BackgroundAccessory pathways may be located in close proximity to the His bundle, resulting in a high risk of heart block during attempted surgical or electrical interruption of these pathways. This study reports the prevalence, ECG characteristics, and results of catheter ablation of parahissian accessory pathways. They were defined on the basis of both the presence of a high amplitude (>0.1 mV) of His bundle potential at the ablation site and an exclusion of anteroseptal or midseptal location of the accessory pathway.Methods and ResultsEight patients with a parahissian accessory pathway were identified among 582 consecutive patients who underwent radiofrequency ablation of an acces-sory pathway. They were six males and two females with a mean age of 21±9 years. During maximal preexcitation, the ECG showed a positive delta wave in leads I, II, and aVF in all patients: six had a negative delta wave in leads V1and V2instead of the positivity usually observed in anteroseptal accessory pathways. This pattern had a sensitivity of 75%, a specificity of 96%, a positive predictive value of 86%, and a negative predictive value of 93% for a parahissian location in comparison with a group of 28 patients with anteroseptal accessory pathways. At the successful ablation site, the mean amplitude of the His bundle potential was 0.2±0.1 (0.12 to 0.4 mV). All accessory pathways were successfully ablated without causing heart block using 5 to 20 W of radiofrequency energy.ConclusionsParahissian accessory pathways have a preexcitation pattern that is distinctive from that of anteroseptal accessory pathways. Catheter ablation of these pathways is feasible using low energy with preservation of normal atrio-ventricular conduction.