Atrial fibrillation is the most common heart rhythm disorder. For patients suffering atrial fibrillation direct current cardioversion is performed to reduce patients symptoms and prevent disease progression. The optimal energy selection for biphasic cardioversion is unknown.

We aim to investigate the efficiency and safety of a high energy shock protocol (360 J) versus a standard escalating shock protocol (125-150-200 J) in cardioversion of atrial fibrillation.

The optimal energy selection for biphasic direct current (DC) cardioversion of atrial fibrillation is unknown. The energy delivered should be sufficient to achieve prompt cardioversion but without the risk of inducing any potential injury e.g. skin burns, myocardial stunning or post-cardioversion arrhythmias. The use of an escalating protocol, with a low energy initial shock, has been considered conventional practice, originally to avoid post cardioversion arrhythmias when using monophasic shocks.(1) This practice has been directly transferred to biphasic cardioversion. The European Society of Cardiology 2016 guidelines (2) and the American Heart Association/American College of Cardiology 2014 guidelines on the management of atrial fibrillation (3) do not recommend any specific energy settings, whereas the European Resuscitation Council 2010 guidelines for cardiopulmonary resuscitation (4) recommend a starting energy level of 120-200 J with subsequent escalating energy setting.

Previously, a non-escalating protocol (200 J) (5) has been found to have a significantly higher first shock success resulting in fewer shock deliveries without compromising safety compared with a low energy escalating shock protocol (100-150-200 J). Further, a study found fewer arrhythmic complications with increasing energy suggesting an 'upper limit of vulnerability'. It is well-established that biphasic shocks induce fewer post-shock arrhythmias (6), skin burns (7) and shorter periods of myocardial stunning compared with monophasic shocks.(8) Importantly, no correlation between increasing biphasic energy delivery and any complications was found in these studies. Nonetheless, the efficiency and safety of a high energy shock (360 J) biphasic protocol compared with a conventional low energy escalating protocol is unknown. Accordingly, this study aims to compare the efficiency and safety of a high energy protocol (360-360-360 J) versus a standard escalating protocol (125-150-200 J). We hypothesise that a high energy cardioversion protocol is more effective compared to standard escalating energy protocol, without compromising safety.

Inclusion Criteria:

• >18 years of age, scheduled for cardioversion of atrial fibrillation. Patients with atrial fibrillation for ≤48 hours may be cardioverted immediately. Patients with atrial fibrillation for >48 hours will be required to have a documented weekly international normalized ratio (INR) ≥2.0 (including within 48 hours of cardioversion) or treatment with non-vitamin K oral anticoagulant for three weeks or longer. Alternatively, a transoesophageal echocardiogram documenting absence of intracardiac thrombi is accepted and cardioversion can be performed on treatment with low molecular weight heparin.

Exclusion Criteria:

• Pregnancy, haemodynamically unstable atrial fibrillation, other arrhythmias than atrial fibrillation, untreated hyperthyroidism