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<< Lancair Builders' Mail List >>
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It is interesting to see the discussions of oxygen at altitude. One recent
posting pointed out an issue I had not thought much about, but which as an
anesthesiologist I deal with every day.
Oxygen is transported in the lungs into the blood through a thin membrane.
This process depends on the matching of ventilation (breathing, symbol "V")
and perfusion (blood flow, symbol "Q"). Even in the very young and fit,
there is a small degree of V/Q mismatch. That is, in the upper part of your
lung, you have too much V and in the lower part too much Q. The result is
that a small amount of blood gets through the lung without picking up
oxygen. This is referred to as "shunting."
As we age, V/Q mismatch increases. There is nothing that can be done about
it. In a healthy person at age 20, the blood can be saturated 100% with
oxygen on sea level room air. Some people can maintain this up to age 40.
But beyond that, for every 10 years, you lose at least 1%. So at 70 years,
the very best sea level SaO2 (Oxygen saturation) possible is 97%.
As we climb to altitude, the absolute pressure of oxygen decreases. At the
same time, the absolute pressure of water vapor stays the same, and the
absolute pressure of carbon dioxide in the lungs goes down slightly as we
relatively hyperventilate to compensate. We sniff on oxygen to compensate.
But this cannot overcome shunting.
I won't go through the alveolar gas equation or shunt calculation, but put
simply, shunted blood passes through the lung without picking up oxygen. If
your shunt fraction is a bit higher than average (you smoke, or used to,
etc.) then more blood shunts. Because shunted blood bypasses the lung, even
breathing 100% oxygen on a pressure mask will not oxygenate the shunt. You
will be able to add some extra oxygen (about a 3% overall increase max) to
the V/Q matched areas, but there will be a decrement in SaO2.
The bottom line? An oximeter is an excellent idea. You may not need it more
than once, but fly with it at least once. Go to altitude and see what
happens. A 90% SaO2 is the normal level of ogygen in your veins, not your
arteries. As such, it should be an absolute lower limit of SaO2 to fly.
Below that you get in trouble FAST. (Again, I won't reproduce the graphs or
equations.)
If you find yourself in SaO2 danger, you may have to set a cabin pressure
altitude limit for yourself. Try increasing the oxygen flow, but also be
aware that it may not help. Also remember to calculate tank life at the
higher flows. And if you want to push the envelope, fly with an oximeter and
make regular checks. You may not recover your judgment in time otherwise.
Gaining altitude is optional. Losing it is not.
Ted Noel MD
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