Initiation and termination of an atrioventricular reentrant tachycardia captured during invasive electrophysiology studies (EPS)

During atrial pacing (S1), a premature atrial beat (S2) is blocked antegradely in the accessory pathway and conducts through the atrioventricular node with a long delay (A2H2 interval), which allows for the recovery of the accessory pathway before retrograde conduction (panel A). The mechanism is an orthodromic atrioventricular reentrant tachycardia (AVRT) using a left-sided pathway, suggested by the atrial activation sequence; earliest retrograde atrial activation during the first echo beat of AVRT (Ae) is in the mid coronary sinus (CSm). A single ventricular premature beat (VPB) terminates the AVRT (panel B). The VPB conducts to the left atrium but encounters the refractory period of the AV node and is blocked, interrupting conduction in the antegrade limb and terminating the arrhythmia.