The V-A-V response to ventricular entrainment during atrial tachycardia: what is the mechanism?

A 46-year-old male was admitted to our emergency department due to a narrow QRS tachycardia of 230 bpm with impeding hemodynamic collapse. His only remarkable medical history was mild hypertension. Adenosine administration was ineffective in stopping the tachycardia, and sinus rhythm was restored using an electrical cardioversion. A few weeks later, during an electrophysiology study an identical tachycardia was immediately induced during the introduction of diagnostic catheters into the right atrium (Fig. 1, upper panel). A rate-related right bundle branch block aberration was present for most of the time. Atrial activation was simultaneous with ventricular activation, ruling out atrioventricular reentrant tachycardia (AVRT), and suggesting atrioventricular nodal reentrant tachycardia (AVNRT) (Fig. 1, lower panel). To confirm the absence of atrial tachycardia, ventricular entrainment pacing using a cycle length shorter by 30 milliseconds than the tachycardia cycle length was performed. The atrial rate accelerated to the pacing rate (Fig. 2), and when ventricular (V) pacing was ceased, the activation sequence was the last entrained atrium (A) followed by the ventricle (V) (i.e., a V-A-V response). Such a response is considered to rule out atrial tachycardia and these findings were repeatable. Therefore, by exclusion of all other possible causes a diagnosis of AVNRT could have been considered irrefutable. However, long VA interval during entrainment and the atrial activation sequence with atrial depolarization later on His catheter than on high right atrial catheter (Fig. 1, lower panel) were not compatible with slow-fast AVNRT, either. Moreover, multiple attempts to stop the tachycardia with fast ventricular and atrial burst pacing failed. This prompted a more detailed assessment of the atrial activation sequence and an analysis of entrainment from the right atrium. Those analyses both led to an unquestionable diagnosis of a macroreentrant right atrial tachycardia with a reentrant circuit identical to that observed in a typical clockwise atrial flutter, albeit slower, which resulted in a rate below the arbitrary thresh-