Deep Stops Increases DCS

[David Doolette]

Well, yeah, I did. But I'm happy for an answer from anyone who knows...

It just seems to me that logistic scoring would be way more appropriate for bubbling. But then, this may very well be a case of "good enough is good enough". (Difference between “Exponential Growth” and “Logistic Growth” – Explained!)

The best place to look is at
Møllerløkken A, Blogg SL, Doolette DJ, Nishi RY, Pollock NW. Consensus guidelines for the use of ultrasound for diving research. Diving and Hyperbaric Medicine. 2016 March;46(1):26-32. available at Diving and Hyperbaric Medicine Journal - DHM Journal, Immediate release articles.

The gold standard grading scale, in my opinion, is one for which the relationship between bubble grade and DCS is well established. With that in mind there is more data published for the Kisman-Masurel scale. The Spencer scale is, however, still recognized as a standard.

 

[Dr Simon Mitchell]

Dr. Mitchell,

Is this (below) the present grading protocol?

Hello,

Thanks for your question. David beat me to it.

Simon

 

[rossh]

For an unbiased look and review of VGE and it role in DCS, readers might like to see this paper:

Venous gas embolism as a predictive tool for improving CNS decompression safety

This is by a research group with many years of work, studies and papers in the topic of VGE.

As they say in the paper, "This illustrates that having detectable bubbles yields only a 4% chance of developing DCS. Even when using high bubble grades as test criterion, the positive predictive value is low..."


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The simple reality: Fast deco has high supersaturation tissue pressure (decompression) stress, fast offgas rates, and it excites VGE into higher growth. BUT... the reverse implied connection is wrong. High VGE does not imply high decompression stress, because high VGE can / do occur even when only lower decompression stress exists. The correlation is not 1:1.

All divers give different VGE scores. You and me can do the same dive together - and we will have different VGE scores - everytime. Some divers make no VGE (0), and some divers make a lot of VGE and bubble like a well shaken bottle of coca-cola. Trying to force all divers to conform to a size 2, is futile.


Do not confuse tissue DCS and VGE ! VGE is not tissue bubbles. VGE does not predict DCS.

Do not confuse Bubble models with VGE ! Bubble models work with tissue micro-bubble growth - not VGE.



Normal tissue DCS is theorized to grow from "extra vascular" bubble growth. This is your standard everyday DCS risk, and its what all deco models try to control and regulate - tissue bubbles - type 2 DCS. Controlling a dive by means of supersaturation stress levels, is the most accurate and reliable prediction / avoidance tool we have.


David Doolette: "Supersaturation is a required condition for bubble formation and growth. There is broad (universal?) agreement that modelling tissue gas uptake with a range of exchange rates - as we do for instance with a collection of compartments with mono-exponential gas exchange - captures the essential processes,..." ref


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The VGE bubble is "intra vascular" (grows in the veins). It grows from gas that has most probably already left the tissue, and is gas that is probably in transit to be exhaled by the lung. They are not tissue bubbles.



VGE does not normally cause us DCS - its usually filtered out in the lung, or dissolves quickly in the arterial side... except.... VGE will cause Pulmonary DCS - if you ascend directly to the surface, or do commercial / mil style surface chamber deco. VGE also has some connection to PFO / pulmonary shunt induced DCS - see DAN report.

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The DAN PFO workshop report describes issues of PFO / shunts and gives good summary suggestions.

However, one aspect of a PFO / shunt has been overlooked.... A PFO recirculates blood - unfiltered blood, that still contains high levels of inert gas. This will lead to an effective increase in inspired inert pressure, and as a result, we probably do not off gas as fast as planned.

Therefore, a good question might be: is a PFO induced DCS caused by rouge arterialized bubbles clogging up the plumbing? Or by higher tissue pressures than is planned? Or ?


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Here is the summary slide from the Dr. Neal Pollock presentation on Decompression Stress at the TekUSA 2016.


Note the salient points: VGE does not equal DCS. It can be a relative, but NOT an absolute indicator of stress, and needs to be used cautiously.

 

[ajduplessis]

Ross,

Your post demonstrates there is almost nowhere you won't go in an attempt to obfuscate the meaning of research whose results are negative to your product.

Simon M

You demonstrated the very same attributes and continue to do so. There is absolute no difference in both you and Ross's approaches IMO.

