From: mayne@ds1.scri.fsu.edu (Bill Mayne)
Newsgroups: rec.scuba
Subject: Re: Nitrox (Was Re: Thermodynamic Decompression)
Date: 9 Sep 92 15:31:50 GMT
Reply-To: mayne@ds1.scri.fsu.edu (Bill Mayne)
Organization: SCRI, Florida State University

In article <pk2a69g@twilight.wpd.sgi.com> story@sgi.com (David (Duis) Story) writes:
>In article <1992Aug21.220829.12246@mailer.cc.fsu.edu> mayne@cs.fsu.edu writes:
>>
>>As for claim (4) (direct benefit to the higher fraction of oxygen),
>>there are at least a couple of factors. One is that if the partial
>>pressure of O_2 is around 1 ATA or more the rate of inert gas exchange
>>is reduced. You would thus absorb more slowly at depth than theory
>>predicts. You also off-gas slower than theory predicts, but still faster
>>than if you were breathing air instead (as nitrox tables or schedules
>>using standard air tables with equivalent air depths assume). 
>
>Could you elaborate?  Why will the partial pressure of O2 affect the
>on and off-gassing of N2 save by reducing the ambient pressure of N2?

I presume that it is well understood that during decompression or
safety stops you off-gas faster breathing mix containing less
inert gas, but that tables, even nitrox tables, assume you are
breathing air on decompression stops. The benefit of breathing
an more oxygen rich mixture is not taken into account and hence
provides an added measure of conservatism on decompression dives.
This effect is not significant when there are no stops.

I therefore assume your question has to do with the effects of
oxygen other than replacing nitrogen. I am not a physiologist and can't
lay my hands on the references from which I read some of this. So take
the following with some NaCl. Perhaps I should never have opened my
mouth, or I should just shut up. But having started it I don't want to
just ignore your question.

In a paper presented at a workshop on nitrox Bill Hamilton writes:

"...it is widely held that oxygen seems to provide a benefit in a
decompression beyond what can be accounted for by its replacement
of inert gas."

My understanding of the reasons for this is limited. I've read that
up to a point increased partial pressure of O_2 makes it easier for
your respiratory and circulatory systems to supply oxygen to your
tissues. If they aren't working as hard, i.e. circulating less,
to provide oxygen there is less exchange of inert gases.
This is essentially the reverse of the well recognized principle
that nitrogen uptake and therefore decompression obligation is
increased if working hard, and therefore breathing harder and
circulating more blood, at depth. It is also claimed that although
using nitrox or pure oxygen on shallow decompression stops does speed
up off gassing and could allow you to make the stops shorter this
effect may be less than Haldanian theory predicts. Again the reason
may have to do with the fact that your body is more easily meeting
its oxygen needs, hence slower circulation and slower off-gassing
of nitrogen, though still better than if you were on air. On the
other hand the extra oxygen itself may have benefits other than
reducing the partial pressure of nitrogen in the breathing mix.
This would be especially true at shallow depths where the normal
partial pressure of O_2 is not so elevated. You are of course
correct that at some point your body can't use extra oxygen anyway.
That point is most likely higher than the oxygen in air at 20 feet
or less, and probably much deeper, though. Note too that oxygen is
given in first aid even in non-diving emergencies and sometimes to
football players recovering from strenuous exercise where there is
no excess of inert gas involved.

I'm sorry I can't readily quote a more qualified source for these
explanations. No doubt there is more to it than I've gleaned from
the articles.

Bill Mayne