Narrow complex tachycardia with cycle length variability—what is the mechanism?

b f t s r o c l b T h t c p p t t r u Case presentation A 32-year-old woman with a history of recurrent palpitations and documented narrow-complex tachycardia was referred for electrophysiologic study. The HV interval during sinus rhythm was 37 ms. The following arrhythmia was initiated (Figure 1) with high right atrial burst pacing. Subsequently, a diagnostic maneuver was performed (Figure 2). What is the diagnosis?