A 52-year-old man with community-acquired pneumonia is admitted to the ICU. On day 3 of hospitalization, despite appropriate antibiotics and oxygen therapy, his respiratory status deteriorates. He is intubated and placed on mechanical ventilation with FiO₂ 0.8 and PEEP 12 cm H₂O. Chest X-ray shows bilateral infiltrates. Arterial blood gas reveals PaO₂ 65 mmHg, PaCO₂ 48 mmHg, pH 7.28. Pulmonary artery occlusion pressure (PAOP) is 16 mmHg. What is the most appropriate next step in ventilator management?