Oxygen Window Revisited

[boulderjohn]

I tried to start aggregating the history and references here a while back.

Gene,

As always, thanks for being such a valuable resource.

As you may remember from previous private conversations we have had, my primary interest in this topic lies in this sentence in your entry:

Many technical divers have chosen to lengthen the decompression stops where ppO2 is high and to push gradient at the shallower depths of the decompression curve, thus creating an S-shaped curve.

As I said earlier, this is the controversial part. Many people do exactly as you describe, claiming that the oxygen window effect at high PPO2s makes those lengthened stops worth while by increasing the speed at which inert gases leave the body. Others disagree, saying that the benefit (if any) from the enlarged oxygen window at that depth is minor and is more than offset by the increased PPN2 of the inspired deco gas. (I am talking EANx 50 here.) As I understand that side of the argument, the chief benefit of the oxygen vacancy is in a minor decrease in the size of existing air bubbles by removing O2 from those bubbles.

I would like to know which position is correct and why.

 

[lamont]

this discussion usually doesn't get anywhere because the different participants are talking past each other.

the "proponets" of the O2 window know how dissolved gas models work, but postulate some kind of effect of higher ppO2s that go beyond the dissolved gas models.

the "skeptics" of the O2 window usually produce an argument based solely around dissolved gas models of dissolved inert gases.

its like arguing that general relativity is wrong because newtonian gravity doesn't predict that. you don't get anywhere in that argument. you have to figure out empirically how they're different and test for that.

also, the proponents of the O2 window theory usually shorten the low ppO2 stops (80 feet for 50% bottle, 30 feet for 100% bottle) in favor of lengthening the high ppO2 stops.

the O2 window theory is also largely based around *practice*, and the theory comes second. arguing that the people doing the dives have the theory all wrong may not change the fact that the practice works (via some other physiological mechanism).

Einstein, Podolsky and Rosen absolutely *nailed* the rationale for why Quantum Mechanics can't work, and Bohr had absolutely no good response to that or any kind of competing theory that made any kind of coherent sense -- but physics sided with Bohr. There's no reason why decompression theory can't side with the divers who have no good concept of physiology to explain why the way they dive works.

 

[lamont]

I would like to know which position is correct and why.

IMO, anyone who thinks they can answer this question correctly doesn't really understand the argument on the other side of the fence, and to truly answer the question you need to go dive the problem.

 

[boulderjohn]

this discussion usually doesn't get anywhere because the different participants are talking past each other.

the "proponets" of the O2 window know how dissolved gas models work, but postulate some kind of effect of higher ppO2s that go beyond the dissolved gas models.

the "skeptics" of the O2 window usually produce an argument based solely around dissolved gas models of dissolved inert gases.

Perhaps that has been your experience, but it has not been mine. I would certainly count Ross Hemingway as a skeptic, and I think he has a pretty good gasp of dual phase models. In my experience, in fact, the skeptics have agreed that there may be a small benefit in terms of bubble size but challenge it on different grounds.

You say that proponents "postulate some kind of effect of higher ppO2s that go beyond the dissolved gas models." Its the "some kind of effect" that is the issue. That is what I am trying to understand.

 

[gcbryan]

Regarding Lamont's post, to have to dive the problem would suggest that proponents can feel the claimed increase in rate of N2 coming out of the tissues wouldn't it?

Since you will never know the "correct" amount of deco in absolute terms other than not experiencing DSC it would seem that feeling the increased rate of offgassing would be the only other way of experiencing this effect.

It's understandable that no one knows exactly what is going on so something could be going on that is outside any of the models. I just don't see how this effect (Oxygen Window) can be claimed with no theory behind it and no possibility of otherwise validating the effect.

How is that done? Is it just a matter that proponents feel better after doing deco this way and simply attribute that feeling to an increase in rate of N2 offgassing rather than to some unknown effect?

 

[lamont]

Perhaps that has been your experience, but it has not been mine. I would certainly count Ross Hemingway as a skeptic, and I think he has a pretty good gasp of dual phase models. In my experience, in fact, the skeptics have agreed that there may be a small benefit in terms of bubble size but challenge it on different grounds.

You say that proponents "postulate some kind of effect of higher ppO2s that go beyond the dissolved gas models." Its the "some kind of effect" that is the issue. That is what I am trying to understand.

The thing is that there's an theory... and lets call it "X-Theory" to give it a name... which is postulated to be the physiological mechanism behind the benefits of diving s-curve-like decompression profiles using longer high ppO2 stops and shorter low ppO2 stops. X-Theory is additional on top of simple bubble models and on top of simple dissolved gas theories -- it probably involves bubble theory, but its beyond what we currently model in bubbles.

- GUE and UTD have claimed that X-Theory is the O2 window.
- lots of people have argued that the O2 window does not give GUE/UTD the theoretical foundation they desire and cannot be X-theory.
- GUE/UTD still claim to get better results diving with S-curves, so therefore still believe in the reality of X-theory (and still keep on calling it the O2 window, which continues to confuse the discussion).
- Ross is an expert on dissolved gas theory and bubble models, but you can't use that knowledge to prove that X-Theory doesn't exist, you can only use it to show that X-theory is outside of the current theoretical framework of decompression.

If you believe that we know all we will ever know about decompression and that our current knowledge is complete then, you can conclude that X-theory doesn't exist. If you allow for incomplete current knowledge of decompression, then there's wiggle room where the S-curve deco practice can actually be better, but we simply don't yet have a theoretical foundation to explain it.

And, on the other hand, all the experimental evidence so far is entirely subjective, and could always be proven wrong by carefully crafted experimentation. There's no natural law that an entire diving agency full of divers could not be wrong about their ideas about how to apply decompression.

