Some of the descriptive terms conventionally used for cardiac structures are inaccurate. Conventionally the heart and its associated structures have been described in accordance with their position when the heart has been removed from the body and positioned with the apex down, rather than according to their in vivo positions. , For example, when defining positions along the mitral and tricuspid valve annuli and the triangle of Koch, the superior aspect of the heart has been described as being anterior, whereas the anterior and posterior aspects have been described as right and left lateral.
When entering the chest through a sternotomy, the most anterior chamber of the heart is the right ventricle (RV). A small portion of the left ventricle (LV) can be seen along the left border of the RV, and the remainder of the LV lies posterior to the RV ( Fig. 3.1 ). , The apex of the heart (point of maximal impulse) is formed by the left ventricular apex, which is slightly superior to its right ventricular counterpart. To the right of the RV is the right atrium (RA), the only appropriately named cardiac chamber with respect to its position. Posteriorly and just anterior to the vertebral column is the left atrium (LA). Therefore, in view of the in vivo position of the heart, the ventricles are anterior to their respective atria and the right-sided chambers are anterior to the left-sided chambers. The LA is the most posterior cardiac chamber and the RV is the most anterior cardiac chamber. ,
Fig. 3.2 presents the inaccurate nomenclature that has been conventionally used for the locations of accessory pathways and the anatomically correct terminology.
Right atrium
The terminal crest (or crista terminalis ) marks the border between the smooth intercaval posterior wall of the RA and the appendage. , It delineates the border between the smooth wall of the venous component and the rough wall of the appendage ( Fig. 3.3 ). A series of muscle bundles known as pectinate muscles arise from the lateral margin of the terminal crest with branches between the crest and the smooth vestibule that surrounds the tricuspid valve orifice. The vestibule itself is composed mainly of a circumferential arrangement of myofibers. The nonuniform arrangement of the terminal ramifications of the terminal crest may account for delay and discontinuity in the spread of an excitatory wave front, leading to reentry that uses the cavotricuspid isthmus (CTI), a critical component of the typical atrial flutter reentry circuit. Surgical scars in the right atrium after open heart surgery can also create the substrate for non–CTI-dependent, macroreentrant atrial tachycardias.
The triangle of Koch ( Fig. 3.4 ), which is the anatomic landmark for the location of the atrioventricular node, is the portion of the vestibule that is located anterosuperior to the orifice of the coronary sinus. Its posterior border is the tendon of Todaro (the hypotenuse of the triangle) within the Eustachian ridge, whereas its anterior border is the annulus of the septal leaflet of the tricuspid valve, and the orifice of the coronary sinus marks its base. The atrioventricular node (AVN) and His bundle lie at its apex, and the orifice of the coronary sinus marks its base. The inferior extensions of the compact AVN are thought to account for the slow pathway in nodal reentry tachycardia, although the arrangement of the cardiomyocytes in this area may also play a role. The valve of the inferior vena cava (IVC) or Eustachian valve is derived embryologically from the right venous valve and can be absent or extremely prominent (in which case it is referred to as a Chiari network ). The Chiari network is a complex remnant of a large valve of the Eustachian ridge present in about 2% of individuals. It can be mistaken for a thrombus on cardiac imaging and occasionally can produce a nearly complete partitioning of the RA (cor triatriatum dexter). The extension of the Eustachian valve continues as the tendon of Todaro medially along the Eustachian ridge.
The coronary sinus (CS) is the cardiac venous system that begins at its ostium in the right atrium and ends at the origin of the great cardiac vein. The major tributaries of the CS include the great cardiac vein (anterior cardiac vein), the left obtuse marginal vein, the posterior (or inferior) left ventricular vein, the middle cardiac vein, and the right coronary vein. In addition, atrial veins and, notably, the vein (or ligament ) of Marshall (or oblique left atrial vein) also enter the coronary sinus vein. From the perspective of electrophysiologists, the CS represents an anatomic structure of particular interest. First, it provides access to epicardial accessory AV connections and to both atria and ventricular arrhythmias. Second, the muscle that wraps around the CS can be a source of atrial arrhythmias.
Left atrium
The interatrial septum is located posteromedially and is slightly angulated given the cephalad position of the LA ( Fig. 3.4 and 3.5 ). It consists of a thin, fibrous portion centrally (fossa ovalis, the remnant of the foramen ovale/septum primum) that is surrounded by a thicker muscular portion (the limbus of the fossa ovalis, the remnant of the septum secundum). The wall of the LA is generally thicker than the wall of the RA. Apart from muscular continuity at the margins and the floor of the oval fossa, there are muscle bundles that allow conduction between the atria. In the majority of hearts, the most obvious muscular interatrial bridge is Bachmann’s bundle, which has a parallel alignment of myofibers in the subepicardium, crossing over the interatrial groove, in parallel fashion to the plane of the atrioventricular junction. The change in fiber orientation and wall thickness at the boundary of these bundles may lead to conduction block during sinus rhythm propagation or may become the substrate for atrial fibrillation.
