The relationship between the right and left atria is highlighted in Figure 78–4. When viewed from the frontal plane (see Fig. 78–4A), the right atrium is rightward and anterior, whereas the left atrium is leftward and posterior (see Fig. 78–4B). The left atrium is also more superior (cephalad) relative to the right atrium. As mentioned earlier, the roof of the left atrium (including the Bachman bundle) forms the inferior aspect of the transverse pericardial sinus. The posterior wall of the left atrium is just in front of the tracheal bifurcation and esophagus. When looking at the right atrium from the right lateral view, one can see Waterston’s groove, which is the sulcus between the pulmonary veins (posterior) and the vena cava (anteriorly) (see Fig. 78–3A).
A. A view of the endocast from the right lateral view shows the characteristically extensive pectinate muscles of the right atrial wall contrasting with the smooth vestibule. The right superior pulmonary vein is posterior to the superior vena cava (SCV, superior caval vein). B. The endocast viewed from the back or posterior view shows the right pulmonary veins passing behind the SCV and the intercaval area. The coronary sinus (CS) is inferior to the left atrium (LA). Abbreviations as before, and ICV, inferior caval vein (inferior vena cava).
The atrial septum runs obliquely from the front extending backward and to the right. The posterior part of the left atrium receives the pulmonary veins. The orifices of the left pulmonary veins are more superiorly located than those of the right pulmonary veins. Typically, there are four pulmonary veins (left superior, left inferior, right superior, and right inferior). Sometimes, there is a fifth pulmonary vein (usually a right middle pulmonary vein), and sometimes, two of the veins will share a common antrum (eg, a left common antrum).8 Coursing close to the lateral and posterior mitral annulus is the coronary sinus/great cardiac vein (see Fig. 78–4). The oblique vein of Marshall enters the coronary sinus; it passes superiorly to inferiorly, between the left atrial appendage and the left superior pulmonary vein. This vein is obliterated in the majority of individuals, and the remnant of this vein (also termed the ligament of Marshall) is contained in the Coumadin ridge. This muscular ridge separates the os of the left superior pulmonary vein (or both left-sided veins) and the left atrial appendage. The mitral valve orifice and left atrial appendage are the anterior structures of the left atrium (Fig. 78–5B).
A. The heart viewed from the left and inferior aspect shows a small remnant of the vein of Marshall (arrow) descending along the atrial wall in between the left atrial appendage (LAA) and the left pulmonary veins to enter the coronary sinus (CS). The left atrial isthmus between the left inferior pulmonary vein and the mitral valve is indicated by the dotted line. B. The long axis cut through the left atrium and left ventricle shows the orifices of the left pulmonary veins and the left atrial isthmus (dotted line). Note the narrow fold (thick arrow) between the os of the left atrial appendage and the orifice of the left superior pulmonary vein in the heart. It may be challenging to keep the ablation catheter stable along this narrow fold without dropping inadvertently into the vein or the appendage. Abbreviations as before, and GCV, great cardiac vein.
The right atrium is best considered in terms of its three components: the appendage, the venous part, and the vestibule.9 A fourth component, the septum, is shared by the atria and is discussed later (see section titled “The Atrial Septum”). During embryonic development, the crista terminalis separates the sinus venosus from the heart. Therefore, the crista terminalis separates the smooth-walled posterior or venous atrium from the anterior trabeculated atrium (Fig. 78–6).10 The crista terminalis is a muscular ridge that is frequently the source of atrial tachycardia. It also has a critical role in atrial flutter as it is a site of functional block, which is necessary to facilitate reenty in cavotriscupid-dependent flutter. The trabeculated anterior portion of the right atrium includes the pectinate muscles. In between these muscles, the wall is very thin.
A. The parietal wall of the right atrium has been cut and deflected posteriorly to show the crista terminalis (arrows). X indicates the distal ramifications of the crista terminalis that lead into the cavotricuspid isthmus. B. A similar display with transillumination reveals the thin areas in between the pectinate muscles, at the inferior isthmus, and the posteroinferior part of the intercaval atrial wall. Abbreviations as before, and, OF, oval fossa/fossa ovalis; TV, tricuspid valve.
