Hi dk11, and thanks for joining in the conversation here. In the absence of a reply / update from , we can only 'guess' as to what may be going on with his dad and, of course, we are not going to speculate.
As for your father, I would suggest you (re)read my comment to . Do you think any of the 'possibilities' may apply to your dad? Remember, there are any number of other circumstances that can result in one's oxygen saturation level decreasing including the BiPAP therapy (settings, mask fit, proper use, etc.), and also, not including the BiPAP therapy (other medical conditions, changes in the present diagnosis, etc.) It may be best for you to bring this to the attention of your dad's physician and other medical staff (therapists, nurses). Sometimes the team, once aware, can perform another assessment to determine what, if anything, has changed to result in the decrease in oxygen saturation.
What do you think?
Please do check back and let us know how this turns out for your dad (and you!).
All the best,
Leon (site moderator COPD.net)