Suggest Treatment For Thoracic Aneurysm

My Dad is 86 years old. He has had open heart surgery and has a thorasic anuryseum. His lungs have been slowly filling up for ayearnow and has had several test including 2 brochoyscopes. They can't find anything. He does not have viral or bacterial disease, there are not tumors or copd, he is not asperating food yet he continues to fill his lungs. He is currently on ogygen. He coughs when he breathes through his mouth only, not his nose. Please tell me what else I can do for him to stop the lungs from filling.