Post Operative Atrial Fibrillation (POAF) is the most common complication of cardiac surgery. POAF incidence can exceed 50% depending on the patient baseline characteristics and surgery type. Patients with POAF tend to have worse acute and long-term clinical outcomes. BoxX-NoAF is a randomized trial to evaluate if prophylactic ablation and exclusion of the Left Atrial Appendage at the time of other routine cardiac surgery can reduce the incidence of post operative AF and clinical AF during long term follow up in patients who have not yet developed AF but are at risk.

Inclusion Criteria:

• Planned and clinically indicated for cardiac surgical procedure requiring cardiac bypass and opening of the pericardium
• Age ≥ 65 years and CHA2DS2-VASc ≥ 3

Exclusion Criteria:

• Documented history of atrial fibrillation or atrial flutter anytime prior to the cardiac surgery
• Prior procedure involving opening the pericardium or entering the pericardial space
• Patients undergoing off-pump surgery
• Prior LAA occlusion, exclusion, or removal (surgical or percutaneous)
• Presence of a permanent pacemaker
• Infiltrative cardiomyopathies (i.e. amyloidosis)
• Planned cardiac surgical procedure using non-sternotomy approaches
• Patients whose planned procedure is a heart transplant or implantation of any long-term ventricular assist devices
• Presence of ventricular arrhythmia
• Active endocarditis
• NYHA Class IV heart failure symptoms
• Preoperative need for an intra-aortic ballon pump or intravenous inotropes
• Active systemic infection at the time of cardiac surgery requiring antibiotics
• Known allergy to Nitinol or nickel sensitivity
• Known medical condition with expected survival of less than 1 year
• Other comorbidities that in the Investigator's opinion make the subject unsuitable candidate to complete the protocol required intervention or visits
• Current enrollment in an investigation or trial or an investigational devices or investigational drug that would interfere with this trial
• Mental impairment or other psychiatric conditions which may not allow the patient to understand the nature, significance, and scope of the trial.
• Pregnancy
• Known severe symptomatic carotid disease