Oxygen window

[Mark Powell]

I thought I had understood the Oxygen window pretty well. Oxygen consumed from the arterial blood is only partially replaced by carbon dioxide in the venous blood resulting in a drop in overall pressure in the venous vs arterial side.

However I just realised that I still have a question.

If we assume that the uptake and release of one gas is not affected by the presence of another gas then what difference does the fact that the oxygen pressure and hence the overall pressure is lower if the pressure of nitrogen stays the same.

Dropping the overall pressure is not going to give the nitrogen more 'room' to offgass. Or is it?

If the arterial and venous pressure of nitrogen stays the same then it is this the gradient between the arterial/venous blood and the tissues that determines the release of nitrogen from the tissues.

If the argument above is true then it seems the oxygen window doesn't help offgassing. What am I missing here?

 

[Dr Deco]

Hello ro:

Window

There will always “room” for the nitrogen to escape into the venous blood, if the nitrogen is lower. This will occur when the pressure is reduced during the decompression process. The addition of oxygen in the breathing gas and its removal by the tissue will make the gradient even larger, assuming the oxygen is metabolized. If the tissue had virtually no metabolism, the oxygen would not be eliminated and there would not be a “window.” Of course, there is no living tissue that has zero metabolism.

The oxygen window plays the biggest role when large fractions of oxygen are in the breathing gas during decompression.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Mark Powell]

Hmmm, not sure that really answers my question. Lets put some numbers on this to try to clarify.

If we assume that the partial pressure (in kPa) in the arterial blood on the surface is
O2 13.3
CO2 5.3
N2 76.4
Total 95.0

And in the venous blood is
O2 5.3
CO2 6.1
N2 76.4
Total 87.8

Then this shows that the partial pressure of the nitrogen is the same and there is no gradient to allow offgassing.

It's true that the O2 level is lower and the overall pressure is lower BUT if the offgassing rate of N2 is ONLY dependant on the N2 gradient then the drop in O2 and total pressure makes no difference to the offgassing of N2.

During an ascent the inspired gas will contain a lower PN2 than the tissues, which will introduce a gradient and allow N2 to offgas but again this is nothing to do with the O2 window. The removal of oxygen by the tissues has no effect on the N2 gradient as the rate of on/offgassing of N2 is independant of the partial pressure of any other gas present.

If the argument above is true than the oxygen window is irrlevant. :11:

Where is the flaw in the argument :06:

 

[Dr Deco]

Hello Mark:

In this situation, there would only be a net exchange of nitrogen if the tissue nitrogen partial pressure were greater than the arterial nitrogen partial pressure. Yes, the exchange depends only on the partial pressures and not on the magnitude of the oxygen window.

Oxygen in the breathing mix will allow the off gassing of dissolved nitrogen because it replaces the nitrogen in the breathing mix. In the limit, if you breathe pure oxygen, the nitrogen partial pressure in the inspired, and arterial, gas is zero.


Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Mark Powell]

Hi Dr Deco,

thanks for taking the time to reply to all the questions.

You said

Yes, the exchange depends only on the partial pressures and not on the magnitude of the oxygen window.

If the exchange depends only on the partial pressure and not on the oxygen window then how does the oxygen window help in decompression?

 

[Dr Deco]

Hello Mark:

“Oxygen Window”

In the breathing mix, it is necessary that the sum of the gas partial pressures equals (or slightly exceeds) the ambient pressure. To a certain point, about thirty feet, you can fill the breathing gas with 100% oxygen. This can be at even greater depths if you are resting in a dry chamber. In these cases, the “window” is 100% of the breathing mix. It must decrease if you go deeper; either nitrogen or helium is added.

There is not any “physiological effect” with the “oxygen window.” It is simply the replacement for inert gas in the breathing mix.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Mark Powell]

There is not any “physiological effect” with the “oxygen window.” It is simply the replacement for inert gas in the breathing mix.

Thank you, that makes perfect sense now.

I was struggling with some of the physiological descriptions I had read which seemed to be arguing that it was all do do with greater drops in PvO2 pressure due to the O2-hemoglobin
disassociation curve.

I wasn't convinced as I came coming back to the inert gas partial pressure gradient as the most important factor and the inert gas partial pressure gradient also seemed to explain the results just as well as the more complicated physiological description.

thanks again for taking the time to help me out on this.

 

[HolgerS]

Dear Mark, dear DrDeco,

I am quite a bit confused about the answer of DrDeco.

I agree on the thesis, that the diffusion gradient of inert gases is independent of the oxygen window if there is no inert gas in the breahting gas.

But there is another reason why the oxygen window should be opened widely, that means 6m O2 deco is better than 3m or 0m O2 deco. The oxygen window leads to an "inherent unsaturation" of tissues and the venous system (by the way: "inherent unsaturation" is the synonym of "oxygen window"). That means the total gas tension is decreased. The total gas tension is one driver for bubble creation and bubble growth!

This means, the wide oxygen window will not foster of inert gas elimination from the tissues in first order, but will help you, to lower bubble growth or prevent bubble creation. This leads to less bubbles and to less trapped inert gas, so that at the end the gas elimination is accelarated.

Dear DrDeco, I am a strong supporter of you and your forum. So please, if am wrong, don´t hesitate to correct me.

Looking forward for a valuable discussion.
) HolgerS

 

[Seabear70]

So, basicly what you are saying is that the levels of saturation of the tisues will balance out with the levels of gases being taken in. If we exceded the levels of gas pressures in the tissues than we can force not just a balance but a complete replacement?

 

[lamont]

I want to see if I understand the oxygen window phenominon at all by throwing out a little hypothetical experiment:

In this experiment you have a closed container which is half full of blood and has a pressure gauge that lets you measure the internal pressure. It has a mix of O2+N2+CO2 which to keep things simple we'll say is 1 atm. Now when you cause a chemical process to convert the O2 to CO2 you are going to see a drop in pressure. My understanding is that this is the 'oxygen window'? Do I have that right? If I've got that right, what is the mechanism behind the lowering pressure? Would this happen if I replaced the blood with H2O?

(And I'm not so concerned if you could or couldn't do this experiment in reality -- this is ideal blood that never clots, etc)

I've got other questions about this thought experiment as well. For example, if you start with zero pressure and then introduce a volume of 21% O2 and close the system off again, you're going to see the O2 taken up by the hemoglobin and the ppO2 and overall pressure in the container will drop? How well does this model what happens when blood absorbs nitrogen and oxygen through the lungs? Is the pressure in the container what is referred to as the "tissue tension"?