Figure 14.

Resetting with fusion as demonstrated by the local electrogram. The tracing shows three surface ECG leads and intracardiac recordings from His, high right atrium (HRA), and right ventricle (RV) during a regular accessory pathway-mediated supraventricular tachycardia. The fourth QRS complex is a left ventricular spontaneous extrasystole that results in resetting of the tachycardia since the interval between the two QRS encompassing the extrasystole (810 ms) is shorter than two tachycardia cycle lengths (850 ms). The fact that the RV electrogram (*) is not changed in timing or morphology demonstrates that there is ventricular fusion.