A 59-year-old man presents to the emergency department with 30 minutes of central crushing chest pain radiating to his left arm. He is clammy and nauseated. ECG shows 3 mm ST elevation in leads II, III and aVF. The catheter laboratory team have been alerted and are expected in 40 minutes. His BP is 135/80 mmHg and oxygen saturation is 96% on room air. Which of the following is the single most appropriate immediate treatment to give now?