Atrioventricular nodal conduction during atrial fibrillation - Role of atrial input modification

Background-Posteroseptal ablation of the atrioventricular node (AVN) has be en proposed as a means to slow the ventricular rate during atrial fibrillat ion (AF). The suggested mechanism is elimination of the AVN "slow pathway." On the basis of the unpredictable success of the procedure, we hypothesize that, in fact, the slow pathway is preserved. Therefore, the slowing of th e ventricular rate results from reduced bombardment of the AVN. Methods and Results-In 8 rabbit heart atrial-AVN preparations, cooling of t he posterior and/or the anterior AVN approaches revealed nonspecific effect s on the slow and fast pathway portions of the AVN conduction curve. In 13 other preparations, simulated AF during posterior cooling (n=6) prolonged t he His-His (H-H) intervals but did not reveal specific slow pathway injury. In the remaining 7 preparations, AF was applied before and after posterose ptal surgical cuts. During AF with posterior origin, the cuts resulted in l onger mean H-H along with slowing of the AVN bombardment rate, However, the re was no change in the minimum observed H-H, suggesting an intact slow pat hway. During AF with anterior origin, the mean and the shortest H-H remaine d unchanged before and after the cuts in all preparations, This was associa ted with the maintenance of high-rate AVN bombardment. Conclusions-Posteroseptal ablation does not eliminate the slow pathway. Ven tricular rate slowing can be obtained if the ablation procedure results in a posteroanterior intra-atrial block leading to a reduction of the rate of AV nodal bombardment.