Everyone naturally breathes through their abdomens when lying down so it takes no effort. You have to train yourself to breathe that way when your body is upright
no problem. I learned that in choir because the teacher was trying to teach us to breathe through our diaphragms. She made us all lay down on the floor and pointed out that our stomachs were going up and down. It was pretty cool.
perhaps you should be asking your doctor that question!