Date:     January 22, 2003
From:    Siegmund, Chris [CSiegmund@illumina.com]
To:        DirQuest List (quest@gue.com)

Subject:  RE: Question on oxygen window

Increasing the ppo2 will increase the oxygen window but the two are not synonymous. The easiest way to understand the oxygen window is to look at a curve of blood o2 concentration vs. ppo2.

Upon inspecting the curve you will notice two very distinct characteristics. The first characteristic is that at low ppo2s the curve is very steep and rises sharply. This is due to hemoglobin in the blood. Since hemoglobin binds to o2 and effectively removes it from the blood plasma the o2 bound in the hemoglobin doesn't contribute anymore to the ppo2 in the blood. You can think this as being two different reservoirs of o2 in the body: in the hemoglobin in your red blood cells, and in the blood plasma itself. At low partial pressures the hemoglobin has a lot of binding sites for o2 and as a result fills up quickly with very little rise in ppo2. In essence, for very little rise in ppo2 you have a sharp rise in blood o2 concentration (hemoglobin + plasma content).

The second thing that you will notice is that with higher ppo2s the curve levels off. This is due to all the hemoglobin binding sites being full of o2 and not being able to hold any more. What then starts to happen is instead of the hemoglobin holding more o2 the blood plasma starts to hold more o2. Blood plasma doesn't hold nearly as much o2 as hemoglobin does for a given rise in ppo2 and as a result the curve flattens out.

So what does this all mean for increasing ppo2? Well this means that at low ppo2s if you increase your ppo2 a fixed amount the amount of o2 in your blood goes up a lot because the hemoglobin binds to it (and when it is bound it no longer contributes to the ppo2 of the blood). When you're at high ppo2s and you increase your ppo2 the same fixed amount the blood content of o2 will only go up a little bit because all the hemoglobin biding sites are full and additional o2 can only be carried by the blood plasma.

Now what does this mean physiologically? When you are doing deco on backg as the main way to off gas is to reduce ambient pressure, i.e. ascend. Now, if you breathe a high ppo2 mix during deco you start to take the curve past hemoglobin saturation to when blood plasma starts carrying the o2. Now let's take a look at these two situations of back gas and deco gas during deco along with physiology. In both cases your body will be consuming the same amount of o2 regardless of what you breath (hopefully this is obvious). Blood passes from you lungs to your tissues (arterial) and back to your lungs (venous). For back gas at low ppo2s you might not saturate all of your hemoglobin so if your body consumes a given volume of o2 the pressure drop from arterial to venous ppo2 will be very small since the hemoglobin will be giving up the o2 it carries which doesn't contribute to blood ppo2. For deco gas at high inspired ppo2s you will saturate your hemoglobin and your blood plasma will start to carry the extra o2. Remember that it will take a much greater increase in ppo2 to increase the amount of o2 in your blood if the plasma is the primary carrier than if the hemoglobin was carrying it. Now, if your body consumes the same amount of o2 as if you were on back gas, you can see that the pressure drop from arterial to venous will be much greater than before because you are not pulling it out of hemoglobin which does not contribute to your blood ppo2 but instead out of your blood plasma. This is the genesis of the oxygen window. You now have a much bigger pressure difference from arterial to venous circulation which can be filled by supersaturated inert gasses that are trying to get out of your tissues. This void, where the o2 was, functions somewhat like a vacuum pulling the inert gasses out of your tissues.

I have not covered basic physiology and I have made many simplifications in this description, and as a result the above functions better as an introduction to the oxygen window than a full explanation of it. Hopefully it is a little clearer now and not more confusing than ever.

Chris Siegmund

-----Original Message-----

Sent: Wednesday, January 22, 2003 8:23 AM

To: quest@gue.com

Subject: Question on oxygen window

I was reviewing some old notes the other night and was missing a few pages of explanations I had formerly compiled. The subject of oxygen window came up so I wanted to get a few details from those knowlegeable here on the list.

Can somebody clarify the difference between the concept of the oxygen window and running high partial pressures of oxygen? Or are these two concepts just two different terms for the same thing? I mean, you open the oxygen window by increasing the partial pressure, so what's the difference?

I'm sure George has the right answers, I'm not questioning that, but he does not always have the time to explain the details & I don't want to just take his word for it. How does this actually work?