If the ambient light is too bright the pulse ox will not function since it
measures the pulsatile light at certain wavelengths against baseline or
background light. If the difference between these is not fairly large the
instrument will not be able to determine pulsatility and will thereby not
work.

In the OR we simply place a dark towel or some other "shade" over it. The
same thing can be done in the glider cockpit. I suppose that if the readout
is on the actual instrument itself (finger) that it would be a bit tough
seeing it if covered. Since all pulse oximeters average several cycles it
may be possible to cover it then pull it out from under the shade and still
see a reading before it shuts down.

Also realize that if your hands are cold or you are nervous or perhaps just
downed an extra cup of coffee just before you took off, that there may be
enough vasoconstriction in your extremities that the instrument may have
trouble getting a reading for that reason too.

Casey Lenox
KC
Phoenix