Cardiology Plus

: 2020  |  Volume : 5  |  Issue : 4  |  Page : 194--202

Cardiac anatomy for electrophysiology

Siew Yen Ho 
 Cardiac Morphology, Royal Brompton Hospital; National Heart & Lung Institute, Imperial College, London, UK

Correspondence Address:
Siew Yen Ho
Royal Brompton and Harefield NHS Foundation Trust, Sydney Street, London SW3 6NP


This article presents some of the important structures relevant to electrophysiological intervention and mapping. Notable structures within the neighborhood of the heart are the respective courses of the esophagus and phrenic nerves. The right atrium contains the sinus node, terminal crest, and cavotricuspid isthmus. The landmarks of the triangle of Koch are a guide to the location of the atrioventricular node, while the central fibrous body and the membranous septum is a guide to the location of the atrioventricular conduction bundle. The arrangement of the ventricular outlets and attachments of semilunar valvar leaflets, as well as the location of aortic sinuses relative to the atria and left ventricular summit, are presented.

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Ho SY. Cardiac anatomy for electrophysiology.Cardiol Plus 2020;5:194-202

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Ho SY. Cardiac anatomy for electrophysiology. Cardiol Plus [serial online] 2020 [cited 2021 Jan 19 ];5:194-202
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A proper understanding of cardiac anatomy has become an integral part of imaging and mapping patients during transcatheter electrophysiological procedures. Following on from our previous article,[1] this article looks at important structures in the vicinity of the heart, components of the cardiac conduction system, and features of the cardiac chambers of particular relevance to interventional electrophysiologists.

 Neighborhood of the Heart

As presented previously,[1] the heart is enclosed inside its fibrous pericardial sac, partially overlapped anteriorly by the lungs, and closely related to the esophagus, thoracic aorta, and branches of the vagus nerve that descend posterior to the left atrium.[2],[3] Moreover, the roof of the left atrium is related to the bifurcation of the pulmonary arteries and to the left bronchus. In patients with dilated left atria, the atrial wall may extend closer to the pulmonary bifurcation.

Located on the surface of the fibrous pericardium are the right and left phrenic nerves, as well as their accompanying pericardiophrenic arteries, which are branches from the internal mammary arteries. The right phrenic nerve descends vertically along the right anterolateral surface of the superior caval vein to become related to the right aspect of the intercaval atrial wall, traversing along the front of the root of the lung, eventually reaching the diaphragm adjacent to the lateral border of the entrance of the inferior caval vein.[3] Along the way, the right phrenic nerve can be <2 mm from the anterior wall of the right superior pulmonary vein, making it vulnerable to damage when ablations are carried out for inappropriate or re-entrant sinus tachycardia or during pulmonary vein isolation in atrial fibrillation [Figure 1]. The left phrenic nerve descends over the left side, close to the aortic arch, then onto the pericardium over the left atrial appendage and over the left ventricle. It may then take variable courses over the left ventricle to the course either over the anterior surface of the ventricle or leftward over the obtuse margin. When implanting pacing leads, it is worth noting that an anterior course of the nerve may overlie the great cardiac vein, while a leftward course is close to the lateral vein or to the left obtuse marginal vein.[3]{Figure 1}

 The Right Atrium

Previously, the forgotten chamber of the heart, the right atrium has since assumed huge importance owing to its role as the first chamber encountered in most procedures. It provides access to the left heart through the atrial septum.[4] Hazards encountered in crossing the septum have been alluded to in our previous article,[1] particularly with regard to the aortic root, for electrophysiologists.[5] Guarding the anterior margin of the inferior caval vein (ICV) orifice is the Eustachian valve, which is usually a thin fibromuscular flap of variable height [Figure 2]a. Occasionally, it is fenestrated, lace-like, and extensive, potentially entrapping an inserted catheter [Figure 2]b. Slightly cephalad and medial is the orifice of the coronary sinus, which is guarded by the variably shaped Thebesian valve. Often, the valve is a small and membrane-like semilunar or fenestrated flap. Rarely, it is large, covering the entire orifice, leaving only a slit-like entry port by its cephalad margin.{Figure 2}

