NEDU Study

[rjack321]

An attempt to discuss. I admit to not having read every single blog post on it.... in these long threads the information is sometimes quite buried between memes, jokes and questions about computer settings, don't you think so? I'd be still curious to read arguments countering mine. How is testing a deco schedule made with one model based on a calculated measure from the same model not circular? How is an experimental study with heavy (possibly aerobic) exercise at depth comparable to civilian tech diving? I'm genuinely curious and hope to learn something.

The models aren't relevant. The fact that the divers who stopped deeper had a higher bent rate than the shallower profile is the salient point. And they had to terminate the study early because those two different bend rates became statistically different (human subjects review won't allow you to hurt people once statistically significant end points are reached).

This explains why, the intermediate tissues end up with more gas loading via the deeper stops.

 

[nadwidny]

by feels welcome, you mean the list of very well known and respected divers that have been unceremoniously banned for no just cause other than having a disagreement with the owner?
RBW and TDS have basically been demoted to classifieds...

Pretty much except for the NEDU study thread.

definitely time to have the discussion here

Because we would expect something different here? I would seriously doubt that.

 

[klausi]

The models aren't relevant. The fact that the divers who stopped deeper had a higher bent rate than the shallower profile is the salient point. And they had to terminate the study early because those two different bend rates became statistically different (human subjects review won't allow you to hurt people once statistically significant end points are reached).

This explains why, the intermediate tissues end up with more gas loading via the deeper stops.

Many thanks for the answer and the interesting video.
The lecture is very well given, the Kiwi accent is pleasant to my ears. It’s an introductory lecture explaining the original reasoning, though, I do not believe it answers my SPECIFIC objections to the original NEDU study (I am not talking about any contribution to the discussion afterwards).

- He states that the ergometer work was easy and set to 115W. The NEDU study writes (p5) that "
set at 65 watts so that divers’ work rate (incorporating the extra power required due to submersion in this diving dress) was approximately 134 watts.", and this is so hard that in fact some divers could not keep it up. This is a physiological regime very different from what is normally encountered during tech dives (anaerobic even?). Has that issue of exercise intensity specifically been addressed?

- The models DO matter. The heat maps he presents after min 34, and the fig. 4 and afterwards in the NEDU study compare deco schedules based on calculations grounded in a gas content model. This whole argument compares models based on a measure calculated from (the same) models, naturally introducing a bias. I have been making computer models of biological processes for the last 15 years, and this struck me right away when reading up on the topic. Has this issue specifically been addressed? Many thanks!

 

[Beau640]

I just want to verify that this is seriously here to generate discussion...looking through OP's post history almost every single post is a thread directing people to his blog...Everything you're replying with has been answered by Simon Mitchell himself in that thread. Like your questions were already stated/asked verbatim over there.

The Ethics of Full Disclosure on ScubaBoard

 

[klausi]

I just want to verify that this is seriously here to generate discussion...looking through OP's post history almost every single post is a thread directing people to his blog...Everything you're replying with has been answered by Simon Mitchell himself in that thread. Like your questions were already stated/asked verbatim over there.

Yes...... I wanted to ask two very specific questions, outlined above. It seems I can't get an answer other than "This has been answered in a very long thread". My questions are certainly not answered in the video linked above. Would you mind to give a very brief summary of the answers if they are so well known and obvious? Thanks in advance.

It's not ok to link to your own blog? I think it might be informative. You don't have to read it of course. And I post other things of course, too.

 

[PfcAJ]

You've got to remember that there had to be a high enough instance of DCS to produce enough data points given the number of man-dives they could do. Hot on the bottom and cold on deco is the combo needed to produce more DCS.

For most divers, hot then cold is how dives are done. Maybe it's not as magnified, but it's there. Youre generally swimming about on the bottom. You're core temp drops a bit over the course of a dive. Deco is generally at rest. I know I've been downright shivering on deco before after doing work relining cave passage for the better part of an hour at 160'. The NEDU conditions are exaggerated perhaps but the general situation holds true. Effort then relax. Warm then cold.

This has been the first study testing the efficacy of deep stops. There's been no experimental evidence to support deep stops being more efficient. Now we have evidence to suggest against that idea.

 

[klausi]

You've got to remember that there had to be a high enough instance of DCS to produce enough data points given the number of man-dives they could do. Hot on the bottom and cold on deco is the combo needed to produce more DCS.

For most divers, hot then cold is how dives are done. Maybe it's not as magnified, but it's there. Youre generally swimming about on the bottom. You're core temp drops a bit over the course of a dive. Deco is generally at rest. I know I've been downright shivering on deco before after doing work relining cave passage for the better part of an hour at 160'. The NEDU conditions are exaggerated perhaps but the general situation holds true. Effort then relax. Warm then cold.

This has been the first study testing the efficacy of deep stops. There's been no experimental evidence to support deep stops being more efficient. Now we have evidence to suggest against that idea.

Many thanks - makes sense. I guess I am spoiled here in the Philippines with 26C at 80 meters and 29-30C during deco.

 

[Dr Simon Mitchell]

In the NEDU study (the 2nd, theoretical part of it), decompression stress is calculated based on "comparing changing gas pressures and development of gas supersaturation in
modeled tissue compartments" - a dissolved gas model. Unsurprisingly, the deco schedules based on a dissolved gas model do better.

I have read too many scientific papers not to pick up that case of somewhat circular reasoning.

Hello Klausi,

It is not circular reasoning. I think you have misinterpreted it.

The second "theoretical" part of the NEDU report addresses the issue of why they got the result they did, and whether there are any decompression profiles that could be expected to give a different result (in particular, they asked the question "are there any deep stop / bubble model profiles that would give a better result than the shallow stops / gas content model profile that we tested").

