A 58-year-old woman with a history of type 2 diabetes mellitus and hypertension is admitted to the ICU following a motor vehicle accident with multiple rib fractures and pulmonary contusion. On day 2 post-injury, she develops acute dyspnea and hypoxemia. Vital signs: BP 128/82 mmHg, HR 102/min, RR 32/min, SpO₂ 84% on FiO₂ 0.50. Bilateral infiltrates are noted on chest X-ray. ABG: pH 7.28, PaCO₂ 52 mmHg, HCO₃⁻ 20 mEq/L, PaO₂ 68 mmHg. She meets ARDS criteria. She is intubated and placed on lung-protective ventilation with tidal volume 6 mL/kg of predicted body weight (PBW) and PEEP 8 cm H₂O. Which of the following is the MOST appropriate next intervention if her oxygenation does not improve after 4 hours of lung-protective ventilation?