Rearrangement of atrial bundle architecture and consequent changes in anisotropy of conduction constitute the 3-dimensional substrate for atrial fibrillation.

BACKGROUND Anisotropy of conduction facilitates re-entry and is, therefore, a key determinant of the stability of atrial fibrillation (AF). Little is known about the effect of AF on atrial bundle architecture and consequent changes in anisotropy of conduction and maintenance of AF. METHODS AND RESULTS Direct contact mapping was performed in left atria of goats with acute AF (n=6) or persistent AF (n=5). The degree and direction of anisotropic conduction were analyzed. Mapped tissue regions were imaged by high-resolution MRI for identification of endocardial and epicardial bundle directions. Correlation between endocardial and epicardial bundle directions and between bundle directions and anisotropic conduction was quantified. In persistent AF, epicardial bundles were oriented more perpendicularly to endocardial bundles than in acute AF (% angles<20° between epicardial and endocardial bundle directions were 7.63% and 21.25%, respectively; P<0.01). In acute AF, the direction of epicardially mapped anisotropic conduction correlated with endocardial but not with epicardial bundles. In persistent AF, the direction of anisotropic conduction correlated better with epicardial than with endocardial bundles (% angles<20° between direction of anisotropic conduction and bundle direction were 28.77% and 18.45%, respectively; P<0.01). CONCLUSIONS During AF, atrial bundle rearrangement manifests itself in more perpendicular orientation of epicardial to endocardial bundles. Propagation of fibrillation waves is dominated by endocardial bundles in acute AF and by epicardial bundles in persistent AF. Together with the loss of endo-epicardial electrical connections, rearrangement of atrial bundles underlies endo-epicardial dissociation of electrical activity and the development of a 3-dimensional AF substrate.