Any reported cases of Ox Tox between 1.4 and 1.6?

[inquis]

There is a summary table on page 135

Thanks for the pointer to Powell's summary. I'd say #3 isn't very useful for our purposes since we don't use PO2 values over 1.6 bar, as well as Powell himself hedging his position because of vasoconstriction. #2 seems to be a misuse of the term to mean "inert gas gradient", so not terribly useful either. (No slight against Powell, as he's just reporting one way that term is (mis)used.)

So #1 seems to be the only real consideration for current diving practices. Lower tissue tension is further from the critical supersaturation level, yielding a reduced tendency to bubble. OTOH, we already have the widespread understanding that deeper has less tendency to bubble. Since the only way to widen the o2 window is to go deeper, it's kind of six of one, half-dozen of the other.

I suspect the term "oxygen window" has fallen out of favor because "deeper" is FAR easier to understand and yields the same benefit.

There's also the fact that the M-Values of the deco models were empirically set, which would have necessarily included some degree of the window-induced tension reduction.

Ultimately, we're still left with the trade-off that deeper is less likely to bubble but off-gassing at the associated elevated inspired PO2 may be impacted by pulmonary stress (vasoconstriction & mucus buildup). Therefore, whether 6m or 3m is "better" will vary.

 

[justinthedeeps]

So included in 'oxygen window' is a prediction that a higher ambient pressure a bit deeper results in reduced inert gas bubble size or occurrence, while keeping the same optimal ambient p[inert] = 0 (oxygen breathing) for off-gassing. Is this right?

The theoretical bubbles are smaller when deeper, but contain the same number of inert gas molecules. Diver is assumed to off-gas just as well but with less perceived risk, or somehow a little better [still unclear on that part]

Sounds like bubble models

Why is a more complex model comparing paO2/pCO2 necessary?

Alternate theory, some venous bubbling is normal & readily handled by the circulatory system and alveoli (but does Buhlmann account for this?) What if venous micro-bubbling actually accelerates deco [in non-PFO divers]?

 

[inquis]

Diver is assumed to off-gas just as well but with less perceived risk

Yes, less risk of DCS when deeper at the same off-gassing rate, assuming pulmonary stress isn't an issue. One can debate the cause (oxygen window or higher ambient pressure), but I don't think it's controversial that it's true.

Buhlmann only implicitly considers venous bubbling since the M-Values were empirically set so as to not cause DCS. From Doppler measurements, we now know that we do bubble a bit, but presumably that hasn't changed from his time.

 

[passeparici]

Thanks for the pointer to Powell's summary. I'd say #3 isn't very useful for our purposes since we don't use PO2 values over 1.6 bar, as well as Powell himself hedging his position because of vasoconstriction. #2 seems to be a misuse of the term to mean "inert gas gradient", so not terribly useful either. (No slight against Powell, as he's just reporting one way that term is (mis)used.)

So #1 seems to be the only real consideration for current diving practices. Lower tissue tension is further from the critical supersaturation level, yielding a reduced tendency to bubble. OTOH, we already have the widespread understanding that deeper has less tendency to bubble. Since the only way to widen the o2 window is to go deeper, it's kind of six of one, half-dozen of the other.

I suspect the term "oxygen window" has fallen out of favor because "deeper" is FAR easier to understand and yields the same benefit.

There's also the fact that the M-Values of the deco models were empirically set, which would have necessarily included some degree of the window-induced tension reduction.

Ultimately, we're still left with the trade-off that deeper is less likely to bubble but off-gassing at the associated elevated inspired PO2 may be impacted by pulmonary stress (vasoconstriction & mucus buildup). Therefore, whether 6m or 3m is "better" will vary.

I am surprised at all those people so quick to have an opinion despite the fact that it seems they don't really have the knowledge to understand very well the question !

 

[Seaweed Doc]

There is a reason that DAN's protocol is to go to an ER, not a chamber, because the chamber you go to may not be available to you.