 

[The Chairman]

For an unbiased look and review of VGE

I don't want to come across as a contrarian here, but the idea of emboli in my blood scares the crap out of me. Isn't the very definition of an embolism an occlusion?

In addition, I was taught and have taught my students that the incidence of DCS is tantamount to noise, statistically speaking. In that regard, an incidence of %4 is way above statistical noise and that also scares the crap out of me.

Do these figures apply ONLY to decompression diving? Only to decompression diving under certain deco programs? I've done way over a hundred deco dives, how have I escaped DCS for so long? I feel like you're telling me I've been playing Russian Roulette for a long time.

 

[RainPilot]

I don't want to come across as a contrarian here, but the idea of emboli in my blood scares the crap out of me. Isn't the very definition of an embolism an occlusion?

In addition, I was taught and have taught my students that the incidence of DCS is tantamount to noise, statistically speaking. In that regard, an incidence of %4 is way above statistical noise and that also scares the crap out of me.

Do these figures apply ONLY to decompression diving? Only to decompression diving under certain deco programs? I've done way over a hundred deco dives, how have I escaped DCS for so long? I feel like you're telling me I've been playing Russian Roulette for a long time.

I suspect that a lot of it has to do with "aggressive" dive profiles as well as individual susceptibility.

For example, as a population each of us may have a 4% chance of having a genetic illness but on an individual basis you either have it or you don't. If you dive in a way that works for you, it SHOULD continue to work for you. That being said, adjusting your deco strategy to take advantage of new research as well as dialing up your conservatism as you age is probably the best way to go.

If you are diving right on your personal deco edge, then its possible that you may get hit when external factors vary. A mate of mine got a hit a few weeks ago, dive was one he'd done many times before with no problems, dive was clean, took a shoulder joint type 1 hit and rode the chamber. In his case he attributes a tight fitting wetsuit that he hadn't worn for a while and a strong current so he was hanging onto the line, leading to a circulation issue in the shoulder and so the hit.

FWIW he dives a deep stop profile, GF15/85. Thats the same profile a friend of ours used till he got a big hit here, spent some time in a bad way, then moved away from deeper stops toward (I believe) 30/85.

Its anecdotal, but several of our dive group here have experimented with higher GFLo since Dr Mitchell's presentation and all have reported feeling a lot less fatigue etc post dive.

 

[manni-yunk]

FWIW he dives a deep stop profile, GF15/85. Thats the same profile a friend of ours used till he got a big hit here, spent some time in a bad way, then moved away from deeper stops toward (I believe) 30/85.

Its anecdotal, but several of our dive group here have experimented with higher GFLo since Dr Mitchell's presentation and all have reported feeling a lot less fatigue etc post dive.

This is the exact frustration I tried to post earlier. Generally speaking when comparing profiles for many depths - a 15/85 will produce what I consider "Deep Stops ". (keep in mind, Im no expert ,so what I consider a deep stop should mean absolutely nothing)..which for some reason experts say is to difficult to actually define (I find it amazingly frustrating that experts can not agree on a definition of deep stops no matter how "difficult" it is to define....) You then mention that people you know have moved toward a 30/85....GUESS WHAT.....30/85 will produce a first stop almost exactly the same depth as most VPM +2 or+3 profiles (depending on the dive). So in reaction to the research that everyone is debating....your friends have moved from an actual deep stop profile to a seemingly now vilified VPM type profile but by calling it 30/85..its not a deep stop.

I still state that without a definition of deep stop - the overall technical diving community will continue to have this debate endlessly because we are debating apples and oranges. We all have our own baseline perception of what the phrase means and until we are speaking the same language.....we cant engage in meaningful conversation on the topic.

 

[RainPilot]

By deep stops, I understand this to mean stops significantly deeper than those mandated by a pure Haldanean model (USN, ZH-16 without GF etc) these will vary with any depth profile so can be very hard to put a number on.

I think the main problem here is that the conversation has devolved into an argument over which profiles are safe and which aren't etc etc whereas the main topic before was the efficiency of the different approaches.

My take-away from the NEDU study and all of the threads on here and RBW etc etc is that its the philosophy that is under question, not specific implementations. To paraphrase the 97 pages on this thread so far, and 1000's on others, my understanding so far:

1. In the past, as a result of numerous factors, including Dr Pyle's experiences and the gang at WKPP, a theory arose that there may be significant advantages to stopping deeper than previously the norm. Much of the evidence was based on "feeling better" after similar run-time dives as well as Doppler imaging after dives conducted by, among others, Dr Wienke.

