Objectives. This study was performed to investigate the prevalence, mechanisms and clinical significance of supraventricular tachycardias inducible in children or adolescents after radiofrequency modification of slow pathway conduction for the treatment of atrioventricular (AV) node reentrant tachycardia. Background. Limited data have been reported with regard to the physiology of AV node reentrant tachycardia in young patients. Radiofrequency catheter ablation allows evaluation of the effects of selective modification of the different pathways involved in AV node reentrant tachycardia. Methods. Selective modification of slow pathway conduction was performed in 18 young patients (12.9 ± 3.4 years old) with typical (anterograde slow-retrograde fast) AV node reentrant tachycardia. Radiofrequency energy was applied across the posteromedial or midseptal tricuspid annulus, guided by slow pathway potentials and anatomic position. Programmed stimulation was performed after modification of slow pathway conduction defined as noninducibility of typical AV node reentrant tachycardia. Results. Modification of slow pathway conduction was achieved in each patient, with a median of four applications of radiofrequency energy. However, atypical forms of supraventricular tachycardia were inducible in 9 of 18 young patients after slow pathway modification: AV node reentrant tachycardia with 2 to 1 AV block (seven patients; anterograde fast-retrograde slow AV node reentrant tachycardia (five patients); and sustained accelerated junctional tachycardia (two patients). In comparison, atypical forms of tachycardia were inducible in only 2 of 59 adult patients with AV node reentrant tachycardia undergoing slow pathway modification in the same laboratory (p = 0.01). Additional applications of radiofrequency energy to the posteromedial tricuspid annulus rendered AV node reentrant tachycardia with 2 to 1 block and the fast-slow form of AV node reentrant tachycardia noninducible. Junctional tachycardia terminated spontaneously in both patients. During 9.8 ± 3 months of follow-up, slow-fast AV node reentrant tachycardia has recurred in one patient, whereas fast-slow AV node reentrant tachycardia has occurred in two patients, both with inducible fast-slow tachycardia after the initial modification of slow pathway conduction. Conclusions. Initial applications of radiofrequency energy may selectively modify the anterograde conduction of slow pathway fibers in young patients with AV node reentrant tachycardia. This may result in AV node reentrant tachycardia with 2 to 1 AV block or a reversal of the reentrant circuit (fast-slow tachycardia). Induction of these tachyarrhythmias indicates that further applications of radiofrequency energy are required for the successful modification of slow pathway conduction in young patients. The increased prevalence of inducible atypical arrhythmias among young patients suggests differences in the anatomic or electrophysiologic substrate of AV node reentrant tachycardia that may evolve as a function of age.
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