But you're trying to work backwards from theory to be able to prove correct one diving practice or the other -- when all the diving theory in the world has come from diving practice, and the only way to prove something wrong or not is via experiment -- actual diving.

 

[boulderjohn]

I then received the following message from DAN's Richard Vann, who sent along a chapter he wrote "Inert gas exchange and bubbles." In Bove and Davis’ Diving Medicine, 4th edition. Chapter 4, Bove AA, ed, WB Saunders, Philadelphia. 53-76, 2004.


The PDF he sent is too large for me to attach here. I will see what I can do about it. I am myself still in the process of digesting it.

OK, I have read it, and my head hurts.

I would quote key sections, but he sent it scanned into PDF as an image file, so it would take a lot of typing that I am not presently up for. If someone wants to digest it personally, I can forward it.

He is saying something similar to the quote from George Irvine above, that the O2 window speeds up the removal of bubbles by creating a greater pressure gradient between he bubbles and the surrounding tissue. The presence of bubbles slows down decompression because inert gas cannot go directly from tissue to blood to lungs but must first leave the bubble and get into solution.

I think I can understand it, and I have heard it before, but it is not the theory that I have seen proposed in the past.

 

[boulderjohn]

- GUE and UTD have claimed that X-Theory is the O2 window.
- lots of people have argued that the O2 window does not give GUE/UTD the theoretical foundation they desire and cannot be X-theory.
- GUE/UTD still claim to get better results diving with S-curves, so therefore still believe in the reality of X-theory (and still keep on calling it the O2 window, which continues to confuse the discussion).

I think that has been my observation as well, especially the part about confusing the discussion, at least for me.

When it comes to something as important as decompression, I like to have confidence that what I am doing works. I don't look forward to being bent, so I don't accept things at face value, and I ask annoying questions. I keep looking for the science behind things, and when people give me science that seems questionable, I get concerned.

I was in fact part of a presentation in which the presenter was asked for the theory and said in response (and this is very nearly a direct quote), "You just have to have faith."

It seems to me that if someone were to say the following...

We have observed that decompression profiles that follow an S-curve starting at the point of a switch to a high PPO2 gas give better results. We don't know why. Could someone check this out?"

...then somebody somewhere could very easily test this by putting divers (even in a chamber) through different profiles and comparing bubbling results.

 

[gcbryan]

This, as an explanation, is something that I would agree with. It would have been much better if it had never been referred to an the "Oxygen Window" which does exist but has a different meaning.

It would also be better if S curves were just called S curves and were thought to produce a better decompression schedule just because of personal experimentation rather than stating that it was because of an effect that the Gas Law say cannot happen.

The thing is that there's an theory... and lets call it "X-Theory" to give it a name... which is postulated to be the physiological mechanism behind the benefits of diving s-curve-like decompression profiles using longer high ppO2 stops and shorter low ppO2 stops. X-Theory is additional on top of simple bubble models and on top of simple dissolved gas theories -- it probably involves bubble theory, but its beyond what we currently model in bubbles.

- GUE and UTD have claimed that X-Theory is the O2 window.
- lots of people have argued that the O2 window does not give GUE/UTD the theoretical foundation they desire and cannot be X-theory.
- GUE/UTD still claim to get better results diving with S-curves, so therefore still believe in the reality of X-theory (and still keep on calling it the O2 window, which continues to confuse the discussion).
- Ross is an expert on dissolved gas theory and bubble models, but you can't use that knowledge to prove that X-Theory doesn't exist, you can only use it to show that X-theory is outside of the current theoretical framework of decompression.

If you believe that we know all we will ever know about decompression and that our current knowledge is complete then, you can conclude that X-theory doesn't exist. If you allow for incomplete current knowledge of decompression, then there's wiggle room where the S-curve deco practice can actually be better, but we simply don't yet have a theoretical foundation to explain it.

And, on the other hand, all the experimental evidence so far is entirely subjective, and could always be proven wrong by carefully crafted experimentation. There's no natural law that an entire diving agency full of divers could not be wrong about their ideas about how to apply decompression.

But you're trying to work backwards from theory to be able to prove correct one diving practice or the other -- when all the diving theory in the world has come from diving practice, and the only way to prove something wrong or not is via experiment -- actual diving.

 

[lamont]

I think that has been my observation as well, especially the part about confusing the discussion, at least for me.

When it comes to something as important as decompression, I like to have confidence that what I am doing works. I don't look forward to being bent, so I don't accept things at face value, and I ask annoying questions. I keep looking for the science behind things, and when people give me science that seems questionable, I get concerned.

I was in fact part of a presentation in which the presenter was asked for the theory and said in response (and this is very nearly a direct quote), "You just have to have faith."

Thing is that deep stops were 'invented' by Pyle just because he was trying to decompress fish swim bladders. He didn't have any physiological explanation at all, he just found it treated him better to do that, and he stumbled across it entirely by accident.

Deep stops could have been invented by divers who stopped every 1/2 distance to the surface to chant prayers to ancient mayan gods. The idea that ancient mayan gods were protecting divers against DCS would have been the wrong explanation entirely, but it would have been just as effective of a decompression strategy.

The way that I view the O2 window is that there's a ritual that is performed in Tech 1 and Tech 2 courses where students and instructors chant about the O2 window. And then they go out and do the dives, secure in their knowledge that their chants will ward off the DCS bubble daemons. And based on my admittedly limited knowledge, I tend to believe that the chants do actually ultimately result in cleaner deco...

I would advise not accepting anything on *faith*, but you should try shaping the curve and not shaping the curve and see what you think...

And at some point perfect assurances of scientific safety end, and we're all just lab rats in one big experimental maze...