A 40-year-old male with rheumatic arthritis and pulmonary interstitial fibrosis is on maintenance steroid therapy. Over the past week, he developed a cough with slightly purulent sputum and low-grade fever. Two days ago, dyspnea progressively worsened, prompting medical evaluation. Examination reveals marked dyspnea, cyanosis, and widespread bilateral basilar rales. Heart rate is 110/min with regular rhythm. Arterial blood gas (ABG) on room air shows pH 7.48, PaCO2 30 mmHg, and PaO2 45 mmHg. Which mechanical ventilation mode is most appropriate?