Low 02 at night

My 35 year old daughter has mild obstructive sleep apnea. It is improved with cpap to under 5 AHI but her oxygen level at night has not improve. It is under 89% most of the night with a low of about 80%. She had a stress test with echo and a ct scan of her lungs. Everything seems normal. She will be going on oxygen at night. Any ideas what other tests might need to be run? Suggestions? She does not smoke .No smokers in the house. My mother and sister had pulmonary fibrosis but there is no evidence of this on ct scan. Trying not to get too worried here. My husband has central and obstru tive sleep apnea, AHI 93 before bipap treatment. His 02 levels are good at night.

What concerns me about the facts you posted is that AHI has improved to under 5. I would have expected something lower at that age. If the CPAP is set up properly and you are not achieving under 1 for AHI, that suggests central apnea may be a problem. Central apnea is not improved with CPAP and may even be aggravated. A BiPap may help, but in most cases it does not. There may be a need for an ASV machine. You may want to discuss the use of an ResMed AirCurve ASV with the sleep doctor. Oxygen can be a solution, but an ASV may be less invasive. Just my opinion as a central apnea sufferer. I am not a medical professional.