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jheimann@sc*.gt*.co* wrote:
> In his nitrox FAQ posting, Alan mentioned a friend who insists that he
> uses less air when diving nitrox due to the higher O2 content.  I don't
> see how this is physiologically possible, [...]

It is entirely possible to use less air when diving nitrox.  However,
the higher O2 content _probably_ has nothing to do with it -- a much
more likely scenario is that the diver is simply more relaxed (for
whatever reason -- he knows he's got more bottom time/less hang time/
lower DCS risk/whatever), so he uses less air.  Personally, I've
noticed fairly dramatic differences in my own air consumption between
dives where I'm completely comfortable vs. dives with even minor
annoyances.  Diving nitrox has on occasion been a significant part of
that comfort factor, so it has resulted in me using less air.

There are some individuals whose breathing is regulated by oxygen levels
rather than CO2, although most of these individuals probably shouldn't
be diving anyway.  This is known as "hypoxic drive", and often occurs
in COPD (chronic obstructive pulmonary disease -- emphysema, etc.)
patients.  The body adjusts to chronic elevated CO2 levels by ignoring
that as a breathing stimulus (it has been theorized that this CO2
tolerance occurs in divers to some extent as well), at which point the
body's normal "backup" stimulus -- lowered O2 levels -- takes over as
the primary means of regulating respiration.  This, in itself, is not
really a problem, although some of these people may stop breathing if
exposed to high concentrations of oxygen.

Of course, we could also argue that you will always use less air
when diving nitrox -- in fact, you won't use any air at all!  _Gas_
consumption, on the other hand... :-)

--tab, picking nits again...

-- 
Tracey Baker              tab@pa*.co* / tab@go*.ho*.at*.co*
  "I don't think safety is the main issue here...
                          You'd be stupid not to be safe."

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