Oxygen window

[ragdon]

Hi All,
I'd like to know what the oxygen window is, can anyone give me an explanation in laymans terms?

[MattS]

Blood coming from the lungs, in the arteries is rich in Oxygen (red) . As the blood travels around the body the various tissues 'feed' off the Oxygen it carries. By the time the blood gets back to the lungs through the veins it contains much less Oxygen (blue). The difference between Oxygen levels in the Arterial and Veneous blood is known as the Oxygen window.

It really is not as complicated as this guy makes out ;-)
http://www.wkpp.org/articles/Decompr...gen_window.htm

[Foggy]

Originally Posted by MattS

Blood coming from the lungs, in the arteries is rich in Oxygen (red) . As the blood travels around the body the various tissues 'feed' off the Oxygen it carries. By the time the blood gets back to the lungs through the veins it contains much less Oxygen (blue). The difference between Oxygen levels in the Arterial and Veneous blood is known as the Oxygen window.

It really is not as complicated as this guy makes out ;-)

And there was I thinking it was the window between the round and the square window..... :-)


Cheers Paul

(you know when you're senile when.......)

[MattS]

Originally Posted by Foggy

And there was I thinking it was the window between the round and the square window..... :-)

No No No The O2 window was between the arch window and square window ;-)

[Mark Chase]

Originally Posted by MattS

Blood coming from the lungs, in the arteries is rich in Oxygen (red) . As the blood travels around the body the various tissues 'feed' off the Oxygen it carries. By the time the blood gets back to the lungs through the veins it contains much less Oxygen (blue). The difference between Oxygen levels in the Arterial and Veneous blood is known as the Oxygen window.

It really is not as complicated as this guy makes out ;-)
http://www.wkpp.org/articles/Decompr...gen_window.htm

Yes but this isnt exactly whats ment by opening the 02 window in diving terms. When GI3 talks about holding te 21m stop on 50% for 5mins to open up the Oxygen window he is refering to the above system in terms of decompresion.

ATB

Mark Chase

[MattS]

Oh alright then

We have established that the there is an Oxygen imbalance between the arterial and venous blood streams. We also know that during decompression the gas dissolved in the blood is at a higher pressure than the gas we are breathing.

The breathing gas is made up of oxygen and inert at ambient pressure. The (partial) pressure of each is relative to the fraction of the gas within the mix. So breathing air at 10m you have 0.42 bar Oxygen and 1.58 bar of inert.

Simple so far.

Now lets take a look at the math Mark posted which describes dissolved gas decompression

pt. i.g. (tE) = pt. i.g. (t0) + [pI i.g. - pt. i.g. (t0)] ·[1 - 2-tE / t1/2]

Anything odd about that...Like something missing...Like where has all the O2 gone?

The math simply ignores any inbalance of Oxygen from the off gassing equation. However things don't just dissapear when we are talking Physics. Just like my books, everything must balance. So we breathed in O2, it was eaten by the tissues and the only thing the tissues have to replace it is a small amount of CO2 and more inert. The inert gas pressure in the venous blood flowing to the lungs must be higher than the inert gas pressure in the arterial blood flowing from the lungs. The tissues do not eat all the available O2 however. Each tissue has an appetite for O2 which varies widely depending on how hard it is working. Typically in a resting adult at the surface the tissues will be consuming very roughly around 0.07 bar of the available O2 in the arterial blood stream. So there is an extra bit of conservatism in the dissolved gas calculations.

The concept of widening the oxygen window basically means breathing O2 at a higher partial pressure and hoping the tissues will eat more of it, exchanging it for more inert. This should not be confused with the difference breathing less inert makes to the traditional maths. If we change our gas from air to 80% at 10m the maths knows full well that we are breathing 1.6 bar O2 and 0.4 bar inert and increases the offgassing rate accordingly.

To believe in widening the O2 window you have to believe that tissues will consume a percentage of the available O2. In simple terms imagine you have gone to the all you can eat buffet. Do you; eat everything or a percentage of what is available or until you are full? The tissues demand O2, the blood supplies O2 but what happens when supply outstrips demand is beyond me.

I really can't follow the pathology at this point. As I understand it there is some evidence to suggest the O2 window exists. By all means have a go at this if you are interested.
http://www.cvphysiology.com/CAD/CAD003.htm

[tom]

Matt, thanks for that. I have no way of knowing whether what you're saying is correct or not, but it was a brilliantly clear explanation that I actually understood! Cheers.

[Mark Chase]

Great reply Matt, I am glad you are as concerned about the GI3 implied conclusions as I am. I thought I had misunderstood the theory but apparently not.

ATB

Mark Chase

[And]

Hi

In the dissolved gas model, diffusion is assumed to be infinite and thus cannot limit tissue gas uptake or removal. Helium and Nitrogen are independent of each other, so whether nitrogen is present or not in the deco gas it does not influence the offgassing of Helium. The same can be said with nitrogen when removing nitrogen from the decompression gas, and has been confirmed with studies using heliox and O2 in hyberbaric medicine.

Deco can be longer on a He based dive when using Nitrogen in the deco mix, as nitogen is diffusing into the tissues as helium is diffusing out.

Gas movement from lung to tissue and back is dependent on a partial pressure gradient. The oxygen window has been called a partial pressure vacancy simply because a part of the o2 is consumed by the tissue. It always exists. What GI discusses is enlarging this window.

Enlarging the oxygen window can only occur when Partial pressure of arterial O2 is increased to a maximum tolerated value, either by increasing depth or increasing Fraction of inspired O2 of the gas mix, or both. Although enlarging the oxygen window may not directly affect tissue gas removal, it does directly affect the ongassing of a tissue during decompression, which affects the amount of time required to decompress the tissue. Decompressing as quickly as possible is a good thing due to other factors such as hypothermia, dynamic ocean environment etc etc.

Breathing 80%, will obviously effect the rate of diffusion from the tissue, and as the dissolved model is only concerned with inert gasses, then it does not recognise the oxygen window at all.

Andy

[Mark Chase]

I understand this point Andy but the issue of opening the 02 window also affects the gas switch at 21m on to 50% 02 50% Nitrogen? Obviously the 1.6PP02 is the preferred max level at this depth but the 21m stop doesn’t last long and you’re immediately into low PP02s again? Why is opening the 02 windows on 50% and finishing the dive on it on a PP02 of 0.81 any better than doing deco on 32% & 80% and finishing the dive on a PP02 of 1.29?

Surely for the O2 window argument to stand up GI3 should advocate the 100% for deco theory on all dives?

Surely holding ANY deco mix at 1.6 for 5mins would open the 02 windows?

As Matt said when is enough enough?

Always puzzled me this one.

ATB

Mark Chase