Hi. My father had open heart surgery in March. Up until then, he also had shortness of breath, low oxygen, and fatigue. For a month or so after the surgery, he felt better, but is now experiencing the same symptoms he had before the surgery. He had a nuclear stress test which showed no blockages and a CT scan of his lungs which showed nothing either. He has type 2 diabetes. Years ago he took medication for an under-active thyroid.