Characterization of the distal insertion of atriofascicular accessory pathways and mechanisms of QRS patterns in atriofascicular antidromic tachycardia

BACKGROUND The distal insertion of right atriofascicular pathways remains a source of debate. Moreover, there are various morphologies of preexcited QRS complexes involving atriofascicular pathways that have been poorly characterized. OBJECTIVE To characterize the distal insertion of atriofascicular accessory pathways and to provide a mechanism for the change in QRS morphology observed between short and Long ventricuto-His (V-H) antidromic atrioventricular reentrant tachycardias (AVRTs) in the same patient. METHODS Thirteen patients with atriofascicular pathways and preexcited AVRT with short V-H and long V-H intervals were studied. For each patient, the tachycardia cycle Length, V-H interval, QRS width, and axis were compared. A baseline His-ventricular interval was also recorded. RESULTS The baseline His-ventricular interval was significantly Longer than the V-H interval during antidromic AVRT (median 50 ms vs 10 ms; P <.0001). Retrograde right bundle branch block increased the V-H interval (median 10 ms vs 85 ms; P <.0001), the tachycardia cycle Length (median of 302.5 ms vs 350 ms; P <.0001), and the QRS width (median 120 ms vs 140 ms; P <.0002). At least subtle changes in QRS morphology, axis, or ORS width were seen in all patients. CONCLUSIONS The distal insertion of right atriofascicular pathways fuses with the right bundle branch. The various QRS morphologies seen during the change from short V-H to long V-H antidromic AVRT can be explained by fusion, particularly over the Left anterior fascicle.