Studies using patients presenting with shockable life-threatening arrhythmias (e.g. ventricular fibrillation) are difficult to perform for ethical and practical reasons. This study will therefore use patients requiring elective cardioversion for atrial arrhythmias, who are able to give consent and also present a more controlled population. Elective cardioversion is a standard treatment for patients with atrial arrhythmias and is performed as a day case procedure under general anaesthesia.