The atria are highly complex multidimensional structures composed of a heterogeneous branching network of subendocardial muscular bundles. The relief of the inner part of the right atrium includes the crista terminalis as well as multiple pectinate muscles that bridge the thinner atrial free walls and appendages. However, a handful of studies have focused attention on the role of the naturally occurring complexities of the atrial subendocardial muscle structures in the mechanisms of cardiac arrhythmias. In accordance with the facts mentioned above, it was decided to examine the morphology and topography of the external interatrial junctions and related structures in order to define the possible anatomical basis of impulse propagation in focal atrial fibrillation. Research was conducted on material consisting of 15 human hearts of both sexes (female — 6, male — 9) from 18 to 82 years of age. In addition we were concerned, on the basis of the history and electrocardiograph tracings, that none of the patients had shown focal and non-focal type of atrial fibrillation. The classic macroscopic methods of anatomical evaluation were used. The walls of the atria were prepared via a stereoscopic microscope, the pericardium and fatty tissue were eliminated from the surface of the atria, visualising muscle fibres linking both of the atria, and the beginnings and the endpoints of fascicles in the right and left atrium were estimated. The structure, large muscle bundle, was present in all examined hearts. The muscle fascicle was descending from the anterior wall of the right atrium just below the orifice of the superior vena cava. The fascicle, running towards the left atrium, divided into two branches, one of which joined with the superior fascicle from the posterior wall and created one running above the interatrial septum and infiltrating into the wall of the left atrium on its superior surface between the superior pulmonary veins. The other branch of the anterior fascicle was running across the anterior wall of the atria and it penetrated into the left atrium muscle in the region of the inferior pole of the left auricle outlet. On the posterior wall of the atria three types of interatrial fascicles were distinguished: unifascicular, bifascicular and trifascicular. The bifascicular type was the most frequent configuration (9 cases — 60.0%), in 5 cases it was trifascicular (33.3%) and finally the unifascicular configuration was observed in just 1 heart (6.7%). On the basis of our study we can conclude that the external interatrial fascicles are the constant structure of the heart, although they may have a variable morphology. Those structures could be responsible for physiological conduction between the atria and may play an important role in patients with atrial fibrillation.
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