The goal of this clinical trial is to learn if an epicardial radiofrequency left atrial posterior wall isolation (box lesion ablation) prevents atrial fibrillation after cardiac surgery. The main question it aims to answer is:

- Does box lesion ablation lower the number of patients experiencing de-novo atrial fibrillation requiring any treatment after cardiac surgery? Researchers will compare box lesion ablation to no ablation to see if ablation prevents postoperative atrial fibrillation.

Participants will undergo ablation or no ablation at the time of other cardiac surgery.

Data will be collected while in hospital. Additional information will be collected on 30th day after surgery by telephone call or during a visit.

Postoperative atrial fibrillation (POAF) is a common complication after heart surgery that occurs in 10-60% of patients. It can cause hypotension, tachycardia, stroke, systemic embolism, or heart failure, and is associated with increased mortality rates, prolonged hospital stays and increased costs. Several studies have investigated the use of radiofrequency ablation (the standard of surgical care for atrial fibrillation during concomitant cardiac surgery when the diagnosis is known preoperatively) to prevent POAF. However, these studies were limited mainly to pulmonary vein isolation and had methodological limitations such as small sample sizes, inclusion/exclusion criteria, and randomization procedures, and have not translated into clinical practice changes. The current study aims to investigate the use of radiofrequency left atrial posterior wall isolation, also known as box lesion ablation, as a more effective treatment compared to pulmonary vein isolation, in a properly designed randomized controlled trial.

Inclusion Criteria:

• All patients undergoing cardiac surgery by a single surgeon (V.K.)
• Informed consent

Exclusion Criteria:

• History of atrial fibrillation/flutter
• Minimally invasive approach
• Off-pump surgery
• Emergent/salvage surgery
• Severe pericardial adhesions