Oxygen window

[Alastair]

Hi All,

I've recently tried to explain the oxygen vacancy concept to someone and struggled to sum it up neatly.

I have got a copy of this document which outlines some of the concepts but stops short of explaining how the oxygen vacancy in the venous blood works:
http://www.e-aquanauts.co.uk/downloa...gen_Window.pdf

As I understand it the venous blood has less 02 in it as it's metabolised which leads to a partial pressure vacancy. However I'm unsure in how this mechanism helps you offgas. The explanation I have read:

Because the bubble exists at ambient pressure and because the partial pressure of oxygen in the bubble is less than the blood, nitrogen will diffuse into the bubble to occupy the vacancy due to the lower partial pressure of oxygen. Thus, the oxygen window is an important force for removal of gas from bubbles.

Is this correct? How would anyone else explain the oxygen vacancy?

Thanks
Al

[Janos]

Al and I were having this discussion at Wraysbury, and I think it's an excellent idea of his to continue it online. Hopefully some more knowledgable other people can chip in too.

Quote: (Originally Posted by Alastair)

As I understand it the venous blood has less 02 in it as it's metabolised which leads to a partial pressure vacancy. However I'm unsure in how this mechanism helps you offgas. The explanation I have read:

Because the bubble exists at ambient pressure and because the partial pressure of oxygen in the bubble is less than the blood, nitrogen will diffuse into the bubble to occupy the vacancy due to the lower partial pressure of oxygen. Thus, the oxygen window is an important force for removal of gas from bubbles.Is this correct? How would anyone else explain the oxygen vacancy?

I agree that breathing a high ppO2 will lead to a partial pressure "vacancy" ie that the sum of partial pressures will be lower than ambient, but I don't see how this affects the rate of off-gassing of the inert. I wouldn't have thought that there is going to be any oxygen in any (micro)bubbles in the blood, as oxygen is too quickly metabolised, and I'm completely at a loss as to why the rate at which Nitrogen diffuses is dependent on anything other than the Nitrogen pressure gradient.

Janos

[Marcin Kaluza]

Quote: (Originally Posted by Janos)

I agree that breathing a high ppO2 will lead to a partial pressure "vacancy" ie that the sum of partial pressures will be lower than ambient, but I don't see how this affects the rate of off-gassing of the inert.

You're absolutely right about off-gassing of inerts and oxygen. There's no relationship whatsoever. More about oxygen window can be found here
and there was a thread on thedecostop.com where I elaborated about the intricacies of the phenomenon so won't bore you again. The thread can be found here
In short: Oxygen Window does not help offgassing, as this is driven by inert gas only, it helps to keep bubble growth under control as it decreases total gas tension on the venous side.

Best Regards

[Joe Hesketh]

Quote:

there was a thread on thedecostop.com where I elaborated about the intricacies of the phenomenon so won't bore you again

You can't view threads in TDS without being a member IIRC, so some further info wouldn't go amiss here?

Quote: (Originally Posted by Marcin Kaluza)

You're absolutely right about off-gassing of inerts and oxygen. There's no relationship whatsoever.

So is it just a terminology issue then? I.e. not so much "opening the O2 window", but "closing the inert window".

GI3's infamous "for all you marines" post (see here) is the one piece of information I see referred to quite often. Are people saying the concept as explained there is wrong?

[mstroeck]

Quote:

You can't view threads in TDS without being a member IIRC, so some further info wouldn't go amiss here?

Registration is free and very straightforward, however.

[Alastair]

The bit I found useful was from a chap named Marcin

Hope a bit of re-quoting is appropriate:

Quote: (Originally Posted by Marcin on TDS)

When it comes to off gassing, gases work "alone" i.e. their diffusion is not influenced by the total gas tension in the tissue or blood. When it comes to bubble formation however, they "work" together because each and every one of them wants to diffuse inside the bubble. Therefore when calculating the deco ceiling total gas tension has to be taken into account, or to be precise, the difference between gas tension and ambient pressure otherwise known as supersaturation gradient. The higher the gradient, the more bubbles get excited into growth. To keep this gradient low we have to either keep the gas tensions low or ambient pressure (depth) high, and this (as pointed out by J.Brian) is BTW the best reason to do the 20ft O2 stop at 20ft. The off gassing gradient is in this case maximized as there’s no inert gas in the mix regardless of the depth, but greater depth acts against bubble formation.

