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Ian Crowe asks what effect atrial fibrillation would have on oxygenation.
First, I doubt that anyone with a-fib would be able to get a medical, so I
assume that this person is a passenger. Now the nitty gets gritty.
The issues of shunting and AMS are not affected by a-fib. But what happens
is a reduction in cardiac output (CO). The heart doesn't push as much blood
around with each beat because the atrium (the upper chamber) isn't pumping
blood into the ventricle (the lower chamber). In theory, this causes about a
15% drop in CO. Since passengers are not exerting, this is not a big deal
usually. Several factors come into play, however.
First, theory and fact don't always meet. In my years of doing cardiac
anesthesia, I have seen patients that drop their CO by 50% when they lose
their atrial kick by going into a-fib. Also, the quality of the ventricle is
in question. A failing ventricle will not pump as much blood. And a-fib is
an irregular rhythm, so that some beats do better than others.
Why worry about CO? Simple. The blood that comes to the lungs through the
veins needs oxygen. It gave some up in the tissues. If CO is lowered, more
oxygen is given up in the tissues, reducing mixed venous oxygen. This blood
now goes through the lungs and picks up oxygen. But some shunts, and that
shunted blood now has a larger effect on arterial oxygenation than venous
blood with a higher oxygen level. It doesn't take a lot to drop SaO2 a LOT.
This is just a glancing blow at some involved physiology. The equations are
only algebra, but the concepts are what is important. (It's that shunt
equation I mentioned 2 days ago.) Because predicting an individual person's
response is very difficult, do what I do in the operating room: measure
SaO2. Pulse oximetry is cheap insurance.
Ted Noel MD
____________
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--- Webmaster --- tednoel@cfl.rr.com ---
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