Atrioventricular block during atrioventricular nodal reentrant tachycardia is not always benign

A 70-year-old Japanese woman was admitted to our hospital because of transient second degree atrioventricular (AV) block. An electrophysiologic study (EPS) was performed, and Mobitz type II infra-Hisian block during atrial pacing at a rate of 130/min was noted. An AV nodal reentrant tachycardia (AVNRT) was induced by ventricular pacing at a rate of 180/min, and 2:1-3:1 infra-Hisian block during AVNRT was observed. The AV block and AVNRT rarely occurred in the clinical setting, and the patient did not complain of any symptoms related to these arrhythmias. Therefore, the patient refused permanent pacemaker implantation, although she continued to be followed in our outpatient clinic. However, the patient was re-admitted one year later because of palpitations and dyspnea upon exertion related to the AV block. The 12-lead ECG showed high degree AV block with narrow QRS complexes. The patient underwent pacemaker implantation during the subsequent hospitalization, and her symptoms improved postoperation. AV block during AVNRT is sometimes observed, and it has been considered as a functional AV block. In the present case, a pathologic conduction disturbance in the His-Purkinje system caused the high degree AV block during AVNRT. The high degree AV block during AVNRT may indicate the existence of a conduction disturbance in the His-Purkinje system in some of these types of cases.