A 62-year-old man collapses at work with severe central chest pain radiating to his left arm, associated with diaphoresis and nausea. Paramedics obtain a 12-lead ECG showing 3 mm ST-elevation in leads II, III and aVF. His BP is 138/82 mmHg, pulse 84 bpm and oxygen saturation 97% on room air. Which pre-hospital intervention is the single most important to give as soon as possible?