A 72-year-old woman with COPD (FEV₁ 35% predicted) and cor pulmonale is undergoing emergency repair of perforated peptic ulcer under general anesthesia. After induction with etomidate (0.2 mg/kg) and rocuronium, the patient is intubated with a 7.5 mm endotracheal tube. Immediately after starting controlled mechanical ventilation (TV 500 mL, RR 14, FiO₂ 0.5), SpO₂ drops from 95% to 78%, and PaCO₂ rises from 48 mmHg (preoperative) to 62 mmHg. Peak airway pressure is 35 cm H₂O. What is the most appropriate immediate intervention?