Atrial flutter and atrial fibrillation are reentrant arrhythmias that occur most frequently (but not exclusively) in patients with underlying heart disease. As discussed earlier (see pages 7.1.28 and 7.1.29), atrial flutter (the upper ECG strip) is most often due to a large re-entry circuit in the right atrium while atrial fibrillation (the lower strip) is believed to result from multiple small re-entry circuits that most frequently originate in the left atrium.