When to call an ambulance....

I am a caregiver for my mother and I am having difficulty defining at what level her condition necessitates bringing her to the hospital. She has severe COPD for a year now and until recently we had a decent system. She would use the home oxygen (2 liters) every few hours for awhile and that was good enough. Her oxygen levels eventually would drift down to 84 and back on the oxygen she went until again it leveled at 90 or so. Ultimately a few hours later it again slowly drifited down. Very recently however she started to dip into the mid 70s soon after taking the oxyygen off. This is alarming me. I never saw her in the 70s before. I hope this is a temporary exacperation and she will baseline higher again soon. She is sleeping most of the day and always tired but otherwise not in distress. The problem is that she keeps taking the oxygen off when I am not watching her. The doctor told me to take her to the hospital if she went down to the 70s but I dont know what they can do that I am not doing at home. She is begging me not to. She likely needs oxygen 24/7 now but will never comply with that . I am at the end of my rope on how to help her.