From the epicardial aspect, the right atrium is dominated by its large, triangular-shaped appendage, which extends anteriorly and laterally. Usually, a fat-filled groove (sulcus terminalis) that corresponds internally to the crista terminalis can be seen along the lateral wall demarcating the junction between appendage and venous components. The sinus node is a subepicardial structure in the cephalad aspect of this groove, at the anterolateral aspect of the superior vena cava–right atrial junction.11,12 Right atrial musculature often extends a short distance into the superior vena cava and can be another source of focal atrial arrhythmias (Fig. 78–7). The pectinate muscles do not reach the tricuspid valve. There is a smooth muscular vestibule surrounding the tricuspid orifice, with the musculature inserting into the tricuspid leaflets (see Fig. 78–6).
This anterior view of the superior cavoatrial junction shows the location of the sinus node (ovoid shape) in the terminal groove (dotted line), and muscular extension around the outside of the venous wall (arrows). Abbreviations as before.
The eustachian valve guards the entrance of the inferior vena cava and is variably developed between individuals. In most hearts, it appears as a triangular flap of fibrous tissue. The eustachian valve is a fibrous extension from the inferior margin of the crista terminalis that inserts medially to the eustachian ridge (Fig. 78–8). In some cases, the eustachian valve is particularly large and muscular and can pose an obstacle to passage of catheters from the inferior vena cava to the inferior part of the right atrium. Sometimes, the valve is perforated or net-like and is often described as a Chiari network.
Simulating a right anterior oblique projection, the right atrium is opened to show the septal aspect that appears extensive, but the area marked by the hexagon is not septal. Termed the aortic mound, it is the anterior wall of the right atrium that abuts the non- and right coronary aortic sinuses. The true septum is limited to the valve of the fossa ovalis/oval foramen (OF) and its immediate muscular rim. The triangle of Koch is delineated posteriorly by the tendon of Todaro (arrows) running in the eustachian ridge, anteriorly by the septal leaflet of the tricuspid valve (TV), and inferiorly by the coronary sinus (CS). The apex of the triangle is marked by the central fibrous body (dotted shape) through which the atrioventricular conduction bundle penetrates (hatched). The atrioventricular node and its inferior extensions are depicted by the white shape. The cavotricuspid isthmus is bordered posteriorly by the eustachian valve (EV) (blue line) and anteriorly by the TV (red line). Within this area are marked three isthmuses: paraseptal isthmus (dots), inferior or central flutter isthmus (dashes), and inferolateral isthmus (dots and dashes). The inferior isthmus passes through the sinus of Keith (triangle). The CS is guarded by a crescentic thebesian valve. Abbreviations as before.
The free border of the eustachian valve continues as the eustachian ridge and then the fibrous tendon of Todaro. The tendon of Todaro penetrates the musculature of the sinus septum, which separates the fossa ovalis from the os of the coronary sinus.13 It is one of the borders of the triangle of Koch, which marks the location of the atrioventricular nodal tissue. As shown in Figure 78–9, the other borders of the triangle of Koch include the septal leaflet of the tricuspid valve anteriorly and the coronary sinus os inferiorly. The apex of the triangle of Koch marks the location of the central fibrous body and its associated structures: the compact atrioventricular node and penetrating bundle of His (see Fig. 78–8).14,15 The region between the coronary sinus os and tricuspid valve, also known as the paraseptal isthmus (see Fig. 78–8), is the area often targeted for ablation of the slow pathway in atrioventricular nodal reentrant tachycardia. Inferior extensions of the atrioventricular node have been observed in this area (see Fig. 78–8). The so-called fast pathway, which usually forms the retrograde limb in patients with slow-fast atrioventricular nodal tachycardia (eg, typical atrioventricular nodal reentrant tachycardia), corresponds to the area of musculature immediately posterior and adjacent to the apex of the triangle of Koch.15,16
Schematic of the right atrium illustrating the triangle of Koch (see anatomic specimen in Fig. 78–8). The slow pathway region, the target of ablation in the treatment of atrioventricular nodal reentrant tachycardia, is just anterior to the os of coronary sinus. AVN, atrioventricular node; CT, crista terminalis; ER, eustachian ridge; EV, eustachian valve; FO, fossa ovalis; IVC, inferior vena cava; TCV, tricuspid valve. Reproduced with permission from Macedo PG, Patel SM, Bisco SE, Asirvatham SJ. Septal accessory pathway: anatomy, causes for difficulty, and an approach to ablation. Indian Pacing Electrophysiol J. 2010 Jul 20;10(7):292-309.
