Transversal crista terminalis conduction suggests ineffective bidirectional isthmus block.

Catheter ablation of the posterior isthmus is an effective tool to cure typical atrial flutter. In some cases, however, bidirectional block cannot be obtained despite extensive RF applications. Anatomic obstacles or abnormalities are thought to be the most common reasons for failed or prolonged procedures. We present a case of recurrent typical atrial flutter that seemed to be refractory to all ablation attempts in the region of the posterior isthmus although no anatomic abnormalities could be detected. Despite extensive RF application, bidirectional conduction was unchanged. Using a novel noncontact mapping system (En-Site 3000) the existence of a fast conducting gap in the region of the inferior terminal crest was revealed. Rapid conduction over this gap to the opposite side of the isthmus led to the impression that bidirectional isthmus block was not established. As a result no further RF applications were necessary because isthmus block was complete at that time. This is the first time that transverse conduction across the terminal crest could be detected by this novel noncontact mapping system masquerading as unchanged bidirectional isthmus conduction.