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Rod and others,

>I may be missing the boat here but what I interpreted the original
>post to mean is that because you are diving at high altitude where O2
>is limited, if you make your dive on an oxygen rich mixture, then upon
>returning to the surface and discarding your dive gear, you might
>become more hypoxic upon breathing ambient air. The rationale I
>suppose is that underwater with the enriched oxygen your body
>acclimates to it. When you surface and breath ambient air that has
>considerably less O2, increased hypoxia might occur.

I need to research this - Joan's quote from Lippman's book contradicts
my thinking.

So, let's discuss 4 scenarios (actually 2 pairs)

1) - air dive at sealevel
2) - nitrox dive at sealevel
3) - air dive at altitude
4) - nitrox dive at altitude

Obviously we have to talking about an altitude sufficient to cause the
ambient PPO2 to start to be of concern as well, especially when under
stress/work.

I don't think anyone would say that 2) would make you more hypoxic
than 1), but, at sea level we're getting 20.9% o2.

If 3) doesn't make you hypoxic then I don't see how 4) could,
especially since 3) will still give you much more O2 than ambient, and
for any depth (e.g. >33') will give you more PPO2 that you'd get at
sea level.

Rod said that you may "become acclimated" to an enriched O2 mixture,
but I do not buy into this scenario, especially if 3) does not leave
you more hypoxic, otherwise, ALL nitrox dives would have the same affect.

In other words, *if* there's a risk of becoming hypoxic I do not
believe that EAN will worsen the condition.

Second, and this is the part that's contrary to the book, I fail to
see how diving w/ reduced atmospheric ambient PPO2 can cause a diver
to become hypoxic AFTER a dive, but not before.  Of course, your body
is getting more O2 under the water so you can run up your metabolism,
perhaps they're saying that there is some hysteresis in your
metabolism and when you exit the water it is still demanding O2.  BUT,
this would still not account for the difference between air and EAN
because air will still have a much greater PPO2 at depth than ambient.

Any medical types care to elucidate before I go off digging on my
own???

-Carl-

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