A 52-year-old man with a history of hypertension presents with palpitations and syncope. Physical examination reveals an irregular pulse. A 12-lead ECG shows a wide complex tachycardia with a rate of 180 bpm. The patient is haemodynamically stable. Which investigation is most appropriate to differentiate between ventricular tachycardia and supraventricular tachycardia with aberrant conduction?