A 27-year-old woman with a history of bulimia nervosa comes to the clinic with a complaint of chest pain. The patient describes the pain as sharp, substernal, and occurring during an episode of vomiting.
On exam, the patient appears ill and diaphoretic. Vital signs show a blood pressure 135/85, pulse of 90, respiratory rate of 30, temperature of 98.6, and oxygen saturation of 98% on room air. The remainder of the physical exam is ordinary. Laboratory tests are normal.
Chest x-ray shows a small left pleural effusion.
What is the next best diagnostic step?