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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. Although this appears in the BSAC manual, I am not sure that there is any data to support that either of these phenomena occur in a clinically significant (read as symptomatic) fashion. Increased FIO2 does decrease blood flow in at least some capillary beds but not at the cost of O2 delivery as far as I know (since the reduction in blood flow is, in principle, due to the increase in avaialable oxygen). Under normal conditions, there is a fair excess of O2 available to tissues so a 15% or so reduction would not cause symptomatic change. The issue of oxygen diffusing into the bubbles may occur but this should be offset by nitrogen diffusing out (which is the reason for using O2 in the first place). There may be some interesting transient phenomena but I have seen nothing documenting them. Maybe John Crea or prime Rat have some data on this. Is there a reference given in the BSAC manual for this statement? >My basic question is if a diver does a dive on air and then transfers to >O2 for stops does the vasoconstriction occur? This will reduce blood flow >and hence increase the risk of DCS. It may indeed reduce the blood flow reducing the ability to remove nitrogen. That would slow the decompression process but not cause bubbles and DCS in and of itself. Tom
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