INCOMPLETE REENTRY AND EPICARDIAL BREAKTHROUGH PATTERNS DURING ATRIAL-FIBRILLATION IN THE SHEEP HEART

Background The mechanisms underlying atrial fibrillation and its initi ation are not fully understood. Our hypothesis is that atrial fibrilla tion results from complex activation involving the subendocardial musc le network. Methods and Results We have used video imaging to study th e sequence of activation on the surface of the right atrium of the Lan gendorff-perfused sheep heart during pacing, atrial fibrillation, and its initiation. We recorded transmembrane potentials simultaneously fr om over 20 000 sites. We observed two types of patterns of wave propag ation during the initiation of atrial fibrillation. The first type res ulted from heterogeneities of refractoriness and transmural propagatio n near the stimulating electrode. The second type involved heterogenei ty in conduction away from the pacing site. During atrial fibrillation , the average period of activation was 138+/-25 ms (n=6), and complete reentrant pathways were never observed. Propagation patterns were cha racterized by a combination of incomplete reentry, breakthrough patter ns, and wave collisions. Incomplete reentry occurred when waves propag ated around thin lines of block and then terminated. Breakthrough patt erns were frequent and occurred every 215 ms on average. The location of these breakthrough sites and the lines of block during incomplete r eentry were not randomly distributed but appeared to be related to pre ferential propagation in the underlying subendocardial muscle structur es. A computer model of atrial free wall connected to a pectinate musc le suggested that subendocardial muscles lead to epicardial breakthrou gh patterns that act to destabilize reentry. Conclusions These results suggest that the complex three-dimensional structure of the atria pla ys a major role in the activation sequences during atrial fibrillation and its initiation.