Gradient Factors and Deep Stops

[tridacna]

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[Dr Simon Mitchell]

Of course, deeper stop style planning and bubble models have been the most successful deco approach in rec and tech dives for the last 15 years.

This is nonsense Ross. It implies that the "success" of deep stops and other approaches have been measured and compared, and that deep stops are better. In fact, the opposite is true. Every human study of the use of deep stops in decompression diving that has emerged so far has found that deep stop approaches are less successful. There are two studies of the effect of inserting a single deep stop in the ascent from no decompression dives on post dive bubble counts, and they draw opposite conclusions. This is hardly a basis for claiming that "deeper stop style planning and bubble models have been the most successful deco approach for the last 15 years". They have been a "popular" approach prior to emergence of the relevant data, but that is very different to "successful".

VPM follows the basic gas kinetic theories and formula (as used in every dive computer), which eliminates the possibility of the above.

This is completely incorrect. The gas kinetic formulae common to all decompression planning give us the ability to track hypothetical tissue gas pressures during decompression, but different models have completely different approaches to what they do with that information (how much supersaturation you allow in what tissues and when). That is why they prescribe different decompression Ross.

I can't believe you keep saying this. Do you really believe it? I suppose it is possible that you do. Your recent claim that it is the tissue half time and not tissue blood flow that largely determines gas uptake and elimination provided a clear demonstration of how little you really understand about "basic gas kinetic theories".

Simon M

 

[rossh]

This is nonsense Ross. It implies that the "success" of deep stops and other approaches have been measured and compared, and that deep stops are better. In fact, the opposite is true. Every human study of the use of deep stops in decompression diving that has emerged so far has found that deep stop approaches are less successful. There are two studies of the effect of inserting a single deep stop in the ascent from no decompression dives on post dive bubble counts, and they draw opposite conclusions. This is hardly a basis for claiming that "deeper stop style planning and bubble models have been the most successful deco approach for the last 15 years". They have been a "popular" approach prior to emergence of the relevant data, but that is very different to "successful".



This is completely incorrect. The gas kinetic formulae common to all decompression planning give us the ability to track hypothetical tissue gas pressures during decompression, but different models have completely different approaches to what they do with that information (how much supersaturation you allow in what tissues and when). That is why they prescribe different decompression Ross.

I can't believe you keep saying this. Do you really believe it? I suppose it is possible that you do. Your recent claim that it is the tissue half time and not tissue blood flow that largely determines gas uptake and elimination provided a clear demonstration of how little you really understand about "basic gas kinetic theories".

Simon M

You just make this stuff up and say anything to counter the truth that goes against your point of view, and think you can get away with it.


The most dominate deco method in the last 15 years, has been the deeper stop / slow ascent method across all types of diving. At the same time the injury rates / numbers / treatment / percentages / what-evers, have gone down, as recorded by every reliable metric, measure, and statistical tracking body.


But hey, why don't you show use again how you think there are less tech divers in the world today....


Model defined tissue perfusion topic:

You lost that argument because were not able to prove your claims.... or provide anything to counter my claims about the way models address this issue. So now you make up straw man arguments, and invent false attributions about my posted comments... as yet another cover up.

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[nadwidny]

The most dominate deco method in the last 15 years, has been the deeper stop / slow ascent method across all types of diving.
.

Ancient history. In the last 2 years most everyone with any experience diving deep has done away with deep stops. I'm even seeing people diving GFs of 80/90.

 

[JohnnyC]

Awww ****....here we go.....

 

[Dr Simon Mitchell]

The most dominate deco method in the last 15 years, has been the deeper stop / slow ascent method across all types of diving. At the same time the injury rates / numbers / treatment / percentages / what-evers, have gone down, as recorded by every reliable metric, measure, and statistical tracking body.

You continue to make schoolboy inferential errors in relation to this issue. Yes, many hyperbaric services (including my own) have noted a decline in DCS case numbers over the last 20 years. But proper scientists recognise that you can make no sense of this trend in the absence of a denominator (number of divers or number of dives) which would allow calculation of a rate. When we looked at this in our own jurisdiction [1] using new entry level certification numbers as a denominator we found that the number of new divers fell in approximate proportion to the fall in numbers of DCS cases (see below).

The publication of the UHMS remote DCI workshop proceedings in 2005 [2] also resulted in a reduction in recompression of milder cases globally. This will have contributed to the apparent reduction in recompressed cases reported by hyperbaric units.

Your notion that the downward trend in DCI case numbers can be attributed to the use of bubble models and deep stops is delusional, not least because all the directly comparative human studies in decompression diving show that deep stops approaches appear to be inferior. Why would you then conclude that a downward trend in DCI numbers is due to the use of an inferior approach? Moreover, the vast majority of the DCI cases that make up these data are from scuba air no decompression diving. Whatever is responsible for the trend, it provides us with little knowledge of relevance to decompression diving.

You lost that argument because were not able to prove your claims.... or provide anything to counter my claims about the way models address this issue.

You have some strange notions about what constitutes "winning" or "losing" an argument Ross. You were claiming that perfusion of tissues does not play an important role in tissue gas kinetics. First me, then the world's foremost decompression modeller came on to the thread citing substantial bodies of published evidence demonstrating that you were completely wrong. In reply you cited nothing except your own flawed perceptions. And here you are again, claiming that you "won" the debate. Why would anyone believe anything you say? For those interested, the discussion is on this board here. It is worth a read.

