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Identical atrio-His interval and A-A intervals during long RP tachycardia: what is the mechanism? Diagnosis: Atypical AVNRT.

A 46-year-old woman with frequent palpitations underwent electrophysiological study. The findings of transthoracic echocardiography were normal, and a 12-lead ECG showed no ventricular pre-excitation. Four catheters were placed in standard locations: high right atrium, His bundle, coronary sinus (CS), and right ventricular apex. All the recorded baseline intervals were within normal limits. Retrograde ventriculoatrial (VA) conduction showed decremental and dual pathways during ventricular pacing and single extrastimulus testing. Antegrade atrioventricular (AV) nodal conduction showed decremental and triple pathways during atrial single extrastimulus testing. Narrow-complex supraventricular tachycardia (SVT) was reliably induced with a V–A–V sequence by a single ventricular extrastimulus (Figure 1). Ventricular premature depolarization (VPD) during tachycardia, when the His bundle was refractory, did not reset the atrial cycle. Retrograde Wenckebach periodicity occurred during right ventricular overdrive pacing at a cycle length (CL) of 500 ms. The tachycardia was terminated with atrio–His (A–H) and VA block during atrial and ventricular entrainment pacing. Figure 2 shows progressive prolongation and abrupt shortening of the A–A interval with block during SVT. In addition, the A–H interval was identical to the A–A interval during SVT. On the basis of these findings, what is the mechanism of the tachycardia?



Figure 1. 
Intracardiac electrogram showing that narrow-complex supraventricular tachycardia was induced with a V–A–V sequence by a single ventricular extrastimulus (600/370 ms). CS indicates coronary sinus; HBE, His bundle electrogram; HRA, high right atrium; and RVA, right ventricular apex.





Figure 2. 
Intracardiac electrogram showing progressive prolongation and abrupt shortening …