**BoxX-NoAF Clinical Trial Summary** This study is about preventing a problem called *Post Operative Atrial Fibrillation (POAF)*, which can happen after heart surgery. POAF is when the heart beats fast and irregularly after surgery. It can lead to more serious health issues. This trial will test if doing *prophylactic ablation* (a procedure to stop bad electrical signals in the heart) and closing a part of the heart called the *Left Atrial Appendage* during regular heart surgery can lower the chance of getting POAF. * **Key Points:** - The trial involves procedures during planned heart surgery. - Participants must be at least 65 years old and meet certain health criteria. - People with a history of certain heart issues or other conditions are not eligible. Inclusion criteria mean you need to be having surgery that opens the heart area and be 65 or older with a specific health score. Exclusion criteria mean you cannot join if you've had atrial problems before, certain heart devices, or other serious health issues.