Bill

I read somewhere, some time [sorry for the appalling reference], that the O2
carrying capacity of plasma starts at a PO2 of around 1.3.  So, at 6m, 80% is
only delivering a PO2 of around 1.28, whereas O2 is around 1.6 - making a
considerably greater difference to deco effectiveness than would at first
appear to be the case.  IIRC, the plasma O2 capacity was around 15% of that
carried by haemoglobin.

Now, the Buhlmann algorithm does not take into account such physical
mechanisms and so such a benefit will not be demonstrated by detailed
analysis of tables.  In fact, whilst I have learnt from some of the detailed
recent posts on this subject, I feel that many are placing too much emphasis
on the Buhlmann algorithm.  Of course, I base my deco on the output of this
algorithm but modify it considerably.

David Shimell
shimell@se*.co* <mailto:shimell@se*.co*> 
DDI: 01932 814096 * Mobile: 07770 282 202 * Fax: 01932 814343
Project Manager, IBM UK Web Server Group, Sequent Computer Systems Ltd,
Weybridge Business Park, Addlestone Road, Weybridge, Surrey, KT15 2UF, UK
registered in England and Wales under company number: 1999363, registered
office as above

-----Original Message-----
From:	Bill Wolk [SMTP:BillWolk@ea*.ne*]
Sent:	Friday, September 01, 2000 5:44 AM
To:	ScottBonis@ao*.co*; swhac@pc*.gu*.ne*; Techdiver
Subject:	Re: Oxygen window & 80/20 Deco

On 8/29/00 1:29 AM, ScottBonis@ao*.co* wrote:

>Let me try to ask my question in a slightly different way.  If I weren't 
>worried about oxygen toxicity (this is just a fictious example here), then I

>get the impression that the theory is that deco'ing on 100% O2 at 30 feet or

>forty feet would be more efficient at offgassing nitrogen than deco'ing on 
>100% O2 at 20 feet.  Is this correct, or am I all screwed up again?


Scott -

The short answer is yes, you're absolutely correct. The textbook 
definition of an "oxygen window" is "the difference between total gas 
pressures in arterial and venous blood; exists because oxygen is partly 
metabolized by the tissues, so venous oxygen pressure is lower than 
arterial oxygen pressure." 

But we use the term a little differently when we talk about 
decompression. We essentially use "oxygen window" to represent the 
difference between the concentration of inert gas dissolved in tissue vs. 
the concentration of oxygen and inert gas in arterial blood. 

If you'll excuse a bit of an oversimplication, I think I can make the 
concept (and deco) a whole lot clearer:  Remember the concept of osmosis 
from biology? If you have a high concentration of a substance on one side 
of a cell membrane and a low concentration on the other, the substance 
will cross the membrane until the concentration on both sides is equal.  
The more of the substance on one side and the less on the other, the 
higher the osmotic pressure -- literally the force that the substance on 
the high concentration side exerts as it tries to push through the 
membrane. And usually, the higher the osmotic pressure, the faster the 
substance moves towards equalization.  Like lots of people pushing 
through a turnstyle.  This is the key.

In deco, you have tissues that are oversaturated (highly concentrated) 
with inert gas -- on one side of the cell membranes.  If you breathe 100% 
oxygen, your arterial blood carries large amounts of O2 in both 
hemoglobin and plasma.  When it reaches the saturated tissues at the 
capillary level (where arterial blood gives off its oxygen and thereby 
becomes venous blood), the high osmotic pressure (lots of inert gas in 
the tissue and none in the blood) forces the N2 and He from your tissue 
in the bloodstream --> decompression.  At the same time the high osmotic 
pressure on the other side (lots of 02 in the blood, less in the tissue) 
forces O2 into the surrounding tissue, which has the effect of 
oxygenating tissue and reducing swelling and clotting -- essentially, 
healing the cellular damage caused by deco diving.  

The more oxygen in your blood, the larger the osmotic pressure 
differential or "window" forcing inert gas out of the surrounding tissue 
and therefore the more efficient (and faster) the deco. Now, at this 
point it sounds like F02 and not P02 is what matters since hemoglobin 
only binds a set number of 02 molecules, but here's another key: higher 
pressures force more oxygen to dissolve in plasma where it's carried 
without hemoglobin. So the higher the P02, the more oxygen is carried 
over all in the blood and therefore the greater the oxygen window where 
gas is exchanged at the tissue level.

Now you can easily see why 80/20 is a poor choice of deco gas compared 
with 100% 02 -- 80/20 reduces the oxygen window because the blood is not 
only carrying dissolved oxygen, it's also carrying nitrogren -- so 
there's lower osmotic pressure and therefore less inert gas moving from 
tissue to blood (and therefore slower, less efficient deco) and there's 
also less oxygen moving from blood to tissue (resulting in more residual 
cellular damage).  It's less efficient all around.

In putting this post together, I ran across a couple of websites that are 
worth bookmarking [Cobber -- add these to your links page]: 
http://www.oxytank.com/quanda.htm
is a terrific resource on hyperbaric oxygen therapy and physiology that 
clears up a lot of concepts and misconceptions about O2 and deco.  The 
other link is a hyperbaric medicine glossary -- 
http://www.gulftel.com/~scubadoc/glssry.html -- which is where I found 
the definition of oxygen window.

Best -

Bill Wolk





On8/29/00 1:29 AM, ScottBonis@ao*.co* wrote:

>And finally my basic question, the one I tried clumsily to ask previously, 
>remains exactly what is the relationship between this level of dissolved 
>oxygen in blood plasma (if that's what the "window" is), or the PP(O2) in 
>the 
>deco gas that gives rise to it (if that's what the "window" is), and the 
>rate 
>of off gassing of nitrogen.  I'm just having a lot of difficulty following 
>the concept.
>



Best regards --

Bill

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