I"m an RT. I usually just leave in the cannula. I always try to run my nebs off RA for every patient, just because I"m too lazy to start another O2 hookup.
For COPDers, (or almost any patient for that matter) you are NOT going to over oxygenate them with a 6 or 8 liter nebullizer running plus a 2-4 liter cannula. Please, please do not worry about "overoxygenating" a COPDer! In all the repiratory therapists I've ever talked to about this issue, only 1 has actually seen it happen - it is so, so so extremely rare. And it happens *very* quiickly. Like, you turn up the O2 and less than a minute later the patient starts to get sleepy and is less responsive. Then you just turn down the O2 and watch them perk right back up again!! It's not as scary as it sounds. I have end stage COPDers on 100% O2 all the time. Never, ever withold oxygen from a patient that needs it!
This is sometimes a big problem at my hospital. I know you nurses mean well, but some of you focus way to much on this "over oxygenating" theory (and it IS just a theory, remember!). I've gone into patients' rooms where they're wearing a non-rebreather at 4LPM. That's doing WAY more harm than good - the patient is slowly suffocating! Give some o2!
I've drawn gases on COPDers where they're PaO2 is like 300 or even higher and they've all turned out just fine.
(usually from being on bipap or vent, rarely NRB's just for the record).