A 52-year-old male with hypertension and type 2 diabetes undergoes elective laparoscopic cholecystectomy under general anesthesia. Thirty minutes into the procedure, the anesthesiologist notices a sudden drop in SpO₂ from 98% to 88% and systolic blood pressure from 120 mmHg to 85 mmHg. Breath sounds are bilaterally present and equal. The capnograph shows a normal waveform with adequate CO₂ elimination. Which is the most appropriate next investigation to differentiate the cause of hypoxia in this setting?