You are seeing a 25 year old male in A&E who has presented with palpitations, dizziness and chest discomfort. He reports that his symptoms are not severe but he is worried about what is causing them. You perform an ECG as part of your assessment and you note that the rhythm is of a supraventricular tachycardia. You have asked the patient to blow through a syringe and this method has failed to convert the rhythm back to sinus rhythm. What is the next most appropriate step in treatment for this condition?