Recently, we have found that atrial fibrillation is associated with increased oxidative stress in a particular part of the top parts of the heart, the left atrial appendage (LAA). Oxidative stress is a condition where abnormal oxygen forms are produced. These forms harm the cells of the heart, causing them to beat abnormally. Also, the inside of the heart becomes sticky and more likely to form blood clots. These clots, when they travel to the head, are thought to be the main cause of stroke in this condition. We have found increased oxidative stress and increased evidence of blood clotting in pig and mouse models of atrial fibrillation. We would like to see if these same findings are present in the human LAA.