The Oxygen Window

Posted to SCUBA-UK by John Brett

Date: Tue, 23 Nov 1999 03:53:47 -0500
From: John Brett 
To: scuba-uk@drogon.net
Subject: RE: Deco gas choices


>
> Could you explain this "Oxygen Window".

I'll try, but I provided the references because they describe it better
than I could.

> Metabolism can change venous pressure by 0.04 bar or so. This
> won't vary much.


(with apologies for stating the obvious)

The blood supply from the lungs to the tissues carries O2 by two
mechanisms: Haemoglobin (Hb) and solvation (i.e. O2 dissolved in
blood plasma).
The blood supply from the tissues to the lungs carries CO2 by two
mechanisms: bicarbonate production and solvation.

When breathing 21% at 1ATM (i.e. air), the blood carries very little
O2 in solution (), but the Hb is 97-98% saturated. At the tissue, the
Hb releases its O2 readily, supplying the tissue's requirements for O2.

The tissues return CO2 to the bloodstream. The bicarbonate buffering system
carries the majority of the generated CO2 (IIRC ~38mEq/L vs 1.2mEq/L
dissolved).
See http://www.mtsinai.org/pulmonary/papers/eq/eq1.html

In this case, the ppO2 and ppCO2 of the blood changes little.

When breathing a higher pressure of O2 (e.g. 100% @ 6m), the quantity of
dissolved O2 goes up, whereas the Hb transport system, which was already
near
capacity, takes very little extra. Consequently, the ratio shifts in
favour of dissolved O2. The result is that arterial ppO2 is elevated. Since
dissolved O2 is easier for the tissues to extract, the Hb contributes little
to metabolism*. The venous ppO2, therefore, drops by whatever the tissues
extract. The generated CO2 continues to generate bicarbonate in preference
to dissolving, however, so the ppCO2 in the venous blood does not increase
significantly.

Put the two together, and there is an overall drop in total gas tension in
the blood. This is the oxygen window, and causes an increase in the rate of
offgassing of any gasses dissolved in the tissues (sort of vacuum-cleaner
effect).
See http://www2.gol.com/users/emaiken/Dive/Bubble_Decompression_Strategies.htm

and http://www.aquanaut.com/bin/mlist/aquanaut/techdiver/display?29397,subject
and http://www.aquanaut.com/bin/mlist/aquanaut/techdiver/display?8282,subject

The magnitude of the pressure drop is highly dependent upon the inspired
ppO2, but measuring it seems to be somewhat problematic. It is insignificant
at ppO2s < 1.0. Empirically, it has a substantial effect on decompression
by 1.6 ATA ppO2.


* Incidentally, this is why CO poisoning during a dive often doesn't
manifest until the _ascent_ phase - at depth, dissolved O2 supplies
sufficient O2 to the tissues, and the compromised Hb-transport mechanism
is not noticed.

HTH,
John Brett