CHAPTER 78: ELECTROPHYSIOLOGIC ANATOMY

Although the study of cardiac electrophysiology centers around the initiation of electrical impulses and their propagation, it is also very much an “anatomic specialty.” The study of electrophysiology requires critical appreciation and understanding of the anatomy of the heart and its relationship to electrophysiologic function. Over the past two decades, advances in electroanatomic mapping and cardiovascular imaging and innovations in both catheter ablation and device-based interventions have led to an improved understanding of cardiac anatomy, normal electrophysiologic function, and mechanisms behind heart rhythm disorders. Accordingly, describing cardiac structures in terms of their function and relationship to cardiac conduction¹ is much more useful to the clinical electrophysiologist than the conventional approach² of describing the heart in a purely anatomic and geographic fashion. This chapter on electrophysiologic anatomy highlights features of particular relevance in heart rhythm disorders.

For the clinician, the heart must be viewed in the context of its location and relationship to surrounding structures (Fig. 78–1). The frontal silhouette of the heart is nearly trapezoidal. The right border of the heart is more or less a vertical line just to the right of the sternum. It is formed exclusively by the right atrium, with the superior and inferior caval veins joining at its upper and lower margins. The inferior border lying horizontally on the diaphragm is marked by the right ventricle. The sloping left border is often described as having three to four “moguls” on the chest x-ray, which correspond to the aortic arch, pulmonary artery, and left ventricle (superior to inferior). Occasionally, the left atrial appendage is visible between the pulmonary artery and the left ventricle. The aortic knob appears above the pulmonary trunk. In the hilum, the aortic knob is superior and rightward and the main pulmonary artery (or pulmonary trunk) is inferior and leftward. The pulmonary trunk crosses over the root of the aorta anteriorly such that the aorta ascends behind or posterior to it. On the frontal silhouette, the left atrium is barely seen; only its appendage curling round the edge of the pulmonary trunk is visible. The left atrium is the most posterior cardiac chamber (closest to the spine), and the esophagus is immediately behind the left atrium (Fig. 78–2). Understanding this spatial relationship is crucial to ablationists in order to reduce the risk of the postprocedural complication of atrioesophageal fistula.³

The heart is enclosed in the fibrous pericardium, which is a protective sac that separates the surface of the heart from adjacent structures and creates a low-friction environment to facilitate cardiac motion. The mediastinal pleura is the outermost lining of the fibrous pericardium. Within the fibrous pericardium, there is a thinner double-layered membrane, the serous pericardium. One layer of the serous pericardium is fused to the inner surface of the fibrous pericardium, whereas the other layer lines the outer surface of the heart as the epicardium. The pericardial cavity is the space between the layers of the serous pericardium. Two recesses are found within the pericardial cavity. These recesses have particular importance for interventional pericardial procedures. The first is the transverse sinus, which is a passage between the superior and posterior reflections of the pericardium, anterior to the superior vena cava, posterior to the arterial trunk, and superior to the left atrium. The second is the oblique sinus, which separates the left atrium from the esophagus. The oblique sinus is a blind cul-de-sac formed by the right pulmonary veins and inferior vena cava on the right side and by the left pulmonary veins on the left side.^4,5 The vein of Marshall is contained within the left margin of the oblique sinus.

The phrenic nerves descend over the pericardial sac. The right phrenic nerve courses along the right anterolateral surface of the superior vena cava down across the intercaval aspect of the right atrium and crosses the diaphragm at the lateral border of the inferior vena cava.⁶ Along this course, the right phrenic nerve can be less than 2 mm from the anterior wall of the right superior pulmonary vein (Fig. 78–3A) and thus can be injured during catheter ablation for pulmonary vein isolation.⁷ The left phrenic nerve descends on the left side close to the aortic arch and onto the pericardium over the left atrial appendage and the left ventricle. It takes one of three courses over the left ventricle, passing over the anterior surface, leftward over the obtuse margin, or more posteriorly over the left atrial appendage and the basal lateral wall. Most commonly, the course of the left phrenic nerve is close to the lateral branches of the great cardiac vein and, thus, can be stimulated during left ventricular pacing for cardiac resynchronization therapy (Fig. 78–3B).^6,7

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).

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).

The Right Atrium

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.

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).

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.

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

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

The Atrial Septum

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.

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.

The Left Atrium

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

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.

