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OK, all you guys out there going down and stuff - building rebreathers and
sticking SPGs on deco bottles. How about some *real* tech answers to some
tough man/woman-size questions that follow?
I think that Phillip is right. You are not going to get a handle on
nitrogen toxicity until you start thinking about the dose in relationship
to body size. When I asked some of the same questions last year, Chris
Parrett told me that the next version of Abyss would take this into
account - I don't think it does - and others mumbled maybe. So, all you
(2-3?) building new fancy constant pO2 computers out there to hang on our
rebreathers - what about it?!
Peter Heseltine
"Still trying to breath longer through smarter phsyiology"
PS. The above will be reformatted in docspeak and transmittted to the
hyperbaric list. Report to follow to those that reply to this.
*******************************************************************
* Peter Heseltine, M.D., F.A.C.P. *
* Professor of Medicine *
* University of Southern California LAC+USC Medical Center *
* Tel: 213/226-6705 1200 North State Street *
* Fax: 213/226-2479 Los Angeles, CA 90033-1084 *
* Eml: heseltin@hs*.us*.ed* USA *
*******************************************************************
---------- Forwarded message ----------
Date: Sat, 21 Sep 1996 11:35:42 -0500
From: Phillip Finch <pfinch@so*.ne*>
To: "Peter N.R. Heseltine" <heseltin@hs*.us*.ed*>
Subject: Deco theory questions
1) Does the volume of a given gas consumed during a dive affect one's
saturation state?
Seems to me that the answer to that one should be "yes," though every dive
table and dive computer I have seen assumes otherwise. We all know the
benefits of EANx at the right depth. By breathing a .36 mixture I am
consuming approximately 80 per cent of the absolute amount of nitrogen that
I would be consuming if I were breathing normox. And that difference
significantly affects deco and saturation status. But would I not enjoy some
of the same benefits if I reduced my air consumption from, say 2.0 cu.ft/min
to 1.6 cu.ft/min?
Conversely, if I raised my consumption by a similar ratio, wouldn't I be
losing some of the benefit of the EAN?
I realize it isn't quite as simple as that: partial pressures directly
affect the solution of gasses, and no reduction in consumption can change
the partial pressure of whatever quantity of being consumed (I think). But
given that even a 10 p.c. variation in deco stop times can be critical, and
that gas consumption rates can vary drastically between individuals, by a
factor of 2 or more, shouldn't a deco model attempt to account for the
absolute quantity of a gas consumed at a given depth?
2) Partial pressure within a tissue, relative to ambient pressure, is a
crucial determinant in Haldanean theory. But doesn't the actual volume of a
given compartment--blood, for example--determine partial pressure within
that compartment, for a given absolute quantity of dissolved gas?
And doesn't the volume of a given tissue compartment vary significantly
between individuals, depending on height, weight, and body type?
Shouldn't a comprehensive deco model attempt to account for these differences?
Certain combinations of these variables can place an individual well outside
the norm which all deco models seem to assume. My breathing is considerably
more efficient than it was two years ago--at the surface, when I'm paying
attention, my consumption is between .4 and .5 cf/min (.08cf per
inspiration, with five or six deep cycles per minute), which I guess is
pretty good. At that rate I might be considered a big guy breathing small.
OTOH, a small person breathing big (say, half my size, comsuming twice my
air, not at all an unlikely occurrence for an inexperienced diver) would
seem to be significantly more at risk for DCS than the norm.
If gas consumption rate has any significant effect on saturation states,
then the argument for air-integrated computers suddenly gets a lot stronger.
And if body mass/type has a similar effect, then the argument for
*programmable* air-integrated computers gets a lot stronger.
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