A 65-year-old man presents to the emergency department with 1 hour of central crushing chest pain radiating to his left arm, associated with diaphoresis and nausea. His ECG shows 1 mm horizontal ST depression in leads V4-V6 and his initial high-sensitivity troponin I is elevated. He is haemodynamically stable and has no contraindications to antiplatelet therapy. Paramedics have not given him any medications. What is the single most important immediate pharmacological intervention?