In the normal heart, the fibrous rings of tissue surrounding the atrioventricular valves provide electrical separation of the atrial and ventricular tissue with the exception of the atrioventricular nodal tissue. However, anomalous muscular atrioventricular connections at the atrioventricular junctions (accessory pathways or bypass tracts) can lead to atrioventricular reentry and pre-excitation, including the Wolff-Parkinson-White syndrome.³² Description of the location of accessory pathways in the literature and clinical arena has been complicated by the use of heterogeneous terminology. This terminology has been particularly complicated by the interchangeable use of the terms anterior and superior and the terms posterior and inferior.¹ Anatomically, the term superior is more accurate and is preferred to anterior, whereas inferior is more accurate and preferred to posterior.

The true septal component is limited to the area of the central fibrous body and immediate musculature. The so-called anterior septum is contiguous with part of the supraventricular crest of the right ventricle, whereas the posterior septum is formed by the muscular floor of the coronary sinus overlying the diverging posterior walls of the ventricular mass and the vestibule of the right atrium overlapping ventricular myocardium (the atrioventricular septum). Anatomically, the atrioventricular junction can be described as comprising extensive right and left parietal junctions that meet with a small true-septal component (Fig. 78–14). The right parietal junction is relatively circular and occupies a near vertical plane in the heart marked by the course of the right coronary artery in the atrioventricular groove. The superior and most medial part of the junction abuts directly on the membranous septum.

The left parietal junction surrounds the orifice of the mitral valve, and part of it is the area of fibrous continuity between the mitral and aortic valves (see Fig. 78–14). On the left side, because of the nature of the aortomitral continuity, accessory atrioventricular connections are mainly limited to the mitral annulus supporting the posterior leaflet of the mitral valve. This portion of the mitral annulus runs from posterosuperior to posterior and inferior when the heart is viewed in left anterior oblique projection. The inferior area harbors the coronary sinus and its tributary, the great cardiac vein (see Fig. 78–5). The inferior paraseptal region, the so-called posterior septum, is the inferior pyramidal space that contains epicardial fibrofatty tissues together with the artery supplying the atrioventricular node (Fig. 78–15).^33,34 The floor of the pyramidal space is formed by the pericardium, and the four walls of the pyramidal space include the triangle of Koch in the right atrium, the left atrium, the right ventricle, and the left ventricle. The central fibrous body forms the peak of the pyramid.

Maintaining a more ventricular position when ablating septal pathways is important to avoid injury to the compact atrioventricular node and subsequent complete heart block. Because the tricuspid valve is more caudal than the mitral valve, there is a tiny portion of the right atrium that shares a common wall with the left ventricle—the atrioventricular membranous septum. The atrioventricular membranous septum is immediately adjacent to the pyramidal space. More extensive than the atrioventricular membranous septum is the so-called atrioventricular muscular septum, which is formed by the right atrial wall overlying the basal left ventricular wall in the medial portion of the atrioventricular junction, and sandwiched in between is epicardial fat of the pyramidal space inferiorly and central fibrous body superiorly. Appreciation of the “atrioventricular septum” is important because a catheter at this location may record a large ventricular signal from the basal left ventricle, leading an ablationist to think he or she is more ventricular with little risk of heart block during attempted ablation of a septal accessory pathway.³⁵ In fact, a catheter on the “atrioventricular septum” is on the atrial side of the junction, with significant risk of iatrogenic heart block.

Each ventricle has three components: the inlet containing the atrioventricular valve, the outlet leading to the arterial valve, and the apical trabecular component. The ventricular septum curves as it is traced from inlet toward the outlet portions, allowing the right ventricle to lie anterosuperiorly over the left ventricle. The curvature places the right ventricular outflow tract anteriorly and slightly leftward to that of the left ventricle, resulting in a characteristic “crossover” relationship between right and left ventricular outflow tracts (Fig. 78–16).

The Right Ventricle

The right ventricle is the most anteriorly situated cardiac chamber in the normal heart. It is located immediately behind the sternum. This anatomic relationship is important to recognize when performing pericardiocentesis. Using an anterior approach, if the needle continues through and past the pericardial space, the needle will encounter the right ventricular free wall, thus creating potential for right ventricular puncture or laceration. When viewed from the front, the right ventricle has a triangular shape. The right ventricular inlet extends from the annulus of the tricuspid valve to the papillary muscles. The leaflets of the tricuspid valve can be distinguished as septal, anterosuperior, and inferior or mural. The septal leaflet, with its cords inserting directly to the ventricular septum, is characteristic of the tricuspid valve. The medial papillary muscle, a small out-budding from the septum, supports the junction (commissure) between the septal and anterosuperior leaflets (Fig. 78–17). A larger papillary muscle, the anterior papillary muscle, supports the extensive anterosuperior leaflet and its junction with the inferior leaflet. The junction between anterosuperior and inferior leaflets is supported by a group of small papillary muscles, the inferior papillary muscles.

