First off, good luck with the surgery
Second, i dont think you really have anything to worry about doing, it is pretty much out of your hands, and so you know, ALOT of people who wake up from surgery have to be on oxygen, evan if they dont have asthma
and sometimes asthmatics get an attck from the anxiety of it all, weather your in pain or anything like that
This time just about the only thing i can suggest is what my asthma specialist recommended for me to do, which i did, and that was get a 'albuterol'? treatment, where they put the oxygen mask over your face with the steam and medicine in it for a few minuites.
say that youve been havin a tough time with your asthma lately, and that youd like one of those, right BEFORE surgery, and explain what happened last time, i had this, and had NO problems after my surgery last time
good luck, im sure everything will turn out great, but let us know how it goes