In this study, we will compare blood and tissue markers of oxidative stress between patients with and without atrial fibrillation who are scheduled to undergo cardiac surgery. It is the expectation that participants with atrial fibrillation will have more abnormal markers of atrial fibrillation. This study requires participants to be seen during their routine preoperative visit, undergo a history and physical examination, give blood only once, and allow use of their discarded LAA. This tissue is routinely removed at surgery because its removal is thought to reduce the risk of stroke in patients who develop atrial fibrillation after the surgery. This happens in up to 50% of patients, thus providing the desire to remove the LAA.