Narrow QRS complex tachycardia with a 2:1 atrioventricular block: What is the mechanism

Abstract

A 65-year-old female patient presented with recurrent episodes of symptomatic tachycardia. A baseline electrocardiogram did not show pre-excitation. The electrocardiogram of the event recorder during tachycardia exhibited a narrowQRScomplex. Since the patient’s tachycardia was resistant to medication, an electrophysiological study was performed after obtaining written informed consent. Tachycardia was reproducibly induced by atrial overdrive pacing after atrioventricular (AV) block (Figure 1). The intracardiac electrocardiogramduring tachycardia showed that a para-Hisian region was the earliest atrial activation sitewith a 2:1 AVblock. AHis bundle potential was not recognized at the time of AV block. Rapid intravenous injections of 4mg adenosine triphosphate (ATP) reproducibly terminated the tachycardia (Figure 2). Based on these findings, what was the mechanism of the tachycardia, and what should we do as the additional diagnostic pacingmaneuver?