A 70-year-old man with known stable angina, usually well-controlled with bisoprolol and GTN spray, presents with a 2-hour history of severe central chest pain radiating to his left arm, associated with nausea and sweating. His pain has not resolved with 2 puffs of his GTN spray. An ECG shows ST elevation of 3mm in leads V2-V4. What is the most immediate and crucial management step in the pre-hospital or emergency department setting?