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>>When teaching on a BSAC Oxygen Administration course we tell our students >>to expect a transient worsening of symptons to a casualty shortly after >>starting to administer 100% O2. This is due to two reasons. Firslty, if >>there is a lodged bubble some of the oxygen in solution will diffuse into >>the nitrogen bubble. Secondly, the body reacts to the higher partial pressure >>of O2 and reduces the blood flow due to vasoconstriction. This can be read >>in the BSAC Safety and Rescue manual. > >This may be a stupid question but I was brought up to believe that the >only stupid question is one that goes unasked... > >Given that the patient will be breathing *some* gas at 1 ata, why would >O2 diffuse into the nitrogen bubble any more than additional nitrogen >(if they were only breathing air)??? > >The way that I see it, if you're breathing pure O2 then the PP N2 in the >lungs and bloodstream should go down (faster than by breathing air, which >would have a PP N2 of .79) causing an increased rate of N2 >offgassing. Eventually, the N2 bubble will diffuse back into solution >because of a low PP N2 in the surrounding tissues. The issue will be >whether this happens soon enough to prevent tissue damage. > >If my logic is flawed I'm sure I will be corrected... > Carl, this is essentially correct, however... Bubbles exist at slightly higher than ambient pressure, their internal pressure must be equal to the sum of ambient pressure + pressure due to surface tension of the fluid they are in + pressure due to compressive forces of the tissue they are in. If the bubbles are small, and it is our duty as divers to keep them so, the pressure due to surface tension can be quite large, as it is inversely proportional to radius of the bubble. In the longer term the pressure due to surface tension may be reduced by surface active compounds-surfactants stabilizing the bubble (this latter comment added to prevent Prime Rat from flaming me). The bubble will contain a mixture of gases approximating venous blood, oxygen, carbon dioxide, water vapour, diluent gas). Breathing 0.79 bar N2 is unlikely the culprit for continued bubble growth as the pN2 in the bubble is higher than this, it is probably the tissue nitrogen. Breathing 1.0 bar O2 increases the bubble-blood pO2 difference and decreases the bubble-blood ] pN2 difference. Briefly, O2 diffusion into the bubble exceeds N2 diffusion out and bubbles may grow transiently before resolving. David Doolette ddoolett@me*.ad*.ed*.au*
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