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Tachycardia atrial cycle length equal to that during sinus rhythm: What is the mechanism?

A 64-year-old woman with frequent palpitations presented for atrial fibrillation radiofrequency catheter ablation (RFCA). She had not been administered any antiarrhythmic medication before admission to the hospital. After RFCA, a routine electrophysiological study was performed. Three catheters were positioned: a quadripolar catheter at the high right atrium (HRA), a quadripolar catheter at the His bundle region, and a decapolar catheter in the coronary sinus (CS) (CS 7-8 at the CS ostium). The baseline sinus cycle length was 628 milliseconds (ms), the atrial-His (AH) interval was 87 ms, and the His-ventricular (HV) interval was 52 ms. Antegrade atrioventricular (AV) nodal conduction showed decremental and dual pathways during atrial extrastimulus testing, but no tachycardia was induced. Retrograde ventriculoatrial (VA) conduction showed VA disassociation. Figure 1 shows a narrow QRS complex tachycardia with AV 1:1 conduction that was induced by two spontaneous ventricular extrasystoles. The corresponding intracardiac recording showed that the atrial cycle length oscillated between 627?30 ms (recorded by the HRA catheter) and 626?33 ms (recorded by the CS 7-8 catheter), which was approximately the same as that during sinus rhythm (SR) (Figure 2). How was the tachycardia induced and what is the mechanism of the tachycardia? This article is protected by copyright. All rights reserved.