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Hi David: On Tue, 19 Sep 1995, David Story wrote: > I was reading in The Physiology and Medicine of Diving (4th ed) and > noted that it says: > > "The oxygen window increases linearly with increasing depth" and then > cites 3 studies from the 60's which I don't have. would they be: hills; hills & lemessurier; van liew...? yount&lally wrote an article in aviat. space&enviornmental medicine june'80: "on the use of O2 to facillitate decompression." van liew, et-al did a paper in the mid-80s also try vann's article in the 3rd edition of B&E (instead of 4th ed.). p 358 shows a figure that plots O2 window vs INSPIRED O2 PARTIAL PRESSURE (increases linearly with depth....). this plot is linearly increasing up to a point, then levels off. now consider that O2 is both dissolved in blood and bound to haemoglobin. the amount dissolved in the blood is: solubility*pressure; so, the more pressure, the more dissolved. different tissues extract O2 from circulation at different rates. once enough dissolved O2 is present to satisfy a particular tissue's needs, the O2 window levels off to a constant. there has been speculation that the excess inspired O2 (once the window is all the way open) may lead to deco problems. this effect could kick in at ~2ata ppO2. of course, george will tell you that you'll have other problems at such excessive ppO2.... the O2 window changes with gas switches and depth. one common use of the term "O2 window" (as vann uses in B&E), is the pressure difference between a alveolar ppO2 and a bubble's ppO2. bubble O2 pp is about the same as tissue ppO2 (by diffusion). as long as we are still in the linear region of the O2 win, the tissue O2 tension will be small and constant. so--it must be the N2 (or other diluents) that make up the bulk of the pressure in a bubble. by recompressing a diver who has formed bubbles, you force the pressure inside the bubbles to increase when the bubbles are squashed(linearly with depth--ignoring skin/volume effects....). even if you breath air (at an aed<5ata, of course), the O2 win will be large when you first descend due to the the large inspired ppO2. as long as the diluent pp in tissue surrounding a bubble is less than the pp's inside the bubble, the bubble will offgas to the tissue by diffusion. it's said that the O2 window drives this off-gassing. besides metabolism, there are other reasons why living tissue is unsaturated. one example is the addition of ~1/15 ata water vapor to your mix when inspired gas enters your lungs and gets hydrated. this addition reduces the O2 and diluent pps from what was delivered by your regulator. did that make sense...? regards, eric _____________________________________________________________.sig Eric Maiken email: eapg243@ea*.oa*.uc*.ed* Dept. of Physics o: 714 824-6621 U of California fax: 714 824 2175 Irvine, CA 92715-4575
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