An unusual long RP tachycardia: What is the mechanism?

Abstract

A 43-year-old woman with frequent palpitations since childhood underwent diagnostic electrophysiologic evaluation. She had failed treatment with both metoprolol and flecainide. Transthoracic echocardiography was normal, and her 12-lead electrocardiogram showed no ventricular preexcitation. At baseline, atrio-His (AH) and His-ventricular intervals measured 103 and 54 ms, respectively, dual atrioventricular (AV) node physiology was not observed, ventriculoatrial (VA) conduction was intermittent at a pacing cycle length (PCL) of 600 ms, and nonsustained runs of tachycardia occurred spontaneously and became incessant on administration of isoproterenol (0.5 G/min) (Figure 1, upper panel). Tachycardia terminated fter administration of 3 mg of adenosine (Figure 1, ower panel). Figure 2 shows a ventricular premature epolarization (VPD) delivered during tachycardia when he His bundle is refractory (upper panel) and an attempt t entrainment of tachycardia from the ventricle (lower anel). Figure 3 shows successful entrainment of tachyardia from both the ventricle (upper panel) and the trium (lower panel) at a cycle length 10 ms shorter than tachycardia. Based upon these findings, what is the mechanism of tachycardia?