Atrial fibrillation is a type of irregular heartbeat that is common after having heart surgery. There may be many different reasons why some people get atrial fibrillation after their heart surgery. These reasons may include that a person is older or that he/she is taking certain types of medications before surgery. Genes may also be a reason. Genes contain the material passed from parent to child that determines the make-up of the body and mind. For example, some genes control the color of your hair or eyes. Some people may have genes that make it more likely for them to get atrial fibrillation after their heart surgery. This research project is being done to find out if the cells of people who develop atrial fibrillation after heart surgery are different from the cells in people who do not develop atrial fibrillation after surgery.