2. This mindset led to the development of several theories to try account for the visible results and so some of the more popular "bubble models" began to appear. These were all designed with different methods but had in common that their profiles tended to have initial stops somewhat deeper than those prescribed by the USN tables etc.

3. Over the years these bubble models have been tweaked and refined to allow different levels of conservatism etc so the profiles have diverged a great deal depending on what the user inputs (VPM+?, GF etc) but all still have deeper stops than USN and Haldanean models (Buhlmann etc) mandate.

4. All these models have been based on theory and mathematical equations, as well as Doppler of divers returning from certain dives, mainly because nobody was volunteering to get bent for the sake of testing. Also, there is a large track record of "safe" dives by which I mean no reported DCS using these algorithms.

5. The US Navy observed all the fuss, and since one of the many claims over the years was that "bubble models get you up cleaner and faster than the old school", decided to experiment with the new (to them) bubble models with attendant deeper stops.

6. In order to design a suitable experiment they decided to keep all variables the same, including temps workload etc. They also designed the experiments to result in a measurable DCS incidence in order to confirm the edges of the envelope. In order to assess the efficiency of deep vs shallow stops, they designed profiles with the same runtime with the stops allocated deeper in one profile (A2) than in the other profile (A1). Their reasoning was that if the deeper stops were more beneficial, they should see a lower incidence of DCS in A2. Doubtless if that had been the case they would have continued the study in order to determine which particular algorithm got their divers out of the water cleanest, fastest.

7. What they found in the initial proof-of concept testing was that the A2 profile was significantly more DCS prone than the model they were currently using so they decided to stop further testing. In trying to decide why this would be (which the navy is not interested in as they won't be going to deep stops) the researchers involved crunched all the data to see if they could isolate a cause. They finally settled on the most likely candidate being that the slower compartments were remaining more saturated for longer and this would lead to more deco stress on the diver after surfacing. This is contrary to the previously prevailing opinion that it is more important to protect the fast tissues than the slow ones, hence deeper stops.

8. Since we are lucky enough to have members of that team on these and other boards, they elected to inform us of the results of that test and their best inferences of how it would be applicable to the diving we do. I know Dr Mitchell has upped his GFLo, leading to shallower first stops for his personal diving so he obviously believes that the test had relevance to him. He also presented a talk entitled "Decompression Controversies" (understatement of the year anyone?) which was disseminated on here and Youtube etc

9. Numerous role players have taken the above as an attack on them and their work (rightly or wrongly) and so the resulting discussion has become like a religious argument, with all the logic and reasoned debate that goes with that.

Me? Ive changed to a higher GFlo now, still deeper stops than a pure Buhlmann but not as deep as previously. I feel no worse than before and my friends who changed have reported at worst feeling no different and at best substantially better afterwards.

I also have many friends diving VPM with varying settings and so far all safe, so there are many ways to skin this particular cat but which one is the most efficient, least messy?

Thanks for the patience, this was almost as much for me to get my head around where we are at on this subject. PLEASE correct my many mistakes, I have no doubt Ive missed a lot.

 

[lowviz]

...//... I think the main problem here is that the conversation has devolved into an argument over which profiles are safe and which aren't etc etc whereas the main topic before was the efficiency of the different approaches. ...

...//... However, one aspect of a PFO / shunt has been overlooked.... A PFO recirculates blood - unfiltered blood, that still contains high levels of inert gas. This will lead to an effective increase in inspired inert pressure, and as a result, we probably do not off gas as fast as planned. ...

Which begs the obvious question:

For the most basic case of a diver with a stable and fixed-size PFO, would it not make more sense to time penalize the first few fast compartments rather than adding conservatism to all of the compartments?

 

[RainPilot]

I would imagine that with a fixed PFO i.e. blood (and bubbles) being recirculated potentially through the brain, it would be really important to keep those bubbles small as possible for as long as possible. That being said, a fixed open PFO would, for me, be a no-go for deco diving and probably require some fairly serious modifications to personal approach to NDL.

My opinion is worth exactly as much as I paid for my medical training i.e. zero. Only answering because you quoted me