Atrioventricular nodal reentrant tachycardia with 2:1 block in pediatric patients

Background Episodic 2:1 block occurs in 9% of adults with atrioventricular nodal reentry tachycardia (AVNRT), but developmental differences in conduction physiology among children could influence this phenomenon. Objective This study sought to characterize the frequency and mechanism of 2:1 block during AVNRT in the pediatric population. Methods Records of 179 patients (mean age 13.5 ± 3.4 years) undergoing ablation for AVNRT were reviewed. Results Periods of 2:1 AVNRT were observed in 31 cases (17%). A His potential was visible on the blocked beats of 13, absent in 17, and undetermined in 1. Compared with 148 patients with exclusive 1:1 conduction, those with 2:1 AVNRT had: (1) longer baseline slow pathway refractory period (351 ± 71 msec vs. 278 ± 65 msec, P =.04), (2) shorter atrial cycle length during AVNRT (303 ± 54 msec vs. 333 ± 62 msec, P =.01), and (3) a higher incidence of bundle-branch aberration (35% vs. 18%, P =.03). Long-short oscillations in atrial cycle length were observed in 55% of patients during 2:1 AVNRT, but not during 1:1 AVNRT. Conclusion A pattern of 2:1 block occurs in 17% of pediatric patients with AVNRT undergoing ablation. Although this incidence is higher than in older patients, the mechanism appears identical. These data provide further evidence that functional block within or below the His bundle is the mechanism of 2:1 AVNRT, regardless of the presence of a His potential. Oscillations in atrial cycle length are common during 2:1 AVNRT in children and may contribute to the block pattern, but are not a requisite.