I was looking up C02 retention and 02 use at home.  Yes, I have been googling again.  But on this site I found wonderful, easy to understand explanations.  I'm having a specific sleep study being done in Jan. to see if I'm crossing over with dx's.  (which I guess is very common with COPD)  My terminology may be a bit off.  Simply put my specialist wants to measure my breathing, and to see if my CO2 is increasing with sleep, and if  during sleep I stop breathing for short periods of time.  (apnea)   

So question for the group who uses C-pap, is this a form of 02 delivery not using your 02 tanks ?  I'm sure they'll explain it at my next appointment with my Dr. for test results.  Just like to be a bit prepared.  Thanks !