Message-ID: <004501c1b36f$79916cc0$6401a8c0@cfl.rr.com>
From: "Ted Noel" <tednoel@cfl.rr.com>
To: <lancair.list@olsusa.com>
Subject: levels of oxygen
Date: Mon, 11 Feb 2002 21:46:30 -0500
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Gearge Braly points out that many people fly safely at SaO2's of 87%. While
this may be true, we must ask whether it is desirable.

On a physiologic basis, there is reason to be concerned. First, it places
you at a position where you must increase cardiac output to avoid reducing
mixed venous oxygen so low that shunting overtakes oxygenation. Most of us
can do that, but it is one part of our reserves used up.

Second, this is where the "knee" in the hemoglobin/oxygen dissociation curve
is. It is a much larger distance to go from 95% to 90% SaO2 than it is to go
from 92% to 87%. And that is a much larger distance than going from 90% to
85%, etc. This is counter-intuitive, but true. Essentially, once you reach
the knee of the curve, your margin of safety is gone. You may be OK where
you are, but it doesn't take much to make things go bad.

To fly at an SaO2 of 78%-83% is IMHO foolhardy. The issue is not cardiac
arrest, it is intellectual function. The brain is able to autoregulate its
flow to match oxygen demand, but if there isn't enough oxygen there, the
brain can't function well. And at that level of SaO2, it begins to be more
difficult to move oxygen off of the hemoglobin molecule to the tissues. Yes,
it does work, but the margin of safety simply is not there.

No, I haven't flown at a cabin pressure altitude over 10k. But that doesn't
alter the physiology. You are welcome to fly at whatever SaO2 suits you. But
I won't recommend anything below 90%.

Ted Noel
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