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Cardiac anatomy for the electrophysiologist


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. 3.1.
(A) View of the heart through the mitral and tricuspid planes, atria, and ventricles using horizontal magnetic resonance imaging (MRI). (B) Schematic reproduction of (A) with appropriate labels. The main direction of the valvar planes, marked by the dotted line in (B), is from the anterior (right) to the posterior (left). (C) Oblique sagittal MR image of the heart, parallel to the plane of the atrioventricular valves ( dotted line in [B]), showing the position of the mitral and tricuspid valvar orifices and the aortic root in the left anterior oblique view. (D) schematic reproduction of (C) with appropriate labels. Ant, Anterior; Ao, aortic root; L, left; LA, left atrium; LV, left ventricle; M, mitral valve; Post, posterior; R, right; RA, right atrium; RV, right ventricle; T, tricuspid valve.

(Cosio FG, Anderson RH, Kuck KH, et al. ESCWGA/NASPE/P experts consensus statement: living anatomy of the atrioventricular junctions. J Cardiovasc Electrophysiol. 1999;10(8):1162-1170.)

Fig. 3.2 presents the inaccurate nomenclature that has been conventionally used for the locations of accessory pathways and the anatomically correct terminology.

Fig. 3.2.
Upper panel: Schematic representation of the atrioventricular junctions in the left anterior oblique projection, showing the anatomically inaccurate nomenclature currently used for the locations of accessory pathways. Lower panel: An anatomically correct nomenclature is shown for the different segments of the junctions.

(Cosio FG, Anderson RH, Kuck KH, et al. ESCWGA/NASPE/P experts consensus statement: living anatomy of the atrioventricular junctions. A guide to electrophysiologic mapping. Working Group of Arrhythmias of the European Society of Cardiology. North American Society of Pacing and Electrophysiology. J Cardiovasc Electrophysiol. 1999;10(8):1162-1170.)

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.

Fig. 3.3., (A) This display of the inside of the right atrium with the lateral wall of the appendage deflected posteriorly shows the atrial septum enface and the terminal crest with its array of pectinate muscles. The sinus and atrioventricular nodes are superimposed. The dotted line marks the position of the tendon of Todaro and the white broken line marks the annulus of the septal leaflet of the tricuspid valve, the borders of the triangle of Koch. (B) This four-chamber section shows three lines in the cavotricuspid isthmus corresponding to the paraseptal isthmus (1), inferior/central isthmus (2), and inferolateral isthmus (3). The short arrow indicates a pouchlike depression crossed by line 2. AV, Atrioventricular; CS, coronary sinus; ER, Eustachian ridge; EV, Eustachian valve; ICV, inferior caval vein; LA, left atrium; OF, oval fossa; SCV, superior caval vein; TV, tricuspid valve.

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.

Fig. 3.4., (A) The right and left atria are separated by the interatrial septum. Note the apical displacement of the tricuspid valve relative to the mitral valve. The septal tissue in between these two valves (dashed lines) is the atrioventricular septum, which houses the atrioventricular node. (B) Right anterior oblique view of the septal aspect of the right atrium and right ventricle. The extension of the Eustachian valve continues as the tendon of Todaro along the Eustachian ridge medially (blue line). It forms the hypotenuse of the triangle of Koch with other boundaries supplied by the coronary sinus ostium (dashed oval) and the septal leaflet of the tricuspid valve (black line). Toward its apex (superiorly located), the triangle of Koch houses the atrioventricular node (asterisk). (C) Right ventricular anatomy is shown (free wall has been removed) demonstrating the many trabeculations (trabeculae carneae), moderator band, anterior papillary muscle, and supraventricular crest. The membranous septum can be seen with transillumination from the left ventricle. (D) Left ventricular anatomy is shown (free wall has been displaced laterally) demonstrating the inferior papillary muscle, aortomitral continuity, and left ventricular outflow tract. The membranous septum is seen from the left ventricular aspect, located at the junction of the right and posterior aortic sinuses of Valsalva. APM, Anterior papillary muscle; AVN, atrioventricular node; CS, coronary sinus; CT, chordae tendineae; FO, fossa ovalis; IAS, interatrial septum; IVS, interventricular septum; LA, left atrium; LV, left ventricle; MB, moderator band; MS, membranous septum; MV, mitral valve; P, posterior aortic valve cusp; PB, parietal band; PM, papillary muscle; R, right aortic valve cusp; RA, right atrium; RV, right ventricle; SB, septal band; SOV, sinus of Valsalva; SVC, supraventricular crest; TV, tricuspid valve.

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.

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