LAAOS pilot study will recruit 50 participants undergoing open-heart surgery in 5 Canadian academic cardiac surgery centers. The study will inform the feasibility of conducting a large RCT and will provide information of the efficacy and safety of appendage occlusion technique. From this study we envision a a large international randomized controlled trial (LAAOS II) to determine the impact of left atrial appendage occlusion on the composite outcome of stroke and non-central nervous system systemic embolic events in at risk patients with atrial fibrillation undergoing cardiac surgery.