Substrates for intra-atrial and interatrial conduction in the atrial septum: anatomical study on 84 human hearts

Technical developments in the management of atrial arrhythmias revived interest into the detailed knowledge of atrial anatomy. The atrial septum (AS), known for its complex structure, has been particularly difficult to study, and our knowledge of the muscular bundles providing routes for intra-atrial and interatrial conduction within the AS remains limited.

The purpose of this study was to describe myocardial arrangement within the AS and adjacent parts of atrial walls for delineation of possible substrates for interatrial and intra-atrial conduction.

Human heart specimens from 84 postmortem studies were studied using conventional morphometric assessment, blunt dissection, and light microscopy of serial histological sections of AS.

Interatrial muscular connections are present anteriorly, posteriorly between right pulmonary veins, and inferiorly between the coronary sinus and the right inferior pulmonary vein. The inferior connections can be more prominent than the Bachmann bundle. Atrial musculature in the fossa ovalis consists of muscular bands isolated by fatty tissue from the endocardium of the right and left atrium. They are arranged along the anterior-posterior axis and have connections with left atrial myocardium. Myocardial fascicles in the posterior-inferior and superior portions of the muscular rim of fossa ovalis originate on the right atrial side and can be traced toward the atrioventricular node.

The general myocardial arrangement in the AS and adjacent regions of atrial walls are important for understanding propagation of atrial activation for selection of the optimal treatment strategy.