Coarse muscular trabeculations crisscross the apical portion. One of them, the moderator band, is characteristic of the right ventricle (see Fig. 78–17). This bridges the ventricular cavity between the body of the septomarginal trabeculation and the parietal wall, giving rise to the anterior papillary muscle along the way. Within its musculature runs a major fascicle of the right bundle branch. The septomarginal trabeculation itself is a y-shaped muscular band that is adherent to the septal surface. In between its limbs lies the infolding of the heart wall forming the ventricular roof, an area also known as the supraventricular crest (see Fig. 78–17). This crest separates the two right heart valves and is an integral part of the outlet, continuous with the freestanding subpulmonary muscular infundibulum, which is a tube-like structure supporting the pulmonary valve. The right ventricular outflow tract is a frequent origin of adenosine-sensitive triggered ventricular extrasystoles and ventricular tachycardia in patients without structural heart disease. Most outflow tract ventricular tachycardias arise in the perivalvular tissue. Although ablationists frequently refer to septal and free wall portions of the right ventricular outflow tract, it should be noted that the infundibulum does not have a true septal component (see Fig. 78–17) because the pulmonary valve can be resected surgically for the Ross procedure without entering the left ventricle.³⁶ The septal component of the right ventricular outlet is only in its most proximal part, where it continues into the supraventricular crest. Furthermore, the right ventricular outlet curves to pass anterior and cephalad to the left ventricular outlet (see Fig. 78–16). Thus, the aortic root is immediately posterior to the “septal” right ventricular outflow tract (see Fig. 78–17). Two of the pulmonary sinuses are adjacent to the right and left coronary sinuses of the aortic valve, although the planes of the aortic and pulmonary valves are at an angle to one another (see Fig. 78–16). Thus, the main coronary arteries can also be at risk when ablating in the so-called septal part of the right ventricular outflow tracts (see Figs. 78–16 and 78–17).

The Left Ventricle

The left ventricle is conical in shape. When the heart is viewed from the front, most of the left ventricle is behind the right ventricle. Its outlet overlaps its inlet. The mitral valve annulus at the entrance to the inlet has limited attachment to septal structures (see Fig. 78–14). Compared to that of the tricuspid valve, the mitral valve annulus is further away from the apex and does not have a septal leaflet (see Fig. 78–11). The larger portion of the valve is hinged to the parietal atrioventricular junction, whereas a third is the span of fibrous continuity with the aortic valve (see Fig. 78–14). The aortomitral continuity is a structure of particular importance and is another site from which triggered idiopathic ventricular arrhythmias can originate.³⁷ At the septal end of valvar fibrous continuity is the right fibrous trigone, and at the parietal end is the left fibrous trigone (Fig. 78–18; see also Fig. 78–14). The right trigone in continuity with the membranous septum forms the central fibrous body. The two leaflets of the mitral valve are disproportionate in size. The anterior leaflet in continuity with the aortic valve is deep, whereas the posterior (or mural) leaflet is shallow. The mitral leaflets are attached via tendinous cords exclusively to two groups of papillary muscles: anterolateral and posteromedial papillary muscles. The papillary muscles can also be sources for ventricular arrhythmias that are often difficult to ablate given their location and motion within the left ventricular cavity as well as the thick nature of the papillary muscles.

The apical component of the left ventricle extends from the papillary muscles to the ventricular apex. At the apex, the muscular wall tapers to a thickness of 1 to 2 mm in normal hearts. The trabeculations are finer than those found in the right ventricle. Occasionally, fine muscular strands, or so-called false tendons, extend between the septum and the papillary muscles or the parietal wall.³⁸ Often, they carry the distal ramifications of the left bundle branch and have been implicated in idiopathic left ventricular tachycardia.³⁹

The left ventricular outlet is bordered by the muscular ventricular septum anterosuperiorly and the anterior leaflet of the mitral valve posteroinferiorly (see Fig. 78–18). The upper part of the ventricular septum leading to the aortic valve is smooth. The common atrioventricular conduction bundle emerges from the central fibrous body to pass between the membranous septum and the crest of the muscular ventricular septum (see Fig. 78–18). The landmark for the site of the atrioventricular conduction bundle is the fibrous body that adjoins the crescentic hinge lines of the right and noncoronary cusps of the aortic valve. From here, the left bundle branch descends in the subendocardium and usually branches into three main fascicles that interconnect and further divide into finer and finer branches as the Purkinje network (see section titled “The Cardiac Conduction System”).

