A 28-year-old man is brought to the emergency department following a motor vehicle collision. He was the restrained driver. On arrival, his blood pressure is 110/72 mmHg, heart rate 102/min, respiratory rate 20/min. Abdominal examination reveals mild left upper quadrant tenderness without guarding or rebound. FAST examination shows free fluid in the left paracolic gutter and pelvis. Focused assessment with sonography for trauma (FAST) of the pericardium is negative. Chest X-ray is normal. What is the most appropriate next step in management?