Deep Stops Increases DCS

[rossh]

Ross, the answer is there.... right in your own graphs. The graphs are flawed in that unlike Kevin's heat maps they don't clearly demonstrate what is happening after surfacing (probably the most important period of supersaturation in slower tissues) and they don't clearly distinguish tissues, but they nevertheless illustrate the important principles. They show that the NEDU deep stops profile (A2) did reduce supersaturation in the faster tissues early in the ascent. But even with the post-surfacing data cut off (why did you do that Ross?) it is actually clear (from your own graphs and contrary to your "it ain't there" claim above) that there is greater supersaturation in in the slower tissues later in the ascent. If protecting fast tissues from supersaturation early in the ascent is so critical to decompression efficacy (as fundamentally assumed by bubble models) why did the profile that achieved that result in most DCS? The answer is that the problems are arising from slower tissues exposed to more sustained supersaturation later in the ascent (as shown in your own graphs).


Simon M

Spare a thought for Commercial and Military divers doing Surface Decompression on this 170ft, 30 min dive: (ascend from 40 ft to surface - no stops, complete the deco on O2 in deck chamber). Here is their supersaturation:





All dive planning is a compromise. Here the SurD procedure is to accept an intolerable supersaturation pressure, for a few minutes, and then correct for that with an abundance of 100% O2 deco. Its an aggressive deco where larger volumes of gas are moved around quickly. Consequently the surface period has rather low supersaturation. This deco works because it's a balance between bad and not so bad factors.


ZHL is a balance between bad and not so bad factors. It has a higher supersaturation to begin with and a modest amount to end with. Consequently its surface period supersaturation can be higher than the SurD process, because the ZHL deco method shifted some of the overall stress out the dive and onto the surface period.


VPM is a balance between bad and not so bad factors. It has lower supersaturation across the whole ascent, and keeps it almost constant throughout, and finishes at the same place as ZHL does. VPM's surface period supersaturation can be higher than the SurD process, and higher than ZHL model because VPM shifted some more of the overall stress out the dive and onto the surface period.


All three procedures work, because they are a balanced compromise between conflicting bad factors. One compromise is no better than the next one. These ideas that one deco is more "efficient" than the next, is misguided.



Our surface supersaturation graphs are perfectly good, and show the important details only - that's the part where off gassing occurs. They show sustained maximum pressures. A scientist might find that of interest too.

The heat maps you refer are colorful eye candy only, that are absent pertinent details. But I can understand the appeal of those flashy marketing materials to trick the pubic into a false belief.


*****

Here is the surface off gas charts that you requested. The Nedu dives, and a VPM-B and a GF 60/85 dive.




Notice how the A1 and A2 dives are well below normal dive levels. That's because the deco was stretched double length. Notice how your "increased integral of supersaturation and time", is not there either.


On the subject of "integral of supersaturation and time" Before you completely shoot your foot off, consider this: A decompression dive that misses all stops, and blasts to the surface for an agonizing death from DCS on the boat deck, this diver will have a low "integral of supersaturation" value. That goes for both in dive and post dive. But he will have some awful big, supersaturation generated, bubbles clogging up the plumbing.

Conversely, a dive that follows the slowest possible ascent rate at the ambient pressure line (GF 0), will have an extremely large "integral of supersaturation and time" value.

Low is bad, high is good. It's an inverse measure. That's the problem when you just "make stuff up" Simon. The way you guys add up 16 concurrent values is flawed, and it includes harmless levels of surface off gassing. This "integral of supersaturation" is worthless as a measure, and in any case, it demonstrates VPM to be the safer approach. But then, I'm more intelligent that to get suckered into this "integral of supersaturation and time" nonsense.


***********

ZHL and VPM both use the Haldane and Schriener equations to track gas pressures (as do many other models). These formula will track concurrently the on gassing, and off gassing, at any point in the dive, including ascent and deco. In any dive from any model, the ascents and deco stops will be both on gassing, and off gassing at different ends of the cell table.
Shallow stop models on gas in deco, deep stop models on gas in deco, multilevel ascent on gas, and extending shallow stop on gas too. They all on gas in deco and ascent. The equations handle this normally, and every deco model recomputes its ascent at each step. The gas tracking calculation in use today of current gas pressures values seems to be correct.

But you keep trying to imply it's not working, or insufficient. So kindly stop dancing around and get to the point. Tell us what is wrong with the Haldane / Schriener equations, and how you propose to change that?