The coronary sinus is a structure of critical importance in cardiac electrophysiology, with particular relevance in cardiac mapping, catheter ablation, and placement of leads for cardiac resynchronization. A small crescentic flap, the thebesian valve, often guards the orifice of the coronary sinus.17 Frequently, the thebesian valve is fenestrated. A complete and imperforate thebesian valve is rare, but can be a cause of inability to cannulate the coronary sinus. The atrial wall inferior to the orifice of the coronary sinus is usually pouch-like and is termed the subeustachian sinus of Keith (or subeustachian pouch). The area between the inferior caval vein and the tricuspid valve (Fig. 78–10B)18 is the cavotricuspid isthmus. The cavotricuspid isthmus is a region of slow conduction in common or typical atrial flutter.19,20 When targeting the cavotricuspid isthmus during catheter ablation of typical flutter, ablationists target the central aspect of the isthmus in order to avoid the trabeculated ridges of the lateral isthmus and the pouch-like aspect of the septal or medial isthmus. This approach also avoids injury to the compact atrioventricular node medially and the right coronary artery laterally. Although injury to these structures is rare, it can occur.21
Computed tomography images of the paraseptal region. A. Volume-rendered image viewed from the right anterior oblique (RAO) 65-degree and cranial (CRA) 10-degree view. The membranous septum (MS) is shown in blue, and the star indicates the location of the compact atrioventricular node at the apex of the triangle of Koch. In part B, the transparency has been increased. The red dotted circle indicates the caudal portion of the superior paraseptal area, and the yellow dotted circle indicates the septal part of the superior paraseptal area. The green dotted triangle indicates the mid-paraseptal area corresponding to the triangle of Koch. The orange dotted circle indicates the inferior paraseptal area. The white dotted oval indicates the cavotricuspid isthmus extending from the tricuspid annulus to the inferior vena cava (ICV; inferior caval vein). Ao, ascending aorta; CB, conus branch; CS, coronary sinus; ER, Eustachian ridge; OF, oval fossa (fossa ovalis); RCA, right coronary artery; SCV, superior caval vein (superior vena cava); SMT, septomarginal trabecula; SNA, sinus node artery; SPT, septoparietal trabecula; TT, tendon of Todaro. Reproduced with permission from Mori S, Fukuzawa K, Takaya T, et al. Clinical structural anatomy of the inferior pyramidal space reconstructed within the cardiac contour using multidetector-row computed tomography. J Cardiovasc Electrophysiol. 2015 Jul;26(7):705-712.
During transseptal catheterization, the left atrium is accessed from the right atrium. Transseptal catheterization requires an appreciation of the extent of the true atrial septum (see Fig. 78–8). Although inspection of the septum from the right atrium suggests an extensive septal area, the true septum separating the right and left atrial cavities is much smaller. The difference between the apparent septum and the true septum is a result of a deep fold between the caval connections and the right atrium and the right pulmonary veins to the left atrium (Fig. 78–11). Enclosed within this fold is epicardial fat. The left aspect of the atrial septum lacks the crater-like feature of the right side. The true septum that interventionalists can cross safely is limited to the fossa ovalis and the immediate muscular rim that surrounds it on the right atrial aspect (see Fig. 78–11). Most hearts have a well-defined muscular rim on the right atrial aspect, allowing the operator to “feel” and observe (in the left anterior oblique view) the “jump” from firm muscular rim to tenting of the fossa ovalis as the transseptal access sheath is withdrawn from the superior vena cava to the right atrium.
A long-axis section through the heart profiles the cardiac septum to show the thin valve of the fossa ovalis (open arrow) on the left atrial side and the muscular rim on the right atrial side. The cut reveals the infolding of the right atrial wall at the superior rim that is filled with epicardial fat (asterisk) to form the interatrial groove. At the ventricular septum, the leaflets of the tricuspid valve (TV) and mitral valve (MV) are hinged at different levels (small arrows) producing offset between the right and left atrioventricular junctions. Thus, the vestibule of the right atrium overlies the ventricular septum. Abbreviations as before.