In the outlet, two leaflets of the aortic valve have muscular support, these being the ones adjacent to, or facing, the pulmonary valve (the right and left coronary sinuses). The third sinus, the noncoronary sinus, does not have muscular support. The facing aortic sinuses give rise to the right and left coronary arteries. Like the pulmonary valves, these two sinuses contain small segments of ventricular myocardium within (see Fig. 78–18).⁴⁰ Thus, ventricular arrhythmias emanating from the aortic sinuses are limited to the right and left coronary sinuses. As a result of the spatial relationship of the subpulmonary infundibulum and the left ventricular outlet (Fig. 78–19), the foci may be ablated from within the part of the right ventricular outlet that overlies the adjacent aortic sinuses.⁴¹ Since the main coronary arteries arise from the arterial part of the sinuses, they are not in the immediate field, and thus ablations can be performed safely without significant risk of injury to the coronaries.⁴² The noncoronary aortic sinus, being immediately adjacent to the paraseptal region of the left and right atriums and close to the superior atrioventricular junction, may be used to map and ablate focal atrial tachycardias that have earliest activation in the vicinity of the His bundle area—so-called “parahisian” atrial tachycardias (see Fig. 78–15).⁴³

The venous return from the myocardium either is channeled via small thebesian veins that open directly into the cardiac chambers or, more significantly, is collected by the greater coronary venous system, which drains 85% of the venous flow.^44,45 The main coronary veins in the greater system are the great, middle, and small cardiac veins. The great cardiac vein is a continuation of the coronary sinus and is also continuous with the anterior interventricular vein, which runs parallel to the anterior descending artery. The middle cardiac vein runs alongside the posterior descending coronary artery. The great cardiac vein and middle cardiac vein (usually) drain into the coronary sinus. The small cardiac vein receives tributaries from the right atrium and the inferior wall of the right ventricle before coursing in the right atrioventricular junction to open to the right margin of the coronary sinus orifice or into the middle cardiac vein. Several other veins from the anterior surface of the right ventricle drain directly into the right atrium. The coronary veins may be surrounded by a cuff of myocardium that gives the potential for accessory atrioventricular connection as the vein passes through the atrioventricular groove.⁴⁶

The coronary sinus is a critical structure in electrophysiology. Its ostium is usually between 5 and 15 mm in diameter. Its transition to the great cardiac vein is marked by several structures including the valve of Vieussens and the vein of Marshall. The entrance to the vein of Marshall, or oblique left atrial vein, marks the venous end of the tube-shaped coronary sinus. The vein is an embryonic remnant of the left superior vena cava and is a fibrous ligament in most individuals. Even when a lumen is present in the remnant vein, it is narrow, rarely exceeding 2 cm in length before tapering to a blind end (see Fig. 78–5). If adequately wide, this channel may be used for ablating the left atrial wall, including the use of alcohol instillation. The other structure that marks the transition from the coronary sinus to the great cardiac vein is the valve of Vieussens. Found in 80% to 90% of hearts, this flimsy valve can provide some resistance to potential catheter placement.⁴⁷ Once past Vieussens valve, a sharp bend in the great cardiac vein can cause further obstruction in 20% of patients.⁴⁷ There is a muscular sleeve around the proximal portion of the coronary sinus. Bundles from the sleeve sometimes run into the left atrial wall and also cover the outer walls of adjacent coronary arteries (Fig. 78–20).⁴⁸

As the great cardiac vein ascends in the atrioventricular groove, it collects tributaries draining the left ventricle, including the lateral (obtuse marginal) veins and posterior veins, which are important targets for left ventricular pacing in cardiac resynchronization therapy (Fig. 78–21). The distribution, courses, and calibers of the left ventricular veins vary from individual to individual. When using these veins for pacing lead implants, it is worth noting that the left phrenic nerve running in the pericardium may pass across or very close to the lateral or posterolateral veins (see Fig. 78–3).⁷ Thus, pacing from these veins can cause phrenic capture and diaphragmatic stimulation. The great cardiac vein continues to the interventricular groove where it becomes the anterior interventricular vein. In most hearts, it passes under the cover of the left atrial appendage. The left ventricular veins may be accessed for ablating ventricular tachycardia from a source close to the epicardium.⁴⁹ When deploying catheters or wires in superficial veins, it is important to note that venous walls are thin and “unprotected” by muscle on the epicardial side.

