BACKGROUND: Anatomic characterization of unidirectional accessory pathways (APs) is inherently limited to the localization of the downstream insertion site. The inability to define the full anatomic course of unidirectional pathways can limit the safety and effectiveness of ablation in the setting of complex pathways, slanted pathways, unstable catheter positioning at downstream insertions, or insertions near the conduction system. OBJECTIVE: We aimed to develop novel pacing maneuvers to localize upstream insertions of unidirectional APs. METHODS: Two methods were evaluated: localizing the shortest transit time from roving pacing sites to a fixed reference in the opposite chamber (upstream transit mapping)
and identifying the site at which the latest atrial or ventricular extrastimulus reset atrioventricular reciprocating tachycardia (late reset mapping). Unidirectional APs were included to test utility and feasibility of the techniques, and bidirectional APs were included to test anatomic accuracy. RESULTS: A total of 13 patients were included, 8 unidirectional APs and 5 bidirectional APs. Blind side mapping was successfully performed in all cases and showed excellent spatial correlation to conventional mapping methods (mean, 4.2 mm
SD, 1.3 mm) as well as to the site of successful ablation (mean, 2.5 mm
SD, 2.9 mm). The upstream transit mapping method was critical for successful ablation after conventional techniques proved inadequate in 2 cases. CONCLUSION: Two novel methods, upstream transit mapping and late reset mapping, were used to localize the previously unmappable upstream insertions of unidirectional pathways. These methods expand the diagnostic toolbox to facilitate successful ablation in challenging cases.