A 58-year-old woman with Child-Pugh Class C cirrhosis is admitted with hematemesis. Upper endoscopy confirms variceal bleeding. After initial resuscitation and variceal ligation, she develops sudden-onset dyspnea, hypoxemia (SpO₂ 88%), and clear lung fields on auscultation. Chest X-ray is normal. What is the most appropriate investigation to confirm the suspected diagnosis?