A Hidden Recess of Atrial Tachycardia.

We present a case of regular narrow complex tachycardia in a 59-year-old woman with frequent paroxysmal palpitations, a normal electrocardiogram (ECG) in sinus rhythm, and a structurally normal heart. During electrophysiology study, a long R-P tachycardia was present at baseline, with P-waves superimposed on the T-waves and appearing to be positive in the inferior leads. Intracardiac recordings showed the atrial activation to be early in the para-Hisian region. The diagnosis of atrial tachycardia was confirmed by ventricular overdrive pacing, which showed ventriculoatrial dissociation without perturbing the atrial rate. The precise P-wave morphology was brought out in the pause, which followed rapidly delivered ventricular extrastimuli during tachycardia. Based on this information, activation mapping was conducted in the para-Hisian region, high atrial septal regions on the right and left sides, and aortic sinuses. Tachycardia was successfully ablated at one of these sites.