Over the past few years, research has focused on the prevention of atrial fibrillation (AF) after cardiac surgery, but highly effective interventions are still missing. Postoperative AF remains the most common complication after cardiac surgery, with an incidence of 10 to 50%. This complication is usually a transient condition that resolves spontaneously but it has major adverse consequences for patients and the health care system, including increased rates of death, complications (strokes), and hospitalisations with inflated costs. Recently, animal studies have demonstrated that neurotoxins such as botulinum toxin (BTX) injected into fat pads could suppress AF inducibility by parasympathetic activation. Botulinum toxin injection in fat pads has been studied in the dog's heart and could be associated with the reduction of atrial fibrillation in postoperative cardiac surgery. One pilot study has demonstrated the feasibility and safety of this technique in the human heart. The investigators hypothesize that botulinum toxin injection may substantially reduce postoperative AF during the first postoperative month after cardiac surgery without any serious adverse events. By the suppression of ganglionic plexi (GP) activity in the epicardial fat pads, mild term antiarrhythmic effects can be achieved with fewer antiarrhythmic drugs and anticoagulant treatment.