Catheter ablation using radiofrequency energy has been established as the most important mode of treatment in patients with accessory pathway. However the ablation of midseptal accessory pathways had been recognized as being more difficult to ablate than other located pathway because of the low incidence and the difficult localization of ablation site. This paper describes the electrophysiologic characteristics of successfully ablated midseptal accessory pathway using radiofrequency energy.

Routine electrophysiologic studies were performed in 13 patients with midseptal accessorypathway. Guided by the recording of VA interval, the ablation catheter was positioned in all patients in an area bounded anteriorly by the tip electrode of the His bundle catheter and posteriorly by the coronary sinus ostium. Local electrograms during orthodromic atrioventricular reentrant tachycardia or right ventricular apical pacing were compared for 13 patients with midseptal accessory pathway and consequent 13 patients with posteroseptal accessory pathway.

13 patients with midseptal accessory pathway; eight with constant Wolff-Parfinson-White syndrome, one with intermittent Wolff-Parkinson-White syndrome and four with concealed bypass track underwent attempts at ablation of their pathway using radiofrequency energy. 11 accessory pathways were successfully ablated without complication during the firstsession. A second attempt at ablation was made in two patients with success(one; recurred case, the other one; failed case at the first session). In the surface 12-Lead ECG, all eight patientswith constant Wolff-Pakinsin-White syndrome had not shownen Qrs complex at lead 3. Two patient with midseptal accessory pathway had transient left bundle branch block during orthodromic tachycardia. The VA interval during left bundle branch block was not change compared to that during narrow complex tachycardia in both. In all patients with midseptal accessory pathway, the VA interval in his bundle electrogram were almost similar to that in the coronary sinus ostial electrogram, which was not observed in the patients with posteroseptal accessory pathway.

We suggest that VA interval during orthodromic tachycardia and right ventricular apcial pacing is the most reliable market for identifying midseptal accessory pathway, especially distinguishing from posteroseptal accessory pathway.