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>
>Richard Pyle wrote:
>> Are you sure about that?  I have always read that O2 is a vasoconstrictor
>> - not an inhibitor of vasodialation.  People don't lose arms & legs in
>> chambers because:
>> 1) vasoconstriction does not equate to zero blood flow (as pointed out by
>> Prime Rat); and
>> 2) there is so much dissolved oxygen in the blood & tissues during
>> hyperoxia that the tissues do not get hypoxic (if they did, the
>> vasoconstriction effect would reduce, and the tissues would get the needed
>> blood flow).
>
>A thought about this occured to me while I was in the shower this
>morning (yes, thinking best in the shower is, unfortunately, about the
>only thing I have in common with Mr. Mickey Mouse, Michael Eisner):
>
>If increased O2 levels really result in significant vasoconstriction,
>thast would imply that diving Nitrox using EAD (Equavallent Air Depth)
>and an air table would be extremely dangerous.  In fact, we would see
>substantially more DCS incidents in EANx/EAD/Air table divers vs
>Air/Table divers.  (This would be due to vasoconstriction imparing
>offgassing.)  Perhaps one could argue that any vasoconstriction effect
>would also impair tissue loading, but I would argue (and, in fact, do
>:-) that the time delay before the O2 level increased to the point
>where vasoconstriction manifested would permit significant tissue
>loading to occur according to the non-vasoconstriction model.
>
>Do I believe that vasoconstriction is an issue?  Not really, but it
>sounds interesting.  I had not included it in the model I built
>earlier, as at the time the vasoconstriction effect of O2 was not
>quantifiable and did not seem very large.  One thing is for sure: the
>interaction of pressure and O2 on the diver seems more interesting and
>complicated every day -- and I think the complex interactions are way
>beyond our capability to reason about using just calculators.
>Remember, the problem here is essentially one of linear diffusion
>through a moving fluid, with feedback.  Not simple at all.
>
>-frank

These issues are a concern and there is a literature about the effects of
increased pO2 on offgassing during decompression (it can cause a relative
decrease in inert gas offgassing) and how this offsets the benefits for 
decompresiion of the increased tissue/inspired inert gas differential
that high pO2 mixtures provide.

David Doolette
ddoolett@me*.ad*.ed*.au* 

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