The middle cardiac vein drains into the coronary sinus just within the sinus os. Occasionally the middle vein enters the right atrium directly via its own separate os. The middle cardiac vein passes just superficial to the right coronary artery at the cardiac crux. It is a useful portal for ablating epicardial posteroseptal accessory atrioventricular pathways located in the inferior pyramidal space.⁵⁰ Ablation at this site can cause injury to the posterior descending artery, given its close proximity.⁵¹ Rarely, the entrance of the middle vein is dilated (or aneurysmal) and surrounded by a cuff of muscle, giving the potential for accessory atrioventricular connections.^52,53 Tributaries from the middle cardiac vein with a lateral course can also be used to access the lateral wall for lead placement in biventricular pacing.

The Sinus Node

The key components of the cardiac conduction system include the sinus node, the atrioventricular node, and the His-Purkinje system. The sinus node is a subepicardial structure, crescent-like in shape, with a mean length of 13.5 mm in the adult heart.^11,12 The “head” of the sinus node is located at the superior margin of the terminal groove near the anterolateral aspect of the junction between the right atrium and superior vena cava. The “tail” of the sinus nodal tissue can extend down more than 50% of the length of the crista terminalis toward the inferior vena cava. The node is richly supplied with nerves from both the sympathetic chains and the vagus nerve. Nodal tissues are penetrated by the sinoatrial nodal artery, which arises from the right coronary artery in 65% of individuals.⁵⁴ The tissue surrounding the sinus node is dominated by calcium channel activation (as opposed to Na⁺), and therefore, sinus node exit conduction is slow and decremental, similar to the atrioventricular node.⁵⁴ The borders of the node are irregular with frequent interdigitations between nodal and ordinary atrial myocytes, facilitating communication between the node and right atrial wall.^11,12 Figure 78–22 shows the area of early endocardial activation as a result of sinus node activity.

Although much has been written about specialized internodal tracts (anterior, middle, and posterior) connecting the sinus node to the atrioventricular node, their existence in the form as originally defined by early anatomists has never been demonstrated.⁵⁵ There are several muscular interatrial connections that are important in normal cardiac conduction and have relevance in a number of heart rhythm disorders. Bachmann’s bundle is a prominent interatrial muscle bridge that extends across the roof of the atria anterosuperior to the fossa ovalis (Fig. 78–23). Conduction over Bachmann’s bundle leads to early endocardial activation in the anterior left atrium in order to help facilitate interatrial synchrony.⁵⁶ Multiple smaller interatrial bridges are frequently present, giving the potential for macroreentry. Some cross Waterston’s groove and connect the muscular sleeves of the right pulmonary veins and the superior caval vein to the left atrium (see Fig. 78–23).^8,9 Finally, as previously mentioned, muscular bridges from the left atrial wall often overlie and run into the wall of the coronary sinus.

The Atrioventricular Conduction System

In the normal heart (without accessory pathway connections), the atrioventricular conduction system provides the only pathway of muscular continuity between atrial and ventricular myocardium. There is an interface of transitional cells between ordinary atrial myocardium and the histologically specialized cells that make up the atrioventricular node. The compact atrioventricular node was described by Tawara⁵⁷ in 1906 and is located near the apex of the triangle of Koch (see Figs. 78–8 and 78–9). In the adult, the compact atrioventricular node is approximately 5 mm long and wide. The atrioventricular node receives its blood supply from the atrioventricular nodal artery, which arises from the right coronary artery in 80%, the left circumflex artery in 10%, and both in 10% of hearts. Most of the time, extensions from the compact node pass to the right and left sides of the artery.⁵⁸ The right extension courses parallel and adjacent to the tricuspid annulus, whereas the left extension projects toward the mitral vestibule. The distance of the right inferior extension to the endocardial surface is approximately 1 to 5 mm. With reference to the triangle of Koch, the compact node (see Fig. 78–8) is near the apex, but the right inferior extensions reach to the mid-level of the triangle and may even extend to the vicinity of the coronary sinus.⁵⁹ This area is the location at which catheter ablation targets the slow pathway in patients with evidence of dual atrioventricular nodal physiology and evidence of atrioventricular nodal reentrant tachycardia, the most common form of supraventricular tachycardia.

