A 58-year-old man presents to his GP with a 30-minute history of central crushing chest pain radiating to his left arm, associated with sweating and nausea. The pain started at rest and has not resolved with rest. He has a history of type 2 diabetes and hypertension. On examination he appears clammy, with a pulse of 96 bpm and blood pressure of 145/90 mmHg. ECG cannot be done immediately in the practice. What is the single most appropriate immediate management step?