Atrial fibrillation (AF) is the most commonly diagnosed cardiac rhythm disturbance and it is characterized by rapid, uncoordinated contractions of the upper chambers of the heart. Although AF occurs in a high percentage of patients who have mitral valve disease, it can also occur in individuals who have no associated cardiac abnormalities. During AF, atrial blood flow can become relatively stagnant, allowing clots to form that can cause strokes. In addition, the atrial arrhythmia can spread to the lower chambers of the heart, causing dangerous ventricular arrhythmias that result in loss of blood pressure, fainting, heart attack, or death. AF is usually classified as paroxysmal, classic, or persistent. Paroxysmal AF occurs intermittently and varies in frequency and duration from a few seconds to several hours or even days. In contrast, chronic AF is persistent and usually fails to respond to drug therapy or other nonpharmacologic treatments.