When atrial fibrillation occurs in younger patients without clinically apparent cardiac disease, it is referred to as ālone atrial fibrillationā and is often paroxysmal, lasting for periods that may range from minutes to several days. This ECG is from a 32 year old female with frequent episodes of paroxysmal atrial fibrillation but no clinically apparent cardiac disease. The tracing was recorded during one such episode at which time she was receiving a beta adrenergic blocking agent to control the ventricular rate. In these patients, the arrhythmia is due to re-entry circuits in the left atrium that are most often at or near the orifice of one or more pulmonary veins.