Within the right atrium is an area of utmost significance to electrophysiologists, the triangle of Koch. Its anatomical borders are the hinge line of the septal leaflet anteriorly, the tendon of Todaro posteriorly, and the orifice of the coronary sinus, together with the vestibular wall, inferiorly [Figure 3].[6] While these are anatomical landmarks, it should be noted that the size of Koch's triangle varies from patient to patient.[7],[8] In a patient with a small triangle, it is conceivable that the body of the atrioventricular node (AVN) is much closer to the inferior border than in a patient with a large triangle. The tendon of Todaro is a fibrous strand that extends from the free edge of the Eustachian valve into the muscle of a ridge-like sinus septum (Eustachian ridge) with variable prominence. It inserts into the central fibrous body at the triangle's apex, where the AVN is sited. The so-called “fast pathway” in atrioventricular nodal reentry tachycardia (AVNRT) corresponds to the area of musculature close to this region. The base of the triangle, the vestibular wall between the orifice of the coronary sinus and the hinge of the septal leaflet, is known as the paraseptal isthmus [Figure 3]. This area is often targeted for ablation of the “slow pathway” in AVNRT.{Figure 3}

Lateral to the septal isthmus lies the cavotricuspid isthmus (CTI), which is the area between the anterior border of the ICV orifice and the tricuspid valve [Figure 4].[9] This is the target area for ablation in cases of isthmus dependent atrial flutter, in which the reentrant circuit is confined to the tricuspid annulus with the wave-front progressing either in a counterclockwise or clockwise direction across the CTI. Clockwise from the paraseptal isthmus lies the inferior isthmus, which is the shortest, and then the inferolateral isthmus, which is the longest [Figure 4]. The composition of the CTI varies along its length. Nearest to the tricuspid valve, the vestibular wall is smooth on its endocardial surface and is related epicardially to the right coronary artery.[9],[10] Toward the ICV orifice, the wall is thinner than anteriorly, comprising slender muscle bundles sometimes separated by thin fibrous membranes, especially in its posterior region. In about 10%–47% of individuals, the wall has a pouch named the sub-Eustachian pouch that may cause difficulty in achieving a complete ablation line.[11]{Figure 4}

The terminal crest (crista terminalis) is a distinctive muscle bundle arising from the anteromedial wall.[12] It courses anterior to the orifice of the superior caval vein before descending posterolaterally to reach the right side of the inferior caval vein entrance, where it continues as an array of progressively finer bundles to form part of the inferior CTI.

Along its course, the terminal crest gives rise to near parallel ridges, termed pectinate muscles, that line the endocardial surface of the wall of the appendage. These pectinate muscles give rise to fine branches and terminate at the atrial vestibule. Of relevance to the placement of leads or devices, the appendage wall between the pectinate muscles is thin as parchment [Figure 4]. Furthermore, there is a prominent pectinate muscle, known as the sagittal bundle, arising from the anterosuperior aspect, which demarcates the anteromedial and posterolateral recesses of the appendage.[1] For instance, when passing a catheter from the ICV, entering the posterolateral recess leads to the tip of the appendage, whereas the catheter would be adjacent to the aortic root upon entering the anterosuperior recess.[13],[14]

 The Left Atrium

On the inside, the smooth-walled body of the left atrium receives the pulmonary veins posteriorly, while a small finger-like appendage extending anterosuperiorly is lined by a network of thin muscular ridges together with intervening membranous walls.[1] This appendage has important relationships with both cardiac and extracardiac structures that are relevant to ablation attempts within its lumen or attempts at closing its opening. The ostium overlies the left atrioventricular groove, which contains the circumflex artery and the great cardiac vein, as well as their branches.[4]

The orifice of the appendage is usually oval, separated from the orifices of the left pulmonary veins by a fold in the atrial wall that appears as a ridge (left lateral ridge) on the endocardial surface [Figure 5]a. On the epicardial aspect, this ridge is filled with fibro-fatty tissues, nerve bundles, a branch from the circumflex artery that may supply the sinus node, and the vein of Marshall or its remnant [Figure 5]b. This left lateral ridge varies in profile, width, and extent, with implications for catheter stability when carrying out procedures to isolate the left pulmonary veins.[15]{Figure 5}

There are considerable variations in the number, location, and size of the pulmonary vein orifices. The orifices of the right pulmonary veins are directly adjacent to the plane of the atrial septum. On the endocardial surface, there is no discrete border between the atrium and vein.[16]

