A Narrow QRS Tachycardia With Alternating R-R Interval.

We report a case of a 21-year-old female manifesting narrow QRS complex tachycardia and alternating R-R intervals. The fixed RP interval suggested ventriculoatrial conduction via an accessory pathway (AP). The alternating PR intervals indicated anterograde conduction through the fast and slow nodal pathways, respectively. The coexistence of AP and dual atrioventricular nodal pathways is not rare, with most of them exhibiting as atrioventricular reentry tachycardia (AVRT) using the fast or slow nodal pathway exclusively as the anterograde limb. We propose that alternating dual nodal pathway AVRT may occur when the fast pathway's effective refractory period is between the cycle lengths of fast-nodal-pathway AVRT and slow-nodal-pathway AVRT.