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Figure 4 Three surface electrocardiographic, one left atrial (LA) and two right atrial (RA) intracardiac recordings are shown during atrial fibrillation, which was induced after complete interatrial conduction block by left atrial appendage burst pacing. The right atrium is in sinus rhythm. The surface electrocardiogram manifests a "hybrid" morphology composed of both regular (arrow) and fibrillatory activity.