Bret Gilliam has sent me a letter further detailing his opinions on pPO2
limits when diving and his opinions of me. I will spare you his
intemperate ad hominem remarks, but would value your thoughts on the
following two points he makes, particularly in view of other posts on this
thread.

Bret says:

"If you don't like the exposures that I have detailed, I suggest you take
your case to the US Navy and NOAA from which my notes were derived. Oxygen
exposures are "dose" related, not simply partial pressure, as anyone with
a shred of experience understands. And you can exceed a dose limit at 1.3
ATA just as easily as you can at 1.6 or any other setting you might
choose."

By this, and our telephone conversation, I infer that he means dose *and
time*. i.e., that a certain minimum time at the dose must pass (e.g., 45
mins at 1.6 ATA for a single exposure) in order for toxicity to occur. Are
you concerned that O2 "hits" are possible in persons who have not exceeded
their CNS clock at pO2s between 1.3 and 1.6 ATA?

Bret also says:

"The flow rates on the (Atlantis I) mechanical injection of nitrox gas
considerably exceed any conceivable metabolic consumption of even the most
conditioned high performance athlete."

The manual (Joe Odom, 1995, rev 1, 3/96) used by TDI in training/
certifying divers for the Atlantis I indicates that the currently
available 7.6 L/min constant mass flow valve using (recommended) EAN50 on
the unit delivers an FiO2 of 32% at a VO2 of 2.0 L/min and an FiO2 of 35%
at a VO2 of 1.75 L/min, using the standard equation.

But, as noted in a previous post by Bernie Woolfrey:

"Diving and Subaquatic Medicine", quotes O2 metabolic rates (VO2)
as ranging from between 0.8 L/min to over 3 L/min, with rates of over
2 L/min "quite common" (these are based on swimming tests, breathing O/C air).
        The book goes on to say that:
        Typical gas consumption rates for a "slow swim" 0.5kt, a diver
would be breathing about 20 L/min of air, with an O2 consumption of
about 0.8 L/min, and

  0.8 Knots     =       30 L/min air breathed =  1.4 L/min O2 consumed
  1.2 Knots     =       50 L/min air breathed =  3 L/min O2 consumed.

This is an FiO2 of 17% at 3 L/min and an FiO2 of 13.6% at 3.2 L/min. Do
you consider that VO2s in this latter range and higher are likely to be
reached by a high performance athlete?

Bret is apparently e-mail challenged at present and I will send him the
above in a letter. As I have written before, I am not an "expert" but a
recreational diver with some advanced training. As a physician who
advises sport divers on health and diving medicine issues, my interest
lies in accurately informing the divers whom I examine on safe diving
practices and reasonably gauging my own diving risks.

Thanks for any replies you may send,

Peter Heseltine

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