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Bob,
Are you sure you're not an anesthesiologist? You
got the alveolar gas equation right, even though you left out the effects of
exhaled CO2 (it makes things worse.). I would like to see a source for the
overpressure limitation you cite.
Ted Noel MD
Diplomate, American Board of
Anesthesiology
N540TF
----- Original Message -----
Sent: Wednesday, September 13, 2006 5:33
PM
Subject: [LML] Re: High Altitude Oxygen
Failure
> 100% oxygen will delay the point at
which the dropping
> pressure results in hypoxia, but eventually you
WILL
> reach an altitude where the ambient pressure no longer
>
forces a gas exchange in the lungs. Not even 100% oxygen
> will help at
that point.
This is true, but the physics behind it has not been
explained properly. The oxygen exchange between the
blood and the
oxygen in the lungs does not require
an overpressure as previously stated.
Instead, the
problem is getting oxygen into the lungs at high
altitude.
Note that we all have about the same body temperature,
98.6 F or 36 C. At that temperature, the vapor pressure
of water is 47
Torr (47 mmHg = 1.85 inHg).
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html#c1
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html#c2
At
what altitude is the outside air pressure 47 Torr?
At that altitude,
expanding your lungs will cause
water from your lungs to evaporate and
completely fill
the lung volume. No air enters the trachea as a result
of lung expansion. That happens at an altitude of 18500 m
or 60,000
feet. At that altitude is is not possible to
get *any* oxygen into the
lungs by exercising the
diaphragm -- while the body is still at 37C, which
won't be long.
At 35,000', the outside air pressure is sufficient
to
push pure oxygen into the lung spaces -- just barely.
Pressure
breathing helps keep some oxygen in the
mixture as well as all that water
vapor. People can
tolerate an overpressure of only about 0.5 psi (1 inHg),
so a pressure mask can only help a little bit.
At 25,000', breathing
100% O2 at ambient pressure is
sufficient for healthy adults to maintain
blood saturation
in the 90+% range. Smokers may be completely hypoxic.
BTW, my vocal cords quit working at about 25,000'.
There isn't
enough air density for me to drive the
vibrations that make voice sounds.
At 25-30K, I can
croak a bit, but cannot make myself understood over
a
radio. I didn't notice this in the Miramar pressure
chamber, but found out
later over the Sierras in a
sailplane.
-bob
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