Atelectasis occurs when there is pressure on the small airways of the lung that causes them to close up. In obesity, the size of the abdomen can be cause of such atelectasis. It is, of course, more prominent when the person in lying down.
A PO2 of 52 is a significant problem. It is worth discussing wearing O2 24 hours a day. This should be discussed with your Pulmonologist.
Without having a complete set of information, including lab tests, chest x-ray and a physical examination, this is not a good question for me to answer. I would, however, suggest from the little information you have presented:
1. Make a significant effort at losing weight and beginning a sustained exercise program
2. Consider 24 hour a day oxygen therapy