My father has congestive heart failure, pulmonary fibrosis, and pulmonary hypertension. He had a heart attack 3 weeks ago. He was improving but was without his Tyvaso medication almost a week. He s been back on it for 2 weeks and was improving again. And all of a sudden oxygen levels drop quickly and then come back up again with proper Oxygen. Then they lower oxygen again and he seems fine, then without notice hes sleeping and exhausted quickly. He s usually on a 3 at home but in hospital was up to 15, then last night was 13, this morning only needed a 5, then back up to a 12 quickly. chest xray fine.