exposure” is in the form of Pressure*sqt(Time). Simply squaring both sides gives a form that matches what I found by purely numerical operations on the USN Air Tables. Thus, my physically meaningless score is really only a scaled version of your “Exposure” squared.

You can tell IANTD France that their “Exposure” equation was validated by the most independent of means.

 

[Patoux01]

Nice to know that it seems to maybe relate to something. I guess it comes from COMEX initially as that article has been written after some discussions between the head of IANTD France and B. Gardette.

 

[rossh]

You have tried this “peer-review” angle a few times lately. Do you really want to go down that path Ross?

Hi David,

I have original copies of those here - I'm subscribed to the journals.

I agree with them, and in particular, the warnings each one puts on the limitations and restrictions, of using VGE data.

You guys have to live up to the same limits. And because this is a public forum and not a science forum, you need to be extra vigilant in exercising cautions and restraint on the use of VGE and it implications.

.