These test profiles should have been fantastically safe by ZHL / GF standards.

And how would you know that Ross? Where is your database of carefully standardized ZHL / GF air dives to 170' for 30 minutes with air decompression, ....

Simon M

Did you forget, or just pretending to forget?

There is the DCIEM tables. These were created by Canadian DRDC, including VGE correlations, and man tested, with known pDCS risks. The DCIEM tables are widely approved for commercial and government work. This DCIEM table prescribes a deco time of 77 minutes, vs the 174 of the nedu test. The nedu test is 2.25 times longer than needed. The astute scientist would see the anomaly of the nedu test results, and realize it's out of context with the bigger picture.


More simply, all you need to do is apply the "plain obvious" test. The nedu profiles took normal profile and extended them out 2x longer in the shallow zone. Never mind all the fantasy descriptions and USN dive politics in this - these are two shallow stop dive profiles.

It's the same process that is applied by millions of divers, and recommended by anyone in deco medicine - to make it safer, make it longer in the shallow part. And that's what the nedu test did.

Which brings us to the conundrum you face. Your suggested fix for this, is to go more shallow, and go longer in deco... towards the higher risk A2 profile attributes.

 

[Dr Simon Mitchell]

Ross,

Re:

1. "shooting my foot off". I feel I am at very low risk of shooting my foot off in an argument about decompression and DCS. Having treated a number of the sort of cases (fulminant DCS) you refer to after omitted decompression 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. Indeed, since you raised the example of surface decompression on oxygen, would you care to explain why the various tables instruct you to get the diver across the deck and recompressed within 5 - 7 minutes? If the integral of supersaturation and time didn't matter, why not come up, have a coffee and a scone, and recompress later (or even at all?). Since you're "more intelligent that to get suckered into this "integral of supersaturation and time" nonsense" I'm sure you can explain this to me.

2. "to trick the public into a false belief": Why on earth would I or any of the other scientists involved in this debate want to do that?

3. The Haldane and Schreiner equations. A complete red herring. There is nothing wrong with them. I have no idea why you raised this.

4. The DCIEM tables. I am well aware of the pedigree of these tables. However, I challenge you to provide any evidence that they tested 170ft / 30 minute air decompression dives with human subjects working at at the same intensity as the NEDU divers, in 30 degree water and no thermal protection, with sufficient repetitions to derive an accurate probability of DCS for the 77 minute decompression profile that their tables prescribe. In other words, do they really know the risk of DCS for their decompression following a 170/30 dive performed under the conditions of the NEDU study?

5. "these are two shallow stop profiles": no they are not. They are profiles that generated tissue supersaturation patterns typical of one that emphasizes deep stops (like your own algorithm) and a one that does not emphasize deep stops. The prevention of fast tissue supersaturation early in the decompression at the expense of greater slower tissue supersaturation later in the decompression was shown to produce more DCS. Sorry Ross. That is the reality.

Simon M

 

[rjack321]

No objection to this conclusion Simon, but I want to know why it is so and what you think is physiologically going on . . . vis-a-vis the Fast Tissues. What makes these tissues so "robust" that protecting these fast tissues from transient peak SS "does not seem to matter"?

Its quite evident that clinically it doesn't matter. Perhaps these led to the low levels of DCS found in the shallow stop profile. But the important part is that from a risk perspective having transient fast tissue spikes, "the bend" in the popular phase "bend and mend" is not important clinically.

 

[ajduplessis]

. The prevention of fast tissue supersaturation early in the decompression at the expense of greater slower tissue supersaturation later in the decompression was shown to produce more DCS. Sorry Ross. That is the reality.

Simon M

Sorry Simom but the reality is that nobody dives anywhere close to the profiles the study tried to simulate wrt to deep stops. It not even deep stop, they are long multi level dives. The bottom line is the study is junk because it based on junk profiles. At least you ticked the boxes to justify the effort.

 

[rjack321]

Sorry Simom but the reality is that nobody dives anywhere close to the profiles the study tried to simulate wrt to deep stops. It not even deep stop, they are long multi level dives. The bottom line is the study is junk because it based on junk profiles. At least you ticked the boxes to justify the effort.

Check out the 200ft for 30mins with 18/45, EAN50 and O2 deco heat maps posted earlier. SAME EXACT phenomenon with increased saturation in slower tissues on surfacing.