The nice part about our local chamber (really chambers, as in plural) is that it's at a regular hospital with an ER of its own. It's really a question of WHICH ER you go to, the closest or the one at the hospital with the chambers? And at what point do you decide the time difference is meaningful?

But I understand DAN's situation: They're trying to deal with any possible situation in any location with a single rule. Another location I commonly dive is distant from chambers. At that site, I'd aim for the local ER, no questions asked.

 

[tursiops]

Considering that fewer and fewer chamber reportedly accept divers, I suspect that at most diving locations there are more ERs "nearby" than chambers.

 

[Seaweed Doc]

Considering that fewer and fewer chamber reportedly accept divers, I suspect that at most diving locations there are more ERs "nearby" than chambers.

Wouldn't surprise me. I've heard of more chambers shutting down than new ones opening up.

 

[Nick_Radov]

I am surprised at all those people so quick to have an opinion despite the fact that it seems they don't really have the knowledge to understand very well the question !

The term "oxygen window" is rather vague and not precisely defined in scientific terms. I think perhaps what some posters are reacting to is the definition that George Irvine posted on the old Techdiver mailing list in 2002. I'm not an expert on this stuff so I'm not going to take a position on it, just providing some historical context.

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As a dope's explanation of the oxygen window concept for you Marines, the
best gas differential would be a vacuum relative to a partial pressure,
right? Oxygen is the next best as it creates a similar effect in that the
sum of the gas partial pressures is unbalanced by the fact that some of the
oxygen is metabolized, more in a fit person. The greater the difference
between the oxygen and the other gases up to the max differential described
by the metabolism ( maximum window ) , the greater the propensity for
whatever is in the cells to come out and be displaced. For a fit person, the
widow is wider and by definition so is his vascularity and perfusion, so he
decompresses better. These things are all tied together.

You open the widow as wide as possible subject to 1) risk of tox or damage,
2) how long before the vaso constrictive effect offsets the benefit, 3) how
long before the asthma like reaction sets in. You then alternate the process
back to open up the vessels and lungs again, and repeat. All part of a good
deco. Also it can be said that the sum of the inert gases is the other side
of the oxygen window minus the metabolism drop of oxygen- there is no
benefit to combining inerts - they act like one gas. Oxygen can be pushed to
above its partial pressure effectiveness as a result of this imbalance for a
"window" that then exceeds what would be the net effect of the partial
pressures of the gases, and this is especially important in diminishing
bubbles of inert gas as the pressure of the bubble can always be faced with
a negative gradient or "tension" on the outside due to the fact that
metabolized oxygen is creating a "vacuum" in the total sum of the partial
pressures of the gases, leaving a consistent imbalance between bubble
pressure and surrounding tension of any given inert.

This is why oxygen ( pure, not 80/20) works so well in DCS cases after the
fact to reduce bubbles, as well as the fact that saturation with oxygen
tends to move that gas to where it is needed even if the vessels are blocked
by damage.

Capice? For you geeks, see Eddie Bryan's explanation. When you are done with
all that, go back and see what I said about how to decompress. How is all
the matters.

 

[justinthedeeps]

metabolized oxygen is creating a "vacuum" in the total sum of the partial
pressures of the gases, leaving a consistent imbalance between bubble
pressure and surrounding tension of any given inert.

I guess we'll take this koan--possibly impossible to understand?--to keep our minds occupied on decos
Would this be any different between ppO2's of 1.4 and 1.6--both saturating levels for metabolism--apart from already understandable depth effects on bubbles?

 

[crofrog]

I guess we'll take this koan--possibly impossible to understand?--to keep our minds occupied on decos
Would this be any different between ppO2's of 1.4 and 1.6--both saturating levels for metabolism--apart from already understandable depth effects on bubbles?

The Oxygen Window and Decompression Bubbles: Estimates and Significance