A 58-year-old man with acute inferior wall myocardial infarction presents with symptomatic bradycardia (HR 38/min) and hypotension (BP 85/50 mmHg). Physical examination reveals cannon 'a' waves in the jugular venous pulse. A 12-lead ECG shows complete dissociation between atrial and ventricular activity with a ventricular rate of 38/min and atrial rate of 100/min. Which investigation is most appropriate to assess the site and nature of the conduction block?