A 64-year-old man presents to the emergency department with a 45-minute history of central crushing chest pain radiating to his left arm. He is clammy and nauseated. His pulse is 96 beats/min, blood pressure 140/82 mmHg, respiratory rate 20/min, and oxygen saturation 96% on air. His ECG shows 3 mm ST-segment elevation in leads V2–V5. He has no known past medical history and is not taking any regular medication. What is the single most appropriate immediate management step?