Atrial fibrillation is an abnormal heart beat that starts in the upper parts of the heart and can cause stroke or death, if untreated. In general, treatments are not very effective, with frequent relapses of the abnormal heart beats. One explanation for the high relapse rate is that the treatments might not address the underlying cause of atrial fibrillation. Recently, the investigators have found that atrial fibrillation is associated with increased oxidative stress. This is a condition where abnormal oxygen forms are produced. These forms harm the cells of the heart, causing them to beat abnormally. The investigators have found increased oxidative stress in pig and mouse models of atrial fibrillation. They would like to see if oxidative stress is present in humans with atrial fibrillation. In this study, they will compare blood markers of oxidative stress between patients with and without atrial fibrillation. It is the expectation that participants with atrial fibrillation will have more abnormal blood markers of atrial fibrillation. This study requires participants to visit their doctors, undergo a history and physical examination, and give blood only once.