The left atrial wall has marked regional variations in thickness and is usually thinner toward the orifices of the pulmonary veins. An anatomical study reported that the muscular thickness of its posterior wall is <3 mm thick in the majority and is significantly thinner in the hearts of patients with known atrial fibrillation.[17] Importantly, both the esophagus and its arterial supply lie behind the posterior wall of the left atria and may be at risk of damage.[2] Descending slightly to the left, between the trachea and vertebral column, the esophagus continues its descent behind the fibrous pericardium that overlies the posterior wall of the left atrium, to the right of the aortic arch and the right side of the descending thoracic aorta [Figure 5]a. An anatomic study demonstrated the relationship of the esophagus with the posterior wall over a length of 30–53 mm (mean 42 ± 7 mm).[2] In many patients, the descending thoracic aorta also runs within the vicinity of the posterior wall, and some cases may be in contact with the fibrous pericardium.

The coronary sinus tracks along the epicardial side of the vestibule forming the posteroinferior wall of the left atrium. There are no anatomical landmarks to separate the vestibule from the pulmonary venous component. However, frequently, a few pits or crevices with thin walls are seen in the inferior atrial wall at the border zone. These may be encountered when constructing ablation lines along the left atrial isthmus, linking the orifice of the left inferior pulmonary vein to the mitral valve hinge line [Figure 5]b.[18]

The anterosuperior wall is usually the thickest because it includes the overlying Bachmann's bundle [Figure 6]a. The anterior wall lies behind the aorta. It usually has a very small thin part, termed the 'unprotected area' by McAlpine,[13] immediately inferior to Bachmann's bundle.{Figure 6}

 Interatrial Connections

Aside from muscular continuity at the site of the atrial septum, there are other connections peripheral to the septum, frequently found as bridges in the subepicardium.[19] The most prominent interatrial bridge is Bachmann's bundle. It is a broad muscular band that runs within the subepicardium and across the interatrial groove onto the anterior walls of both atria. The muscular fibers in Bachmann's bundle, as in the terminal crest, are well aligned, allowing for preferential conduction. Smaller bridges are often present, giving the potential for macro re-entry. Further bridges are often found posteriorly and inferiorly [Figure 6]b.

 The Cardiac Conduction System and Atrioventricular Junction

The cardiac conduction system comprises histologically specialized myocytes that are different from ordinary working myocytes. Beginning with the sinus node, the impulse is generated, which is then transmitted through the myocardium of both atrial chambers to be received by the AVN. The AVN continues into the penetrating atrioventricular bundle of His and on to the branching bundle that divides into the right and left bundle branches which, in turn, branches further into fascicles that further ramify into fine twigs and the Purkinje fiber network, sending the specialized bundles deep into the ventricular walls [Figure 7].[20] The atrial components of the conduction system, the sinus node, and the AVN are in contact with ordinary atrial myocardium whereas the conduction bundles distal to the AVN are ensheathed in fibrous tissue, thereby insulating the specialized myocytes from ordinary ventricular myocardium until at the ends of the Purkinje fiber network, where Purkinje cells make contact with ventricular myocytes.{Figure 7}

 The Sinus Node

The anatomic landmark for the sinus node is the terminal groove close to the superior cavo-atrial junction [Figure 8]. In most individuals, this node is shaped like a tadpole (or comma), with a mean length of 20 + 3 mm in adults, and is located in the anterolateral margin of the junction.[21] In approximately 10%, the node is horse-shoe shaped and located medially as well as laterally in the terminal groove. In tadpole-shaped nodes, the head is positioned subepicardial nearest to the junction, from where the body and the tail of the node proceed to penetrate deeper into the musculature of the terminal crest as the node passes inferiorly to lie closer to the endocardium. This relationship is relevant to procedures for modification of the sinus node for 'inappropriate' sinus tachycardia. The node is supplied by a branch of the right coronary artery in about 60% and from the left coronary artery in about 40%, with dual supply in a small number of hearts. Most frequently, the branch ascends the interatrial groove. When arising from the left coronary artery, it passes anterior to the left atrial appendage or from the left lateral atrial “ridge” to cross along the anterior wall to reach the cavoatrial junction.{Figure 8}

Although the node comprises specialized myocytes within a fibrous matrix [Figure 8], its borders have multiple prongs of specialized myocytes that interdigitate with the musculature of the terminal crest and may even extend into the muscular sleeve surrounding the superior caval vein. This arrangement facilitates communication with ordinary atrial myocardium.

