A 58-year-old man with no prior cardiac history presents with retrosternal chest pain starting at rest and lasting 30 minutes. The pain radiates to the left arm and is associated with diaphoresis and dyspnea. His blood pressure is 150/90 mm Hg, pulse 100/min, the heart sounds are normal, and the lungs are clear to auscultation. Which of the following is the next most appropriate investigation?