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At 08:43 PM 26/2/99 -0500, Rich Lesperance wrote:
>Remember, the O2 'pushing' the nitrogen out is really just a figure of
>speech. The nitrogen 'wants' to come out due to diffusion (and the pressure
>gradiant, since we are on ascent, and 'decompressing' in the pressure-sense
>of the word). The nice thing about 100% O2 is that we are substituting a gas
>that has a _minimal_ propensity to bubble, because it is metabolized
>normally, while we are uptaking zero percentage of N2.
>
>>>if it does or does
>>>not, up to what depth does it diffuse the N2 from the tissues?<<
>
>Remember, dive tables / computers get you out of the water with a
>(hopefully) _safe_ tissue gradiant of N2. That is, you _still_ have a
>buttload of N2 in your tissues, and are still off-gassing for hours after
>the dive. It's just that the _gradiant_ between the tissue N2 pressure and
>atmospheric gas pressure is low enough that you are not bubbling up.
>
>Two things I want to bring up right now- one, is that this thread is
>starting to get rather involved with decompression theory. even though I may
>talk a good game, I do not feel completely confidant explaining deco theory,
>espescially with regard to controlling tissue compartments. I have a pretty
>good idea, mind you, it's just that when discussions like this start getting
>theoretical, the fistfights over deco models begin <g>, because this is NOT
>an exact science.
>
>The second is that bubbles occur after most dives. There _seems_ to be a
>threshold at which the bubbles 'cause' DCS, but that threshold is poorly
>pinpointed; ie, there is a theoretical, but poor, correlation between the
>two.
>
>>> if i chose to deco on 50% O2, we know the MOD is 22 meters with a
>>>PO2 of 1.6.<<
>
>A pO2 of 1.6 is the recommended limit due to the risk of CNS O2 toxicity.
>There is a fascinating thread going on at the same time as this one, where
>people are yelling at each other because of a poorly written post, where it
>seems as if a poster is advocating keeping the pO2 at 1.6 for the entire
>dive.
>At a pO2 of 1.6, you are relatively safe from CNS O2 toxicity, but you
>are 'burning' up your lungs from pulmonary O2 toxicity.
Rich, you got that ass-backwards.
1) During the working part of an immersion dive, running at
or near pp02 1.6 carries an unacceptable risk of CNS toxicity.
2) In the water, if you run 1.6 for the whole dive, you
are more likely to take a CNS hit before you'll sustain pulmonary
damage.
In the chamber, where the likelihood of a CNS hit is lower, the
administration of a USN table 6 treatment, which includes a total
of 70 minutes at pp02 2.5 and a total of 2 hours at pp02 1.6
generally does NOT cause pulmonary oxygen toxicity. That's over
three hours at 2.5 to 1.6 WITHOUT 'burning' up your lungs.
I think, if you are going to use these lists to think things
thru, you should clearly state that your ideas are just
speculation on your part. Otherwise, dangerous misconceptions
get started and spread around.
>>>secondly,
>>>since the po2 has decreased and with 50% O2 we still have 50% N2, does that
>>>mean we still absorb N2 in our tissues?<<<<
>
>Yes, which is why using mixes other than 100% for deco is less effective.
>Some would say rather silly. You are simply not getting the benefit from the
>increased oxygen content (and therefore decreased nitrogen). While it would
>be better to use a 50/50 mix than air at a certain level, the added benefit
>from 100% O2 is worth it.
>
>So why not use both? Why not go to 50/50 at a depth of 70 fsw (1.6 ata of
>O2), and then switch to 100% at 20 fsw? The answer is that the concensus of
>smart people says that you are exposing your lungs to high pressures of O2
>with less of a benefit, since you still have 50% N2 in the mix.
Rich, if the consensus of smart people that you refer to ever
did exist, it would not argue against using 50 and 100%. And
it would not use the argument that "you are exposing your lungs
to high pressures of O2 with less of a benefit, since you still
have 50% N2 in the mix."
50% is a useful decompression gas. Waiting till you are at 6
metres before applying a high pp02 (1.6) as you would have to
with 100% as your only deco gas, does not make good deco sense.
You should take advantage of the oxygen window right where your
decompression goes from being depth dependent to time dependent.
50% does it nicely.
>I know, this seems rather contradictory: on one hand, I say the only thing
>you have to do is lower the N2 percentage to reap benefits, and then the
>next paragraph, I'm saying you must have pure O2! The best explanation I can
>offer, is that if you consider risk vs. benefit, the ratio is most favorable
>when using 100% O2 to deco.
I think you should have another attempt at that one. In
private.
>Again, these are layman's explanations. if any of our esteemed readership
>thinks I'm a stroke for my terminology, keep it to your bloody selves <g>!
>OTOH, if I have an error in my mental model of how this stuff works, PLEASE
>bring it to my attention; knowledge is power.
>
>And thanks for asking this question, Joel, you forced me to articulate and
>think about things I have not had to mull over in quite awhile. They say you
>don't really understand something unless you can teach it <g>. Hopefully my
>explanation above isn't too confusing or poorly written.
Rich, there are some very good texts on decompression theory
and practice which will help clear up a lot of your confusion.
Try Bennett & Elliott - The Physiology & Medicine of Diving,
Edmonds, Lowry and J Pennefather - Diving and Subaquatic Medicine,
A.A. Buhlmann - Decompression / Decompression Sickness. Bruce
Wienke has written some action packed articles on bubble
mechanics. Also check the archives here. George Irvine has
consistently provided a stream of very useful information based
on the WKPP's successful regimen for decompressing from deep
exposures, long and short - the stuff that really works.
rgds billyw
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