Between the sinus and AVNs, the internodal myocardium does not show insulated tracts of specialized tissues but is composed of ordinary atrial myocardium arranged around the venous and valvar orifices. Bands, like the rim of the oval fossa and the terminal crest, tend to have an orderly longitudinal alignment of their myocardial strands.

 The Atrioventricular Conduction System

Normally, the atrioventricular conduction system, comprising specialized myocytes, provides the only pathway of muscular continuity between atrial and ventricular myocardium. There is an interface of transitional cells between the ordinary atrial myocardium and the histologically specialized cells that make up the AVN [Figure 9]. Transitional cells are arranged to provide anterior, inferior, and deep inputs to the compact AVN. The anterior input sweeps from the anterior margin of the oval fossa into the tricuspid vestibule overlying the AVN. The inferior input approaches the compact node from the musculature near the floor of the coronary sinus and the Eustachian ridge. The deep input bridges the compact node with the left atrial vestibule and inferior rim of the oval fossa.{Figure 9}

Located at the apex of the triangle of Koch, the knob-like compact AVN, approximately 5 mm long and wide and 1 mm thick in adults,[20] has two inferior extensions [Figure 9].[22] The right extension passes in the subendocardium parallel and adjacent to the hinge of the septal leaflet of the tricuspid valve toward about midlevel of the anterior border of the triangle. In some, especially in hearts with small Koch's triangle, it reaches up to the level of the coronary sinus orifice. The left extension projects toward the mitral vestibule.

The compact node extends anterocephalad to become the penetrating atrioventricular conduction bundle of His that then passes leftward through the central fibrous body at a distance of 0.5 + 0.2 (range, 0.1–1.1) mm from the endocardial surface of the right atrium to reach the ventricles. Here, the atrioventricular bundle is directly related to the membranous septum and to the aortic outflow tract. It courses between the membranous septum and often toward the left side of the muscular ventricular septum crest, although there are slight variations.[23] After a short distance, the bundle bifurcates into the left and right bundle branches [Figure 9]. In a few hearts, however, the atrioventricular bundle itself continues beyond the bifurcation as a third bundle, termed the dead-end tract, that runs over the crest of the septum.[24] The left bundle branch usually fans out into three interconnecting fascicles as it descends superficially and further branches in the subepicardium of the left ventricle septal surface as it reaches the apex, continuing into the Purkinje fiber network. In some hearts, thin strands of muscle bridges, termed false tendons, carry fine branches of the left bundle branch from the septum across to the papillary muscles. These have been implicated in idiopathic left ventricular tachycardia.[25]

By contrast, the initial part of the right bundle branch is cord-like. From the branching bundle, it passes through the musculature of the ventricular septum before emerging within the subendocardium at the base of the medial papillary muscle to then descend in a cord-like manner into the septomarginal trabeculation before branching into multiple smaller branches, twigs, and into the Purkinje network toward the apex [Figure 10]. From the apical part of the ventricles, this network spreads extensively in the ventricular walls, reaching its base and the outflow tracts. Triggers of ventricular fibrillation have been mapped to the Purkinje system within the right ventricular outflow tract and have been successfully ablated.{Figure 10}

 The Atrioventricular Junction

The atrioventricular junction provides an insulating tissue plane between the atrial and ventricular myocardium. Breaches of this plane by accessory atrioventricular connections allow a cardiac impulse to bypass the normal atrioventricular conduction system. In brief, there are two anatomic forms of accessory pathways.[26] The first bypasses the entirety of the specialized atrioventricular system and is the most common. Typically, these pathways are composed of muscular strands of varying thickness and extent that connect the atrial wall to the ventricular wall. Some, however, arise from a node-like structure in the vestibular atrial wall as an insulated bundle that then inserts to the ventricular wall. Occasionally, accessory atrioventricular pathways are comprised of muscle around venous walls. The second anatomic form is a muscle bundle that directly connects to the AVN or bundles at various levels, thereby bypassing the normal distribution pattern.[26]

These junctions surround the tricuspid and mitral orifices and, in part, involve the cardiac septum. In human hearts, the mitral-aortic curtain, where the mitral leaflet meets with the aortic valve, rarely contains ventricular myocardium, making it a rare area for accessory pathways. Mainly comprising of fibrous tissue, the mitral-aortic curtain is thickened at each end to form the left and right fibrous trigones. The latter, together with the membranous septum, forms the fibrous accretion, known as the central fibrous body, through which the bundle of His penetrates.

