Unusual response to a ventricular extrastimulus during a narrow QRS tachycardia: What is the mechanism?

Abstract

F IGURE 1 (A) Ventricular extrastimulus (St.) from the right ventricular apex (RVA) during the tachycardia. Abbreviations: A, atrium; CS, coronary sinus; H, His; HBE, His bundle electrogram; HRA, high right atrium tachycardia with the earliest atrial activation site of coronary sinus ostium (CSos). During the tachycardia, atrio-His andHis-atrio intervals were 138 and 180 ms, respectively. A single ventricular extrastimulus applied from the right ventricular apex (RVA) during the tachycardia is shown in Figure 1. This finding appeared with reproducibility. What is themechanism of this tachycardia?