Intermittent orthodromic capture of the earliest activation site during atrial pacing in a case with reentrant atrial tachycardia originating from the atrioventricular node vicinity.

Constant pacing (S1-S1: 520 ms) from the right atrial appendage (RAA) during reentrant atrial tachycardia originating from the atrioventricular node vicinity. RA-FW 1-2 was constantly captured antidromically. However, the earliest atrial activation site (EAAS = His1-2) was orthodromically (red circle) and antidromically (blue square) captured at a ratio of 3:1. The electrogram morphology at the EAAS was different between orthodromically and antidromically electrograms captured during pacing. Moreover, the interval between the S1 and orthodromically captured electrogram was gradually prolonged.