So now you make up straw man arguments, and invent false attributions about my posted comments... as yet another cover up.

Ross, you categorically stated, multiple times, that tissue gas kinetics do not depend on tissue perfusion, they depend on the tissue's half time or "tissue absorption rates". The thread is at the link above and is definitely worth a read, but here are some examples:

Exciting model gas kinetics theory and formula, follow the concept that the individual tissue is the limiting component of the uptake / off gas rate, as represented by a tissue half time value set. Its the balance of partial pressures against the tissues density and its ability to absorb excess inert gas, represented by a half time value. That is the basic theory you will find in all current models in use (VPM-B, ZHL, VVAL and more). The parallel tissue models do not support a perfusion limit in their calculations. Same can be said for the various serial tissue model designs

and this classic....

Our experience shows that on/off gassing rates / quantities, seemingly are not controlled by perfusion limited. Instead we appear to be limited by tissue absorption rates, exactly as the models are designed and operate.

Your position that it is half times or "tissue absorption rates", not perfusion that limits gas uptake is like saying its not oxygen that keeps us alive it is air. The most important component of a tissue's half time / gas absorption is its perfusion. Whilst everyone has a right to speak on public forums, I believe you go beyond those rights in commentating in such an authoritative manner on something you clearly don't understand on a forum where divers come to get educated.

Simon M

1. HAAS RM, HANNAM JA, SAMES C, SCHMIDT R, TYSON A, FRANCOMBE M, RICHARDSON D, MITCHELL SJ. Decompression illness in divers treated in Auckland, New Zealand 1996-2013. Diving Hyperbaric Med, 44, 20-25, 2014

2. MITCHELL SJ, DOOLETTE DJ, WACHOLZ C, VANN RD (eds). Management of Mild or Marginal Decompression Illness in Remote Locations – Workshop Proceedings. Washington DC, Undersea and Hyperbaric Medical Society, 240pp (ISBN 0 9673066 6 3), 2005

 

[Kevrumbo]

Going back to Erik C. Baker's excellent article explaining the trade-offs of utilizing Deepstops:

This is what GF 20/75 would look like for a 90msw depth, 20min BT (and by the implications of the NEDU Deepstop Study would still claim to be too deep for a first deepstop, with the adverse effect of the slow tissues still either on-gasing or persisting in a supersaturated state):

The NEDU Deepstop Study concluded, "Results indicate that slower tissue washout and continued gas uptake offsets the benefits of reduced bubble growth at deepstops." Applied to the Figure 3 graph above, this means that some of the intermediate and slow tissues compartments 9 through 16 will have critical surfacing supersaturated tensions and hence increases the risk of essential DCS.

The only practical way for recreational sport technical divers to counter the inert gas loading of these intermediate & slow tissues within a deepstops paradigm is to arbitrarily extend the shallow Oxygen stop profile longer than what a particular deepstops decompression algorithm prescribes, in order to reduce these surfacing supersaturation tensions.
-----

Conversely, again from the Baker article link above, this tissue compartment graph below could be construed as a radical interpretation & practical application of the results of the NEDU Study, with a discarding of deepstops all together, a non-GF classic Buhlmann dissolved gas algorithm implementation, and an earlier much more aggressive approach in the profile to the leading tissue compartments' M-values before the first deco gas switch. Note the huge over pressure gradient and supersaturation of the first five Fast Tissue Compartments. (The mitigating physiological assumption here though suggested by @Dr Simon Mitchell is "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. . .").

(Anybody try this 90msw profile in figure 1 above yet?)


The point is that GF 50/80 seems to be a happy medium and better conservative starting value of gradient factors used in the de-emphasis of deepstops compared to the two graphs above. IMHO, the trend is going to eventually lead to something like GF 75/75.

 

[leadduck]

I think the detrimental effect of deep stops on the slow compartments depends on the gas you breathe at the stop, and this is often ignored in these discussions, trying too much on finding out "what's the best choice of GFlow".

For example, say for one dive you calculated the deco profile with GF20/80 and the first deepest stop is at 21msw (70fsw) with EAN50. I think that's fine. The fast tissues will off-gas a bit slower with fewer bubbles during deco compared to GF80/80 while the slow tissues don't get a lot of inert gas load thanks to breathing EAN50 (inspired inert gas partial pressure at first stop is 1.55bar).

Whereas for a different dive profile the same GF20/80 may result in a first deepest stop at 39msw (130fsw) with bottom gas Tx18/45. Now you run into the problem of loading the slow tissues with 4.0bar inspired inert gas partial pressure in exchange for only little protection of the fast tissues. Increasing GFlow is probably a good idea then.

I expect that we cannot pick a new optimum GFlow, but should adjust it in a range per dive depending on the depth and inspired inert gas partial pressure at the deepest stop.

 

[DevonDiver]

I think the detrimental effect of deep stops on the slow compartments depends on the gas you breathe at the stop,...

This has been my approach for some time now, and I've been very happy with it.

If the gas breathed on a given stop permits off-gassing across all tissue compartments, then i really see no drawbacks to initiating staged decompression from as deep as possible.

Nonetheless, applying that principle at deeper depths becomes problematic. For a start, you'd need to know that ALL tissues were clearing... and that can't necessarily be presumed just because you've switched to a richer mix.

 

[boulderjohn]

If the gas breathed on a given stop permits off-gassing across all tissue compartments, then i really see no drawbacks to initiating staged decompression from as deep as possible.

All compartments? None of them will still be ongassing?