"Craig Prouse" said

Cannulas are not effective for delivery above 18,000. I've flown a couple
times at 17,500, and I had to open up the needle valve wide open in order
to
stay alert.

According to the Jepp instrument manual, "If you use a cannula, you should
be aware that the FAA restricts its use to a maximum altitude of 18,000
feet. Above this altitude, you must use an oronasal mask that provides an
adequate seal to your face."

I couldn't find this in the FAR's - 91.211 doesn't mention cannulas. Aside
from the good sensibility of the idea, does anybody know if this is in the
regulations?

-Scott