Oxygen window misunderstanding

[BubbleJet]

Hi there,

From my understanding of the oxygen window (OW) phenomenon mainly from [1] and [2], it seems to me that the width of this partial pressure vacancy has no effect on the inert gas off-gasing process. Although it may reduce the risk of bubble growth since the sum of the tensions of all the gases will be more easily lower than the ambiant shrinking pressure, the blood or tissue inert gas load only depends on solubility and concentration gradient.

For example, looking into Bühlmann's algo [3] M-values are built as a limit for ambiant pressure in comparison with total inert gas pressure in tissues. Therefore, it has nothing to do with the OW.

However, reading this article from DAN, I discovered this contradictory statement :

So, what is the oxygen window? Essentially, it is the 'missing' gas tension created by the conversion of oxygen to carbon dioxide (as a result of their different solubility in blood); this allows more nitrogen (or inert gas) to be dissolved in venous blood to take 'in its place' and increases the rate of nitrogen (or inert gas) gas elimination.

It seems to me that this is wrong, at least from a classic Bühlmann point of view. Is that correct ?

References
[1] Behnke AR. The isobaric (oxygen window) principle of decompression Trans Third Marine Technology Society Conference; San Diego, USA: Marine Technology Society; 1967.
[2] Van Liew, H. D. (1993) The Oxygen Window and Decompression Bubbles: Estimates and Significance. In : Aviation, Space, and Environmental Medicine, p. 859.
[3] Bühlmann, A. (1984) Decompression-decompression sickness. Berlin, New York : Springer-Verlag.

 

[dmaziuk]

I'm not sure it follows that having more nitrogen in venous blood equals faster elimination: it still has to get from there to the exhaled air, and that is controlled by delta-pressure and time. If anything. you have lower N2 delta-pressure between venous blood and breathing gas, so elimination should in fact be slower.

That said, in the implementation it is rolled into "alveolar gas pressure" where the pressure of inert gas is basically reduced based on respiratory quotient (between 0.7 and 1, pick one). Alveolar gas pressure is then used to calculate gas pressure in tissue compartment (Schreiner equation). My guess is resulting reduction in calculated tissue loading is "good enough" to cover the real-life phenomena. (The actual numbers are an order of magnitude smaller than the fraction of inert gas in the breathing mix.)

 

[CuriousRambler]

Mark Powell's book Deco for Divers points out and explains three distinctly different definitions of "oxygen window," and spends some time explaining each different use.

So yeah, sometimes people will define "oxygen window" quite differently from one another.

 

[BubbleJet]

I'm not sure it follows that having more nitrogen in venous blood equals faster elimination: it still has to get from there to the exhaled air, and that is controlled by delta-pressure and time. If anything. you have lower N2 delta-pressure between venous blood and breathing gas, so elimination should in fact be slower.

Thank you for your answer ! I am questionning the fact that there is indeed more nitrogen in venous blood. The dissolution of inert gas in a tissue, whether it is arterial blood, a muscle or venous blood, does not depend on the "room" left empty by other gases, there is no such room. Each gas saturate independantly, do you agree ?

 

[dmaziuk]

Each gas saturate independantly, do you agree ?

Sorry, I'm not that kind of doctor. I only do numbers.

 

[Duke Dive Medicine]

Thank you for your answer ! I am questionning the fact that there is indeed more nitrogen in venous blood. The dissolution of inert gas in a tissue, whether it is arterial blood, a muscle or venous blood, does not depend on the "room" left empty by other gases, there is no such room. Each gas saturate independantly, do you agree ?

They do saturate independently but each also contributes to the total tissue gas tension. This the principal behind isobaric counterdiffusion. You have to differentiate between diffusion of gas into and out of the tissues; and the difference between total tissue gas tension and ambient pressure. The O2 window you're referring to decreases the likelihood of bubble formation when ambient pressure is reduced relative to total tissue gas tension; think of it as a cushion. I'd echo @CuriousRambler 's recommendation for Mark Powell's book; it explains the phenomenon much more elegantly than I can. @Dr Simon Mitchell is a decompression ninja. Tagging him in the hopes that he can jump in.

Best regards,
DDM

 

[BubbleJet]

Thank you @Duke Dive Medicine and @CuriousRambler, you gave me an opportunity to dive back into Powell's excellent book. Here is a quote (Powell, M. (2014) Deco for divers. A diver's guide to decompression theory and physiology. 2e édition : Aquapress.)

So what advantage does the oxygen window give us? Well one thing it doesn’t do is that it doesn’t help us to off-gas any quicker. The rate of off-gassing is dependant only on the individual inert gas gradient, in other words gases off-gas at the same rate no mater what the other gases are doing. So the fact that the oxygen partial pressure is lower on the venous side has absolutely no impact on the rate that nitrogen (or any other inert gas) off-gasses. Where the oxygen window does give us an advantage is in controlling bubble formation. Remember that we said bubbles formed when the supersaturation level – the ratio between tissue gas pressure and ambient pressure exceeded the M-Value. Bubble formation is different to off-gassing in that we must consider all of the gas pressures together when calculating the supersaturation ratio. If the sum of all of the gas pressures exceeds the M-Value then bubbles will start to form.

It seems to me then that the original quote from DAN website which led me to raise this thread is indeed wrong, do you agree ?

Another question pops to my head reading Powell : if bubbles start to from when the sum of all gas pressures exceeds the M-Value, (i.e. including O2), why are only inert gases taken into account in Bühlmann's algorithm for supersaturation criteria ?

 

[Duke Dive Medicine]

Thank you @Duke Dive Medicine and @CuriousRambler, you gave me an opportunity to dive back into Powell's excellent book. Here is a quote (Powell, M. (2014) Deco for divers. A diver's guide to decompression theory and physiology. 2e édition : Aquapress.)



It seems to me then that the original quote from DAN website which led me to raise this thread is indeed wrong, do you agree ?

Another question pops to my head reading Powell : if bubbles start to from when the sum of all gas pressures exceeds the M-Value, (i.e. including O2), why are only inert gases taken into account in Bühlmann's algorithm for supersaturation criteria ?

I think the wording of the DAN article opens it up for some individual interpretation.

M values are strictly theoretical. They're a model upon which some decompression algorithms are constructed. Dissolved O2 is not typically taken into account in decompression algorithms because (a) there's very little of it when compared to dissolved inert gas and (b) it's a metabolic gas and is constantly in a state of flux in the body.

Best regards,
DDM

 

[dmaziuk]

if bubbles start to from when the sum of all gas pressures exceeds the M-Value, (i.e. including O2), why are only inert gases taken into account in Bühlmann's algorithm for supersaturation criteria ?

Remember, the goal is not to exactly model what's going on, it is to get you out of the water not bent.

There was a study back in the 50s where they managed to bend goats on oxygen; it was hard and the bends "miraculously resolved themselves within minutes". Yes, a scientific article used the word "miraculous". Perhaps because oxygen bubbles are not seen as foreign bodies and don't provoke inflammatory response.

So between oxygen window and oxygen bubbles not being a problem, simply not counting seems to work well enough for all practical EAN mixes.

 

[dmaziuk]

PS I complained about "disappearing oxygen" a lot myself, as I recall it was @rsingler convinced me that metabolism switches to dissolved oxygen and @huwporter guesstimated that the absolute amounts of gas in question look plausible.