In approximately 20% to 25% of normal hearts, the fossa is patent, even though on the left atrial side the valve is large enough to overlap the rim. This is because the adhesion of the valve to the rim is incomplete, leaving a gap usually in the anterosuperior margin corresponding to a C-shaped mark in the left atrial side just behind the anterior atrial wall (Fig. 78–12). The gap in adhesion allows a probe, or catheter, in the right atrium to slip between the rim and the valve into the left atrial chamber. A catheter lodged in this crevice will have its tip directed toward the anterior wall of the left atrium. The anterior aspect of the fossa ovalis is adjacent to the noncoronary cusp, an important relationship to appreciate in order to avoid inadvertent puncture of the aortic root. The use of intracardiac ultrasound helps facilitate accurate and safe crossing of the true septum.
A. Displayed in right anterior oblique view, the anterocephalad margin (open arrow) of the fossa ovalis (OF; oval fossa) will allow a probe to pass into the left atrium behind. B. The left atrial view of the same heart shows the crevice through which the probe will exit and its proximity to the thin part of the left atrial wall anteriorly (open arrow). Abbreviations as before.
As with the right atrium, the left atrium has three components and shares its septum. The left atrial appendage arises from the anterolateral primordial atrium. The atrial appendage is a highly crenellated structure of variable shape, frequently resembling a chicken wing, windsock, cactus, or cauliflower.22,23 In fibrillating atria, thrombi may form in the appendage as a result of low flow and stasis. Unlike the right atrium, virtually all the pectinate muscles in the left atrium are confined to the appendage. The lumen of the appendage is lined by a complicated network of muscular ridges and intervening membranes that form its wall. Its tip can be directed anteriorly overlying the pulmonary trunk, superiorly behind the arterial pedicle, or posteriorly. When its tip is directed anteriorly, the body of the appendage usually also overlies the main stem of the left coronary artery. As described previously, the remnant of the vein of Marshall or ligament of Marshall runs on the lateral epicardial aspect of the neck of the appendage, anterior to the left pulmonary veins (see Fig. 78–5).
The venous component of the left atrium receives the pulmonary veins posteriorly (Fig. 78–13). The orifices of the right pulmonary veins are directly adjacent to the plane of the atrial septum (see Fig. 78–11). The venous orifices are oval shaped with a longer superoinferior diameter than anteroposterior diameter. The musculature of the atrial wall extends into the veins to varying lengths, with the longest sleeves along the upper or superior veins.8,24,25,26 Close to the venous insertions, the sleeves are thicker and completely surround the epicardial aspect of the vein. The veins often have electrical continuity with bridging muscle fibers between the superior and inferior veins, which are present in approximately half of left vein pairs and one-third of right vein pairs.27 The distal margins of the sleeves, however, are usually thinner and irregular as the musculature fades out. The muscle sleeves are composed predominantly of circularly orientated myocardial fibers with interdigitating longitudinally and obliquely orientated fibers (see Fig. 78–13B).8,25 Triggered activity from the pulmonary vein muscle sleeves can initiate atrial fibrillation,28 and these sources of ectopic activity are targeted during pulmonary vein isolation procedures for the treatment of drug-refractory atrial fibrillation.29
A. Posterior view of the left atrium from a computed tomography scan. Shown are the four pulmonary veins draining into the posterior left atrium. Note that the left superior (LS) and left inferior (LI) pulmonary veins share a common antrum in this heart. RI, right inferior pulmonary vein; RS, right superior pulmonary vein. B. Posterior view of an anatomic specimen showing atrial fiber orientation around the pulmonary vein sleeves and posterior wall of the left atrium. LIPV, left inferior pulmonary vein; LSPV, left superior pulmonary vein; RIPV, right inferior pulmonary vein; RSPV, right superior pulmonary vein.
The anterior vestibular component leads to the mitral valve. There are no surface anatomic landmarks that separate the venous left atrium from the vestibule. The region between the left inferior pulmonary vein and the mitral valve annulus is the mitral isthmus and is frequently a zone of slow conduction that favors the development of atypical left atrial flutter, particularly in patients with prior left atrial ablation (see Fig. 78–5).30,31 The mitral isthmus is occasionally targeted in substrate ablation during ablation procedures in patients with advanced atrial fibrillation. The tissue of the mitral isthmus is very thick and, therefore, complete transmural ablation can be difficult to achieve.