 

[Kevrumbo]

Its quite evident that clinically it doesn't matter. Perhaps these led to the low levels of DCS found in the shallow stop profile. But the important part is that from a risk perspective having transient fast tissue spikes, "the bend" in the popular phase "bend and mend" is not important clinically.

Richard, since this is so clinically evident that you dismiss it as not important (and perhaps you really meant "statistically apparent phenomena" vs "clinically evident"), then explain from a physiological basis why allowing supersaturation of Fast Tissues in this instance is less risky to those tissues and results in a lower overall incidence of DCS in the shallow stops trials of the NEDU Study. . . ?

(Same question again to Simon, and any insight from Ross is welcome too).

 

[Dr Simon Mitchell]

Sorry Simom but the reality is that nobody dives anywhere close to the profiles the study tried to simulate wrt to deep stops. It not even deep stop, they are long multi level dives. The bottom line is the study is junk because it based on junk profiles. At least you ticked the boxes to justify the effort.

Hello AJ,

Both myself and others have put considerable effort into explaining why the NEDU study is relevant to the profiles we dive. You have seen these discussions. The vast majority of observers of the deep stops thread on RBW were convinced by these arguments. You, obviously, are not. Fine by me. However, allow me to say that I find that your statement above to be academically unpersuasive, and I wonder how hard you have really tried to understand the issues. In any event, stop as deep as you like. But be aware that there are no (and never have been) human data that support this approach. In contrast, there is a growing evidence base suggesting that emphasizing deep stops is disadvantageous. Your choice.

Simon M

---------- Post added February 28th, 2015 at 08:47 PM ----------

Richard, since this is so clinically evident that you dismiss it as not important (and perhaps you really meant "statistically apparent phenomena" vs "clinically evident"), then explain from a physiological basis why allowing supersaturation of Fast Tissues in this instance is less risky to those tissues and results in a lower overall incidence of DCS in the shallow stops trials of the NEDU Study. . . ?

(Same question again to Simon, and any insight from Ross is welcome too).

Hello Kev,

The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum I attended last week. Hopefully this will find its way into the literature at some point soon. In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).

Simon M

 

[ajduplessis]

Academically unpersuasive is the exact same words that comes to mind when studying NEDU. At least we agree on something

 

[Nasser]

Academically unpersuasive is the exact same words that comes to mind when studying NEDU. At least we agree on something

What would you do differently? Can you be a little more specific about how you would design a study on this subject ?

 

[Kevrumbo]

. . .
Hello Kev,

The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum I attended last week. Hopefully this will find its way into the literature at some point soon. In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).

Simon M

Alright, very good & fair enough on the Physiology.

IMO from a basic anatomy & physical chemistry view as it relates to general deco theory, it simply looks like the classic dynamic interplay between (Blood) Perfusion, and (Inert Gas) Diffusion Models. The aqueous Fast Tissues are naturally "overperfused" with blood vessels/capillary beds, and so can washout & transport the inert gases to the lungs more efficiently, while the lipid Slow Tissues are "underperfused" and rely more on the inert gas physical property of diffusivity, and that the rate of off-gassing by diffusion can be limited by inert gas solubility in tissue.

In the NEDU Study the breathing mixture was Air, with the naturally high inert gas constituent of Nitrogen, which we all know is very soluble in lipid tissue. In short it appears then as a result, again as noted above from the study's conclusion, that these Slow Tissues supersaturated with persistence because of Nitrogen's greater solubility along with ambient pressure inert loading effects of the Deep Stops, with statistically significant incidence of DCS upon surfacing. Whether this was a fair test for the Dual Phase/Bubble Models (VPM, RGBM, and Ratio Deco Method), is open for debate and hopefully will inspire further stochastic testing. (At the very least for now, we get more interesting "eye candy figures & charts ! I can hardly wait for the results of UTD's Ratio Deco Experiment . . )

Finally, the practical reaction of divers here & elsewhere -besides bashing Ross- is to go with and adjust Buhlmann Algorithm GF's to moderate or eliminate Deep Stops, some even applying it with Trimix bottom gas. (These Lo/Hi GF's are all over the place & wildly variable, reading & looking back through the posts in this thread and over there on Rebreather World --I agree with Simon's recommendation posted on RBW of starting out with GF's 40/70, and then end up with whatever works well for yourselves).