A 68-year-old woman is admitted to the ICU following an emergency laparotomy for perforated peptic ulcer. On postoperative day 2, she becomes disoriented and agitated. Her family reports she was alert and oriented that morning. She is now unable to focus attention, repeatedly asking the same questions. Her oxygen saturation is 88% on room air, BP is 110/70 mmHg, HR is 115/min, and temperature is 37.2°C. Arterial blood gas shows PaO₂ 65 mmHg. She has no prior history of cognitive decline. What is the primary mechanism underlying her acute mental status change?