A 6-hour-old term newborn with meconium aspiration syndrome is now on mechanical ventilation with FiO₂ 0.6 and mean airway pressure 12 cm H₂O. Arterial blood gas shows pH 7.28, PaCO₂ 52 mmHg, PaO₂ 65 mmHg, HCO₃⁻ 24 mEq/L. Chest X-ray shows bilateral patchy infiltrates without pneumothorax. The infant is on routine antibiotics and supportive care. What is the most appropriate next step in management?