The arrangement of muscle fibres in the region of the subthebesian fossa in the aspect of atrial flutter.

The typical atrial flutter is the most common supraventricular tachycardia with reentrant circuit. This tachyarrythmia is based on macroreentry wave going around established anatomic landmarks. The reentry in the inferior right atrial wall passes through narrow isthmus, which is the goal for ablative therapy. The isthmus area is bordered anteriorly by the tricuspid valve and posteriorly by the inferior vena cava, coronary sinus, and eustachian ridge. Near to this area we can find anatomical structure, which can be very important during arising, perpetuation and curing of atrial flutter. The concavity, so-called subthebesian fossa, is on the way of typical atrial flutters' reentrant circuit. Regarding the facts mentioned above we decided to examine the morphology and the arrangement of the muscle fibres in this fossa. Research was conducted on material consisting of 70 human hearts of both sexes from the age of 34 to 72 years. 50 hearts came from patients whose death was not cardiologic in origin. 20 hearts came from humans in whose common atrial flutter was confirmed. We observed the arrangement of muscle fibres in the area of subthebesian fossa. Besides we measured the size and deepness of the subthebesian fossa in both groups of hearts. We found that regular arrangement of muscle fibres within subthebesian fossa was present in 23 healthy human hearts (46%) and 7 cases (35%) of hearts with atrial flutter. The irregular arrangement of muscle fibres was observed in 27 hearts (54%) of control group and 13 hearts (65%) with dysrrhythmia. The thickness of the right atrial wall within the subthebesian fossa was very thin in 8 normal hearts (16%) and in 5 dysrrhythmic hearts (25%). The sizes of examined structure were variable in both groups of hearts, and are presented in the table. It seems that the subthebesian concavity can be the substrate for reentrant circuit during atrial flutter, and there could be such special arrangement of muscle fibres, which allows for microreentrant circuit to arise in this area.

Preliminary study of external interatrial muscle fascicles.

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.