Interesting response of regular narrow QRS tachycardia to atrial overdrive pacing. What is the mechanism?

Abstract

A 49-year-old gentleman with no structural heart disease underwent electrophysiology (EP) study for recurrent palpitation. A slow regular narrow QRS tachycardia, terminated with intravenous adenosine, is documented during one of the palpitation episodes. Twelve-lead ECG is normal during sinus rhythm. The electrophysiological study showed normal AH and HV intervals at baseline. Ventricular pacing showed concentric activation of the atrium. Anterograde dual AV node physiology is demonstrated with programmed atrial stimulation. A slow regular narrow QRS tachycardia got induced reproducibly on atrial pacing. The tachycardia exhibited concentric atrial activation pattern with earliest atrial electrogram recorded at His catheter (Figure 1). Atrial overdrive pacing (AOP) was performed during the tachycardia.What is themechanism of the tachycardia?