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---------- Forwarded message ----------
Date: Fri, 14 Jun 1996 09:28:43 +0930 (CST)
From: David Doolette <ddoolett@me*.ad*.ed*.au*>
To: "Peter N.R. Heseltine" <heseltin@hs*.us*.ed*>
Subject: Re: More on a *safe* pPO2

>I found a paper by the US Navy Medical Research Unit: A model for
>predicting CNS toxicity from hyperbaric O2 exposure in man.
>I would be very interested to know your opinions and thoughts about the
>paper, as it seems to be at least, a thorough meta-analysis of the
>available data.

Peter,

I read the paper yesterday, somewhat hurriedly between teaching commitments,
I didn't notice who had actually written it, a bit of an ommission by the
magazine publisher.

I thought the paper was interesting and thourough and it has prompted me to
re-read a couple of the references cited or look them up for the first time.
 I had a couple of thoughts about the paper that you might be interested in:

1.  I wonder why they excluded a couple sets of data (including Young, I
think).  This has an unscientific sound to it , as if it didn't suit their
needs.  I will have a look at this study later today.

2.  The 1.6 bar regression line in figure three appears to be based on three
data points, aside from being a bit of a leap of faith, it underscores my
comments on the rarity of problems at these relatively low PO2s.

3.  Figure 4 is one of my favorites, illustrating that there was significant
difference between curves for exposure time and probablity of toxicity of
2.2 bar PIO2 dependent on how and when the studies were conducted.  Again
this seems to suggest that CNS O2 toxicity at these low PO2s is a highly
variable occurance.  This, in part, is my reason for believing the CNS O2
clock concept (as taught by Technical diving agencies) is not particularly
useful.  I believe that if you are going to exposure yourself to the risk of
drowning, you need to limit your PO2 to a level that is highly unlikely to
ever cause a problem (somewhere below 1.8-1.4 bar, choose your own poison)
and at these levels I don't believe a safe exposure time can be predicted.

4.  One has to exercise some caution in extrapolating the results of these
studies to recreational diving.  Most of these studies used rebreathing
apparatus, and, while the data is interesting, and certainly indicative, it
is not completely transferable to open circuit demand diving.

regards,

David Doolette
ddoolett@me*.ad*.ed*.au*

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