As described in our previous article, the so-called atrioventricular septum is not a true septum because epicardial tissues interpose between atrial and ventricular walls.[1] This is the area of the inferior pyramidal space through which the arterial branch supplying the AVN passes.[27] The true septal area is considerably smaller than alluded to in the previous terminology. In its parietal parts, the right atrioventricular groove tends to be deeper than the left groove. Consequently, right-sided accessory pathways may be located further away from the hinge line of the tricuspid leaflets.[26]

 The Right Ventricle

The supraventricular crest is a muscular fold that separates the tricuspid valve orifice from the pulmonary valve. It is not septal but continues into the subpulmonary muscular infundibulum cephalad and has parietal extensions. When mapping the right ventricular outflow tract, for example, in some cases of idiopathic right ventricular tachycardia, some forms of arrhythmogenic right ventricular cardiomyopathy, or some scar-related ventricular tachycardias, the operator should be aware that the area usually termed septal or high septal is not septal in location per se. Instead, it is part of the muscular subpulmonary infundibulum that raises the pulmonary valve above the septum [Figure 10].[1] More appropriately termed the paraseptal area, this part of the infundibulum lies anterior to the aortic root and is related to the right and left coronary aortic sinuses and therefore, is situated close to the courses of the main coronary arteries. The semilunar hinge lines of the pulmonary leaflets cross the border between the ventricular wall and arterial wall, thereby enclosing in the nadirs of the hinge lines, small areas of ventricular myocardium that form the inferior portions of the pulmonary sinuses.

When mapping within the right ventricle, it is worth noting that the right bundle branch runs very close to the surface of the septomarginal trabeculation and that the moderator band carries one of its branches across the cavity to the anterior wall [Figure 10]. The right ventricular wall is normally considerably thinner than that of the left ventricle, and it diminishes from 2–4 mm to 1 mm or less at its apex.

 The Left Ventricle

Apart from its thicker muscular wall, the LV has a different configuration from the RV. The aortic and mitral valves are hardly ever separated by muscle [Figure 11]a, [Figure 11]b, [Figure 11]c, [Figure 11]d. A common atrioventricular conduction bundle emerges from the central fibrous body in the left ventricle, which is formed by the right fibrous trigone. This is a fibrous accretion located at the right border of aortic-mitral valvar continuity that adjoins the membranous septum lying between the crescentic hinge lines of the right and the noncoronary leaflets of the aortic valve [Figure 11]d.{Figure 11}

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 their earliest activation in the vicinity of the bundle of His area; the so-called “para-Hisian” atrial tachycardias.[28],[29]

While half of the aortic valve is in fibrous continuity with the mitral valve, the other half, consisting of the right coronary sinus and the medial part of the left coronary sinus, contains ventricular myocardium that can be important in some cases of ventricular tachycardia [Figure 11]. Significantly, these are also the sinuses that abut the subpulmonary muscular infundibulum externally. Mapping and intervention of ventricular tachycardia in the outlets requires distinguishing the so-called high septal right ventricular outflow tract types from those arising from the aortic sinuses. The musculature in the aortic sinuses may be a source of repetitive monomorphic ventricular tachycardia. The noncoronary aortic sinus, being immediately adjacent to the paraseptal region of the left and right atriums and close to the superior atrioventricular junction, has been used to map and ablate focal atrial tachycardias that have their earliest activation in the vicinity of the bundle of His area [Figure 12].[29]{Figure 12}

In the recent decade, there is much interest in the LV summit as a site to target a proportion of patients with ventricular arrhythmias. Described by McAlpine[13] as the highest part of the LV, this area is bordered on its epicardial aspect anteriorly by the left anterior descending coronary artery and posteriorly by the circumflex artery, where it enters the left AV groove. Alongside the anterior descending coronary artery runs the anterior interventricular vein, which turns laterally to become the great cardiac vein as it passes through this area [Figure 13]. The part of the summit superior to the vein is usually covered with epicardial fatty tissues limiting epicardial access. The septal and diagonal branches of the coronary artery and veins may be used for mapping.{Figure 13}


This article reviews some of the structures particularly relevant in electrophysiological intervention and mapping. Whether utilizing an epicardial or endocardial approach, it is important to be aware of the structures within the cardiac chambers as well as the structures within the neighborhood of the heart.

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