Multiple points of instruction discussed practically in the context of a challenging case make for a memorable learning experience. In this installment of Teaching Rounds in Cardiac Electrophysiology, Nayyar et al1 elucidate the unique findings and analysis in such a manner and take us through to diagnosis and a treatment plan that eliminated arrhythmia without having to ablate all of the abnormal structures identified in their patient. Their case in which 2 unusual arrhythmogenic substrates coexist and interact to result in what appears to be a random and inexplicable series of electrophysiological findings is broken down in logical steps defining each aspect of the arrhythmia.1 These steps include defining the antegrade and retrograde limbs of the putative tachycardia circuit, defining the mechanism of tachycardia, and coming to a conclusion keeping in mind certain difficult differential diagnostic situations and the importance of interpreting His bundle refractory extrastimuli maneuvers. See Article by Nayyar et al When anterograde conduction over the AV node and an accessory pathway is present, the primary maneuver to determine the mechanism of AV conduction is incremental atrial pacing or delivery of coupled atrial extrastimuli. With usual accessory pathway activation, the ventricular beats represent fusion between AV node and accessory pathway conduction, and premature atrial beats progressively delay AV nodal conduction with no significant effect on the pathway. As a result, the stimulus to earliest ventricular activation remains fixed, but the H–V interval becomes progressively negative and the QRS progressively more preexcited. Nayyar et al1 noted that while subtle but significant increase in preexcitation and a more negative H–V interval did occur with atrial extrastimuli, there was a clearly discernible increment in overall conduction time to the ventricle. This finding may be seen with atriofascicular or AV pathways with AV node-like characteristics (incorrectly referred to as …