The His-Purkinje System

Superiorly, at the apex of the triangle of Koch, the penetrating bundle of His passes through the central fibrous body coursing leftward. This short bundle of specialized myocardium encased in a fibrous sheath is a direct extension of the compact atrioventricular node, enabling atrial activity to be conveyed to the ventricles. The emergence of the bundle in the ventricles is directly related to the membranous septum and the aortic outflow tract. In relationship to the aortic valve, the landmark for the atrioventricular conduction bundle is immediately inferior to the fibrous area between the right and noncoronary aortic sinuses. The bundle is sandwiched between the membranous septum and the crest of the ventricular septum. After a short distance, the bundle bifurcates into the left and right bundle branches. The left bundle branch fans out as it descends in the subepicardium of the septal surface the left ventricle (see Fig. 78–18). In contrast, the right bundle branch is cord-like and descends through the musculature of the ventricular septum to emerge in the subendocardium at the base of the medial papillary muscle to run in the septomarginal trabeculation (see Fig. 78–17). Along its descent, a prominent branch crosses to the parietal wall within the moderator band. Both bundle branches are insulated as they descend toward the apical parts of the ventricles. The branches then ramify as the Purkinje fibers, running in the subendocardium and into the myocardium in a net-like fashion (Fig. 78–24).

The heart is heavily influenced by autonomic innervation. Moreover, cardiac innervation includes both extrinsic innervation as well as influence from the intrinsic cardiac nervous system. The intrinsic cardiac nervous system is largely an atrial network of neural ganglia.⁶⁰ The vagal inputs to the heart arise from various nuclei in the medulla. Sympathetic inputs come from the paravertebral, superior cervical, middle cervical, cervicothoracic (stellate), and thoracic ganglia. The stellate ganglion is the primary source of cardiac sympathetic innervation. Generally speaking, the innervation of the atria is predominantly parasympathetic, while the innervation of the ventricles is predominantly sympathetic. The extrinsic cardiac nerves course through the hilum at the base of the heart and branch into different autonomic ganglia. These ganglionated plexuses contain neuronal inputs from the atrial myocardium and the extrinsic cholinergic and adrenergic neurons. Between 6 and 10 collections of ganglia or ganglionated plexuses have been described in the human heart.^61,62 Approximately half of the plexuses are located on the atria and the other half on the ventricles. Occasional ganglia are located in other atrial and ventricular regions of the epicardium.⁶³ The ganglionated plexuses are generally associated with islands of adipose tissue referred to as fat pads that serve as visual landmarks to cardiac surgeons.⁶² The atrial fat pads are located in the interatrial groove, at the cavoatrial junctions, and on the left atrial wall in the vicinity of the venoatrial junctions (Fig. 78–25). The ligament of Marshall is densely innervated with both parasympathetic and sympathetic inputs. During catheter ablation of atrial fibrillation, ablation of ganglionated plexi in the ligament of Marshal often results in vagal events during radiofrequency application. Pauza et al⁶¹ found up to 50% of all cardiac ganglia on the posterior and posterolateral surfaces of the left atrium, whereas Singh et al⁶² reported that the largest populations of ganglia are adjacent to the sinus and atrioventricular nodes, reflecting different methodologies of studies. The ganglia within each plexus are interconnected by thin nerves, whereas ganglia of adjacent plexuses are also interconnected, forming the meshwork of the epicardiac neural plexus.⁶⁴ Further nerves penetrate into the myocardium to become thinner and thinner and devoid of ganglia.⁶⁵ Autonomic influences from cardiac ganglia are known to exert important roles in both atrial and ventricular arrhythmogensis.^66,67,68 For example, surgical left thoracic stellate ganglionectomy is often employed to treat ventricular arrhythmia storm refractory to medical therapy.⁶⁹

Whereas the structure of the human heart has not changed, understanding of its anatomy has evolved over time as electrophysiologic diagnostic and therapeutic strategies have evolved, each calling for a revisit of cardiac anatomy. In recent decades, the development of electrophysiologic mapping and catheter ablation techniques has outpaced the work of cardiac anatomists in many ways. Nevertheless, better understanding of detailed anatomy is relevant to clinical electrophysiologists, not only to improve outcomes and therapeutic efficacy and avoid or minimize complications during interventional procedures, but also to provide the anatomic background for different substrate and mechanisms behind many heart rhythm disorders.