As I said in bubble excitation total gas tension plays the major role but because oxygen is kind enough to be metabolized, it lowers the total tension and thus the decompression "stress". In our 70ft example the tension on the arterial side was 1.4 ppO2 + 1.5 ppN2 = 2.9 ATA and on the venous side it was only 1.5 ATA (ppN2 only) to start with, as of course blood gets saturated with gas leaving the tissue.

Because ambient pressure is ~3.0 ATA we have 1.5 ATA vacancy left for inert gases before blood gets supersaturated. In this case there is a good chance that we may get lucky and this vacancy will be big enough to accommodate nitrogen diffusing out of tissue and blood will never get supersaturated, thus “zero supersaturation ascent”, and if there is no supersaturation, there is no bubble growth: as simple as that. The bad news is however that as tempting as it may seem there is AFAIR no way to calculate how to stage the decompression to maintain this “zero supersaturation” of blood as both diffusion and perfusion play vital role in the process.

At the end we have to deal with two different aspects of decompression: bubble growth and off gassing. To increase the off gassing rate we have to minimize inert gas content in the mix and bring the diver close to the surface. When it comes to bubble formation it’s better to keep the diver deep but it also pays off to minimize inert gas content.
Which brings us back to the original question: is it worth doing more deco at 70ft. From the bubble growth perspective yes, from the off gassing perspective no as the overall deco time may only get longer. If you take into account that it takes at least a minute to pump whole content of the blood through the body, and even longer through the extremities it’s IMVHO worthwhile to spend there a little bit extra after a gas switch.

[Marcin Kaluza]

Quote: (Originally Posted by Alastair)

The bit I found useful was from a chap named Marcin

Now I think I've seen it somewhere before On the more serious note though sorry guys for being nonresponsive but I'm extremely busy at work setting up my wife's TOMTOM
The fragment that Alastair quoted summarizes the discussion pretty well, but in case of any further questions do not hesitate to ask. I'll try to help as much as I can.

Regards

[Janos]

Hi Marcin,

Do you really think that there is O2 in any bubbles in the blood? I would have thought that any O2 would have quickly been metabolised.

I'm not convinced that the pp of O2 should be included when thinking about bubble formation.

Janos

[Marcin Kaluza]

Quote: (Originally Posted by Janos)

Do you really think that there is O2 in any bubbles in the blood?

I can't remember saying anything along these lines?

Quote: (Originally Posted by Janos)

I would have thought that any O2 would have quickly been metabolised.

That's right

Quote:

I'm not convinced that the pp of O2 should be included when thinking about bubble formation.

It should. Not because it contributes to bubble growth, but because it does not. You may affect outcome of a vote by voting "yes", "no" or by not participating. By being metabolised oxygen choses the last option

Regards

[Alex]

As I said in bubble excitation total gas tension plays the major role but because oxygen is kind enough to be metabolized, it lowers the total tension and thus the decompression "stress". In our 70ft example the tension on the arterial side was 1.4 ppO2 + 1.5 ppN2 = 2.9 ATA and on the venous side it was only 1.5 ATA (ppN2 only) to start with, as of course blood gets saturated with gas leaving the tissue.

Because ambient pressure is ~3.0 ATA we have 1.5 ATA vacancy left for inert gases before blood gets supersaturated. In this case there is a good chance that we may get lucky and this vacancy will be big enough to accommodate nitrogen diffusing out of tissue and blood will never get supersaturated, thus “zero supersaturation ascent”, and if there is no supersaturation, there is no bubble growth: as simple as that. The bad news is however that as tempting as it may seem there is AFAIR no way to calculate how to stage the decompression to maintain this “zero supersaturation” of blood as both diffusion and perfusion play vital role in the process.

If I understand this right, the following should be true true.

1) Let us consider two scenarious. After a heliox dive, the diver arrived at 22 meters and switches to a) 50% nitrox; or b) heliox 50/50.
In the first case the off-gassing will be faster, however the level of blood supersaturation can be higher than in the second case.

2) Let us assume that a diver is at 66 m on say 18/45, and switches to air. Than the supersaturation of blood can occur because of fast helium off-gassing (which iis probably faster than on-gassing of nitrogen).

3) It seems that the whole story regarding the oxygen window is relevant only in the case of presence of two inert gases. In case of just one inert gas, the level of supersaturation of blood does not depend on the mix we are swithing to, only on the partial pressure of the inert gas in tissues and the ambient pressure. The speed of off- gassing of course does.

Regards
Alex