Unusual features of intermediate septal bypass tracts

Introduction: Intermediate septal (IS) AV bypass tracts, located along the tricuspid annulus between the His bundle and coronary sinus os, lie in clos e proximity to the AV node. Surgical or catheter ablation of IS bypass trac ts incurs Increased risk for development of complete heart block, We report additional unusual features of some IS bypass tracts that distinguish them from typical bypass tracts in other anatomic regions. Methods and Results: We analyzed a consecutive series of 150 patients with a history of Wolff-Parkinson-White syndrome and supraventricular tachycardi a who underwent ablation of bypass tracts. We studied the incidence and cha racteristics of AV conduction of IS bypass tracts compared with bypass trac ts in other locations. Of the 150 patients in the study, 21 had an IS bypas s tract (all had anterograde AV conduction). Ten (48%) of these 21 IS bypas s tracts demonstrated anterograde decremental properties with atrial pacing versus 3 (2%) of 129 non-IS bypass tracts (P < 0.001). During ablation, a change in delta wave morphology before total loss of conduction in the IS b ypass tract also occurred in 3 (14%) of 21 IS bypass tracts versus 0 of 129 non-IS bypass tracts (P = 0.0004). During ablation, a change in P wave to delta wave interval occurred in 4 (19%) of 21 IS bypass tracts versus 0 of 129 non-IS bypass tracts (P < 0.0001). One IS patient exhibited retrograde Wenckebach block in the bypass tract, and two IS patients showed loss of re trograde bypass tract conduction after ablation attempts that first changed the delta wave morphology. No non-IS patient had these features (P < 0.000 1 for each comparison). Conclusion: Some TS bypass tracts have unusual properties that distinguish them from bypass tracts in other locations, perhaps due to the presence of multiple ventricular insertions of the bypass tract, It is possible that so me cases represent true "nodoventricular" pathways.