Hi, Thanks for asking. Based on your query, my opinion is as follows. 1. Her oxygen saturation does fall which rises the carbon dioxide. 2. It is dependent on her lung capacity and the exchange across the alveoli. If she requires more oxygen percentage, sometimes through ventilator to keep carbon dioxide down, it is possible, she has some lung problems. 3. X-ray or CT of lung would definitely show some abnormalities. 4. Breathing exercises with spirometry can be helpful. However, until lung disease is completely treated, I am afraid, CO2 level cannot be brought down easily. Hope it helps. Any further queries, happy to help again.