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Background-: The response to right ventricular (RV) entrainment is useful to distinguish atypical AV node reentrant tachycardia from AV reentrant tachycardia using a septal accessory pathway. Whether entrainment can differentiate between AV node reentrant tachycardia and AV reentrant tachycardia in patients with long-RP tachycardia has not been systematically validated.

Methods and Results-: Twenty-four patients with concealed septal accessory pathways who had an electrophysiology study between January 1, 2000, and January 1, 2010, were included (age, 38 /-17 years; men, 17). Entrainment was performed from the RV apex pacing at cycle length 20 to 40 ms shorter than tachycardia cycle length (TCL). The mean TCL was 390 /-80 ms, the mean AH interval during tachycardia was 151 /-57 ms, and the mean ventriculoatrial (VA) time was 182 /-103 ms. Twelve patients had typical accessory pathways (VA/TCL <40%), and 12 had slowly conducting accessory pathways (VA/TCL >=40%). In all patients with typical accessory pathways, the postpacing interval minus the TCL (PPI-TCL) was <115 ms and the difference in the VA interval during pacing and tachycardia (StimA-VA) was <85 ms. On the other hand, in 6 of the 12 patients in the slowly conducting group, the PPI-TCL was >115 ms, and the StimA-VA was >85 ms.

Conclusions-: Slowly conducting accessory pathways frequently yield RV entrainment criteria traditionally attributable to AV node reentry. Distinguishing AV node reentry from AV reentry in patients with long-RP tachycardia requires other criteria.

(C) 2011 American Heart Association, Inc.