RE: [PSUBS-MAILIST] O2/ lung gradient

["Sean T. Stevenson"<sts@telus.net>]

I'm not entirely clear on the physiology behind this, but it has been demonstrated
that a person in a constant pressure environment can tolerate higher PPO2s
before toxicity symptoms are evident than a diver in the water can.  I read
an article (I can't recall where) which mentioned the pressure differential
as a factor - it had something to do with the effective alveolal surface area
in the presence of a pressure gradient.  There may be other factors at work
here (thermal, stress, etc).

For practical purposes, it makes sense to lower the exposure for the sole reason
that the consequences of convulsions in the water are potentially much more
serious than in the chamber/habitat, even for a hard-hat diver.

Also, for practical purposes you need to consider the effectiveness of the
decompression.  A lot of recent research is lending weight to the notion that
the application of high PPO2 is not the entire answer, and that cycling of
the high PPO2 with low PPO2 "back gas breaks" actually makes the lungs work
more efficiently, as they allow an interval to reset from the oxygen exposure
and offgas more effectively when you again raise the PPO2.  1.6 is a good limit
not only because it may be a little conservative, but it accomodates the cycling
behaviour as you move up in the water column, such that when you switch decompression
gases to the next higher PPO2, you are essentially doing so after a break that
was effectively achieved on the previous gas.

In this case, the last (oxygen) stop at 20 fsw is the only one that you need
to insert gas breaks on.

Industry standards have tended toward 1.4 during the working phase of a dive
and 1.6 maximum on decompression.  I usually mix for 1.2 on the bottom to be
a bit conservative.

-Sean

>At 09:28 AM 5/7/2002 -0800, you wrote:
>> Higher PPO2s can be tolerated in dry environments (chambers, habitats,
>etc.) since
>>there is no pressure gradient across the lungs as there is when in the water.

>
>Hi Sean, why is that a factor? Even vertical, wouldn't the gradient be very

>slight? Lung length, let's say 12"; 1/33' = .44 psi.
>
>William Alford
>walford@dbtech.net
>
>Tout comprendre c'est tout pardonner
>
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