Dual pathways have a crucial role in causing atrio nodal tachyarrhythmias (AVNRT). The aim of the present study was to determine the role of slow pathway (SP) in the concealment zone and also the protective role of AV node during atrial fibrillation (AF). Methods: In 7 isolated nodal rabbit preparations, zones of concealment and concealed conduction were determined by specific pacing protocols. The differences between effective refractory period of atrial and AV node in the basic cycle length

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... d after introducing concealed beat was considered as the zone of concealment. A F was simulated by high-rate atrial pacing with random coupling intervals. Small miniature lesions were made in superior compact node by delivering constant voltage100-110 v. Results: Fast pathway (FP) ablation resulted in longer minimum nodal conduction time, but did not induce any change in nodal effective and functional refractory period. Zone of concealment was determined as 10 ± 4.8 ms. After fast pathway modification, nodal characteristics during AF, including average H-H intervals, number of concealed beats and zone of concealment did not change significantly. Conclusion: Fast pathway has not a determinant role in the mechanism of concealed conduction and zone of concealment. Our findings indicate that during AF, the wave fronts did not use fast pathway to conduct to ventricles.