To answer the latter question they started from the premise that tissue supersaturation is the fundamental determinant of decompression stress and bubble formation, and that bubble formation produces symptoms of DCS. There would be few (if any) decompression scientists who do not believe in this paradigm. With that in mind, their analysis calculated supersaturation in a representative fast tissue, a slow tissue, and a combination of the two for 500,000 possible ways of decompressing from the NEDU test dive profile. These possible decompression approaches inevitably included many that would look like a bubble model (with more emphasis on deep stops) and many that would look like a gas content model (with less emphasis on deep stops). No profile that emphasised deep stops more than their tested gas content model decompression (their "shallow stops profile) produced lower combined tissue supersaturations. They therefore concluded that no profile emphasising deeper stops than their gas content model profile would be likely to produce better outcomes.

I believe you are mistakenly equating the basic mathematical tools used to calculate supersaturation in tissues with gas content models (or dissolved gas models as you put it). In fact, all models (bubble models and gas content models) use these same basic tools to calculate dissolved gas tissue pressures and tissue supersaturation. What differs between models is how they then interpret allowable supersaturation in different tissues at different stages of the ascent. But it is perfectly legitimate use these basic tools to compare the supersaturation patterns in tissues produced by different models. This does not amount to evaluating a bubble model using a gas content model as you seem to believe. It amounts to comparing a bubble model and gas content model using mathematical tools common to both of them. It therefore does not create circular argument.

Can I say before this goes any further that I have little appetite for debating this entire issue again. Ross Hemingway and I have finished such a discussion just weeks ago on TDS, and both of us had a fair amount of air time. You can evaluate that for yourself - it is not hard to find. The discussion has also taken place previously on this forum.... on every forum in fact. I really don't see the point in trundling it all out again.

Simon M

 

[klausi]

Hello Klausi,

It is not circular reasoning. I think you have misinterpreted it.

The second "theoretical" part of the NEDU report addresses the issue of why they got the result they did, and whether there are any decompression profiles that could be expected to give a different result (in particular, they asked the question "are there any deep stop / bubble model profiles that would give a better result than the shallow stops / gas content model profile that we tested").

To answer the latter question they started from the premise that tissue supersaturation is the fundamental determinant of decompression stress and bubble formation, and that bubble formation produces symptoms of DCS. There would be few (if any) decompression scientists who do not believe in this paradigm. With that in mind, their analysis calculated supersaturation in a representative fast tissue, a slow tissue, and a combination of the two for 500,000 possible ways of decompressing from the NEDU test dive profile. These possible decompression approaches inevitably included many that would look like a bubble model (with more emphasis on deep stops) and many that would look like a gas content model (with less emphasis on deep stops). No profile that emphasised deep stops more than their tested gas content model decompression (their "shallow stops profile) produced lower combined tissue supersaturations. They therefore concluded that no profile emphasising deeper stops than their gas content model profile would be likely to produce better outcomes.

I believe you are mistakenly equating the basic mathematical tools used to calculate supersaturation in tissues with gas content models (or dissolved gas models as you put it). In fact, all models (bubble models and gas content models) use these same basic tools to calculate dissolved gas tissue pressures and tissue supersaturation. What differs between models is how they then interpret allowable supersaturation in different tissues at different stages of the ascent. But it is perfectly legitimate use these basic tools to compare the supersaturation patterns in tissues produced by different models. This does not amount to evaluating a bubble model using a gas content model as you seem to believe. It amounts to comparing a bubble model and gas content model using mathematical tools common to both of them. It therefore does not create circular argument.

Can I say before this goes any further that I have little appetite for debating this entire issue again. Ross Hemingway and I have finished such a discussion just weeks ago on TDS, and both of us had a fair amount of air time. You can evaluate that for yourself - it is not hard to find. The discussion has also taken place previously on this forum.... on every forum in fact. I really don't see the point in trundling it all out again.

Simon M

Many thanks for taking your time to write this out.
"This does not amount to evaluating a bubble model using a gas content model as you seem to believe" - Yes that was/is exactly my argument. Just math-wise this seems the case to me. I will think about it some more.
I can imagine that you've had enough of this topic - I hope we'll get the chance to discuss decompression someday in person. Many thanks again!

 

[J-Vo]

Many thanks for the answer and the interesting video.
The lecture is very well given, the Kiwi accent is pleasant to my ears. It’s an introductory lecture explaining the original reasoning, though, I do not believe it answers my SPECIFIC objections to the original NEDU study (I am not talking about any contribution to the discussion afterwards).

- He states that the ergometer work was easy and set to 115W. The NEDU study writes (p5) that "
set at 65 watts so that divers’ work rate (incorporating the extra power required due to submersion in this diving dress) was approximately 134 watts.", and this is so hard that in fact some divers could not keep it up. This is a physiological regime very different from what is normally encountered during tech dives (anaerobic even?). Has that issue of exercise intensity specifically been addressed?

- The models DO matter. The heat maps he presents after min 34, and the fig. 4 and afterwards in the NEDU study compare deco schedules based on calculations grounded in a gas content model. This whole argument compares models based on a measure calculated from (the same) models, naturally introducing a bias. I have been making computer models of biological processes for the last 15 years, and this struck me right away when reading up on the topic. Has this issue specifically been addressed? Many thanks!

As a professional cyclist, I have to say that 135 watts should not be difficult for a navy diver. I wouldn't expect that to be anywhere near their anarobic threshold and should nicly similate the effort of swimming agains a light current in dive gear.