Why Do My Sister’s Oxygen Levels Drop When Sedation Is Reduced After Brain Aneurysm Surgery?

My sister has suffered a brain aneurysm and is currently on a ventilator and sedation alongside a paralysing agent. They clamped the bleed and her follow up ct scan was clear however each time they reduce the sedation her oxygen levels decline and they have to stop. Do you have any kind of idea why this is happening?