Slow tissue on gas from stops

[boulderjohn]

Perhaps it would be helpful to think of different decompression strategies as falling on something like a fuzzy archery target. Somewhere in the middle is a perfect bullseye--the ideal decompression profile. It is not a pinpoint, because it will vary a bit for each individual and for each dive, but there is a fuzzy bullseye that is such a good profile that you are pretty much guaranteed to come out OK. As you move away from the bullseye, your profile will fall into one of the following rings, starting next to the bullseye and moving outward:

1. Pretty darn good--divers using this profile should be fine almost every time.
2. Not as good--there is a higher likelihood that a dive will end in DCS, but by far most divers will be fine.
3. There is a pretty good likelihood that a diver will be bent.
4. You're screwed.

Of course, we don't yet know where the bullseye is, but we are getting closer with each new study. The problem is that there are so many variables in so many ascent profiles that it is hard to separate one part of the strategy from another. The NEDU study attempted to isolate deep stops as a strategy, and it did a pretty good job. It is pretty clear that taken by itself, a deep stop strategy is not beneficial, and the approach that was tested lies somewhere in an outer ring. What about coupling deep stops with longer shallow stops? Where on the target would that fall? We don't know--it wasn't part of the test.

 

[UWSojourner]

Yes I know, it was a rhetorical question, see my paragraph below that hypothesis. That was exactly my point, that lower supersaturation of the fast tissues shouldn't be called "protecting the fast tissues". Although the fast tissues are less supersaturated with the deep stop schedule and may produce fewer bubbles, they eventually see more bubbles and higher DCS risk than with the shallow stop schedule.

Sorry leadduck. I misread your post. I guess not all brain tissue is equally fast .

 

[UWSojourner]

I don't think people are really getting this. The NEDU study was testing the EFFICIENCY of different ascent strategies. They were NOT testing if a deep stop in and of itself was good or bad.

I think they were testing exactly that -- assuming we're all still talking about bubble models and the types of deeper stops they generate. The whole NEDU test was designed to determine whether their bubble-model deeper stops were "good or bad", although I think "better [worse]" or "more [less] effective" would be preferable language. The term "efficiency" is just the technical term for that.

All the research seems to be pointing in one direction -- that bubble-model-style deep stops are: if you're a scholar, "inefficient"; if you're tactful, "less effective"; if you're blunt, "bad".

 

[Dr Simon Mitchell]

So shouldn't I protect my fast tissues (brain) at the expense of the slow ones (joints) by shifting shallow stop time to depth, even if all studies counting only short term DCS and bubbles tell me otherwise?

But the DCS mechanisms are not so simple. The location of the lesions in the Reul/Weis paper suggests occlusion of small blood vessels as a cause, i.e. it can be caused by any bubbles created anywhere in the body passing as VGE through shunts into the brain, rather than bubbles created in the brain. So again, if I want to protect my brain better then I must reduce bubbling overall. The frequent statement that deep stops "protect" the fast tissues like brain can be easily misunderstood.

Yes, exactly.

The brain is an extremely fast tissue. It outgasses very quickly and does not need protection from deep stops. The associations between cerebral and spinal DCS and PFO suggest that the most important vector of harm to these organs is venous bubbles that cross the PFO (or another right to left shunt) and travel to the target organ in the arterial blood. The most important source of these venous bubbles are the slower tissues which tend to be disadvantaged by deep stops.

Simon M

 

[Rollin]

Dr. Mitchell,

I'm very interested in this discussoin. Thank you for taking the time to explain things.

Can you give an example of how slow tissues can form bubbles that can migrate to the veins? For now, I don't understand how this can happen.

The other way of shunting, is that by doing a heavy effort after the dive?

Regards,
Rollin

 

[Dr Simon Mitchell]

What I DO conclude from the study is that IF you do deep stops, you need to compensate for that with more time shallow.

Hi,

David and I have said much the same thing many times, viz: deep stops can work, so long as you 'compensate' for them. But a circular argument arises if you ask yourself, "if I took the total time of my decompression with compensated deep stops, and did that without deep stops, would I be even safer"? Based on current evidence, the answer is probably yes. Put another way, whatever fixed time you are prepared to invest in decompression, the most efficient use of that time almost certainly includes distributing the decompression time shallower than bubble model-style deep stops.

Simon

 

[Dr Simon Mitchell]

Dr. Mitchell,

I'm very interested in this discussoin. Thank you for taking the time to explain things.

Can you give an example of how slow tissues can form bubbles that can migrate to the veins? For now, I don't understand how this can happen.

The other way of shunting, is that by doing a heavy effort after the dive?

Regards,
Rollin

Hello Rollin,

If slower tissues like skin, fat, and to a lesser extent muscle are heavily loaded with inert gas and become supersaturated during ascent and after surfacing, then bubbles will tend to form in those tissues. The capillaries running through the tissue are effectively part of the tissue, and subject to the same prevailing supersaturation conditions so bubbles can form in the capillaries too. These can migrate into the veins, and eventually cross a right to left shunt if one exists. This is illustrated in the diagram below.

This depicts a notional tissue (call it a slow tissue if you like) that has become supersaturated during decompression. Bubbles are shown forming in the top left corner of the tissue in the space outside the blood vessels, but also within the capillaries themselves. These latter bubbles can then pass into the venous blood leaving the tissue and could subsequently cross a right to left shunt like a PFO if one exists.

Hope this makes sense.

Simon M

 

[Rollin]

Thank you!

I guess the blood in the slow tissue arteries stays there longer, so it gets supersaturated more than blood that passes the lung filter every 1-2 minutes.

 

[UWSojourner]

But a circular argument arises if you ask yourself, "if I took the total time of my decompression with compensated deep stops, and did that without deep stops, would I be even safer"? Based on current evidence, the answer is probably yes.

Your time exposed to supersaturation ("integral supersaturation") would seem to support that. See the chart below.

If you try that strategy to assist VPM-B, you have to drive your in-water GF higher to ultimately stay at the shallow stops longer. So, for example, your GF is 105 when you arrive at your 20ft stop under the padded VPM-B+0 schedule. That sounds like bend and mend, the old critique against shallower profiles. "Fate, it seems, is not without a sense of irony."

 

[dberry]

If someone claims to have "all the answers", I'd be extremely careful about listening to them. Particularly if the topic is science, because science will never, ever have "all the answers". The Final Definitive Answer (tm) is frequently found in religion, never in science.

Science is always wrong, but luckily it gets less wrong with time. And in most fields today, the amount of wrong is in the order of tolerable to negligible (at least for the majority of cases).

The Relativity of Wrong by Isaac Asimov

This Asimov (non-fiction) article is a gem that I regularly assign to graduate students. It explains the iterative nature of scientific discovery and progress in a wonderfully approachable style. For example, the earth is not exactly spherical, thus the flat-earth model and the perfectly spherical earth model are both "wrong". But one is clearly more "wrong" than the other. Scientific models and theories continually iterate towards a more complete understanding of a complex universe.