Clinical and Electrophysiologic Characteristics of Intra-Isthmus Reentry: Report of Three Cases

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Background: Intra-isthmus reentry (IIR) denotes a reentry circuit within the cavotricuspid isthmus (CTI). The purpose of this study was to define the electrogram and surface ECG characteristics of IIR. Methods and Results: Three patients underwent electrophysiologic examination and were found to have IIR; the IIR in 1 patient was found during a redo procedure. Detailed electrogram mapping of the CTI and electroanatomic mapping (EAM) were performed in all 3 patients. Entrainment mapping in all patients during tachycardia proved reentry and showed that the anteroinferior CTI was outside the circuit and that the septal CTI was within the circuit. During tachycardia, fractionated or double potentials were recorded at either the septal CTI and/or the region of the coronary sinus ostium (CSos). The surface ECG showed a typical counterclockwise (CCW) pattern in 2 patients and an atypical pattern in 1 patient, and EAM showed a CCW pattern in 2 patients and a focal pattern in 1 patient. The one patient with atypical ECG pattern in spite of the CCW activation pattern showed a low voltage area at the inferolateral tricuspid annulus. Conclusions: IIR presents as a tachycardia circuit confined to the septal CTI/CSos region, and surface ECG characteristics show either typical CTI-dependent AFL or atypical AFL patterns.