Regular supraventricular tachycardia with irregular atrial activations.

A 19-year-old woman developed a recurrent supraventricular tachycardia (SVT) 3 months after catheter ablation of a typical slow-fast type atrioventricular nodal re-entrant tachycardia (AVNRT). During the procedure, the slow pathway was mapped and ablated anterior to the bottom of the coronary sinus (CS) ostium. She was referred to our institute and underwent electrophysiological testing a year after the first ablation. At baseline, the atrio-His (AH) and His-ventricular (HV) intervals were normal. Rapid ventricular pacing revealed irregular atrial activations with variable ventriculoatrial (VA) intervals and P wave morphologies (Supplementary material online, Figure-1). Atrial extrastimulation induced a regular SVT following a sudden prolongation of the AH interval. During the SVT, VA block occurred with irregular atrial activations (Figure 1). A premature atrial contraction (PAC) that occurred during the His-refractory period advanced the following His bundle (HB) activation during the SVT (Supplementary material online, Figure-2). A premature ventricular stimulus delivered during the His refractory period did not reset the following HB activation during the SVT. Rapid ventricular pacing could terminate the SVT.