Abstract
Cardiac anatomy remains a cornerstone for successful and safe interventional treatment for cardiac arrhythmias. The importance of appreciating detailed anatomy and correlating the specifics real time with fluoroscopy and intracardiac echocardiography as well as the sensed electrograms have become even more relevant with complex arrhythmia management. In this chapter, the regional anatomy of the heart is discussed in general, and after reviewing the tenets for anatomic correlation, specific points of interest to the principal arrhythmias targeted today in the electrophysiology laboratory are discussed. Where pertinent, key anatomic points for the electrophysiologist are highlighted.
Keywords
ablation, cardiac anatomy, imaging modalities, mapping
Key Points
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Anatomic terminology presently used and pertinent for the interventional electrophysiology defers from classic anatomy descriptions. It is more important for electrophysiologists to be able to correlate the anatomic view in a consistent fashion with real-time imaging.
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Intracardiac ultrasound, along with fluoroscopy, is the primary real-time imaging modality used during mapping and ablation.
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Preoperative transthoracic echocardiography, 3-dimensional computed tomography, and magnetic resonance imaging with registration (fluoroscopy or ultrasound) are an important visual aid for planned procedures.
Cardiac anatomy remains a cornerstone for successful and safe interventional treatment for cardiac arrhythmias. The importance of appreciating detailed anatomy and correlating the specifics in real time, with fluoroscopy and intracardiac echocardiography as well as the sensed electrograms, has become even more relevant with complex arrhythmia management. In this chapter, the regional anatomy of the heart is discussed in general, and after reviewing the tenets for anatomic correlation, specific points of interest to the principal arrhythmias targeted today in the electrophysiology laboratory are discussed. Where pertinent, key anatomic points for the electrophysiologist are highlighted.
Basic Orientation and Terminology
The heart is situated within the middle mediastinum with its base directed superiorly marking the horizontal plane at the sternal angle, right border between the third and sixth ribs, left border between the second rib and fifth intercostal space, and apex projecting toward the left in an anterior, inferior direction. Structurally the cardiac chambers consist of an inner endocardium, muscular myocardium, and a superficial epicardial layer; on the outside, the heart is surrounded by the pericardium.
The terminology historically used for anatomic orientation of the cardiac structures was developed by cardiac pathologists and anatomists and used by cardiac surgeons. With advances in transcatheter ablation techniques, electrophysiologists typically navigate the heart using catheters under fluoroscopic guidance, rather than observing the heart in the direct visual field. The previously used terminology was often inaccurate for the heart in its undisturbed anatomic position. Thus in 1999, an attitudinally appropriate nomenclature was proposed, which considers the orientation of the heart within the chest in vivo ( Fig. 5.1 ). This terminology is consistent when used for the electrocardiogram, the fluoroscopic projections, and other adjunctive imaging modalities. Under this scheme the body and the anatomically situated heart within the chest are viewed in the standing position and described by three axes, namely, the superior–inferior, anterior–posterior, and right–left axes.
Imaging Modalities for Mapping And ablation
Computed tomography (CT), magnetic resonance imaging (MRI), and echocardiography allow the operator to visualize the anatomy before ablation. The fundamental imaging modalities for real-time guidance and trouble-shooting during catheter manipulation and ablation include fluoroscopy and intracardiac echocardiography (ICE).
Fluoroscopy
Fluoroscopy is a noninvasive and relatively inexpensive method that offers real-time imaging. The drawbacks of this method are its inability to visualize and discriminate soft tissues such as the myocardium and contiguous structures, and it exposes both the patient and the operator to ionizing radiation. Fluoroscopy provides overlapping 2-dimensional projections of 3-dimensional structures, and the views obtained are operator dependent, which may obfuscate interpretation. Fluoroscopic orientation can be determined by a number of anatomic references in the image such as the cardiac silhouette, the calcified coronary artery, the vertebral column, the diaphragm, the mediastinum, the fat stripe demarcating the atrioventricular (AV) groove, implanted devices, and orientation of standard catheters ( 
).
Two routinely used fluoroscopic views of the heart are the right anterior oblique (RAO) and left anterior oblique (LAO; Fig. 5.2 ). The RAO projection portrays a profile view looking from the side of the heart and allows for good AV differentiation. The ventricle is anterior (closer to the sternum) than the atrium (closer to the vertebral column) in the RAO view. However, in RAO there is an overlap of the right and left chambers, and the septal and lateral aspects of the chambers; such spatial differentiation is not possible. The LAO projection looks at the heart front from the apex toward the base, thus allowing for good left versus right and septal versus lateral differentiation. However, it does not allow discrimination between the atria and ventricles as they are superimposed. Both RAO and LAO provide superior–inferior delineation. Catheters may be positioned at specific locations to serve as fluoroscopic landmarks for this purpose.
Intracardiac Echocardiography
Intracardiac cardiac ultrasonography using ICE is now commonly available. ICE can supplement fluoroscopy by accurately delineating cardiac structures such as the interatrial septum, Eustachian ridge, cardiac valves, moderator band, papillary muscles, false tendons, and coronary artery ostia ( 
). In addition, it allows for visualizing catheter contact and transmural lesion formation in real time ( 
). It enables the operator to avoid complications and to identify them when they arise, for example, formation of thrombus on catheters, accumulation of pericardial effusion, damage to valve leaflets, and decrease in left ventricular (LV) systolic function ( 
). Three-dimensional ICE has been described but has not yet been adopted for routine electrophysiology procedure.
Structural Anatomy of the Left and Right Atria
Right Atrium
The left and the right atria (LA and RA, respectively) have pronounced gross and histological differences, and both chambers are intricately involved in initiation and propagation of cardiac arrhythmogenesis. The RA is marked laterally by a line joining the superior and inferior vena cava (IVC) with its superior border terminating medially in the RA appendage (RAA). Medially, it is bound by the right AV groove and continues inferiorly with the inferior margin of the heart. The interatrial septum forms the posterior wall of the RA superior to the opening of the coronary sinus (CS). Inferiorly, a depression known as the fossa ovalis marks the remnant of the primary septum of the fetal heart. The upper margin remains crescent shaped and is identified as the limbus ( Fig. 5.3 ).
Sinus Venarum
The smooth-walled sinus venarum or the intercaval area is the rightward portion of the posterior RA, encompassing the posterior RA wall between the orifices of the SVC and IVC. The venous component of the RA continues medially as the interatrial septum that has a central rounded thin-walled fossa ovalis. The RA vestibule is the smooth-walled area on the left aspect of the RA between the venous component posteriorly and the tricuspid valve orifice anteriorly.
Right Atrium Appendage
The RAA is the trabeculated part of the RA formed by the pectinated muscle extending anteriorly from the crista terminalis. It is the most anterior and medial part of the RA with the tip of the appendage projecting anteriorly and leftward over the aortic root ( Fig. 5.4 ). The appendage wall is nonuniform with variable myocardial thickness and orientation. The inferior vestibular part of the appendage has a thinner wall that overlies the epicardial fat surrounding the right coronary artery (RCA) coursing around the tricuspid annulus. The RAA is larger and more muscular than the left atrial appendage (LAA) and has a triangular shape. The arrangement of the pectinate muscle fibers in the RAA is quite variable. Loukas and coworkers have suggested that 10% of RAA have an arborizing arrangement of pectinates.
Clinical Correlation . RAA thrombus can form in patients with atrial fibrillation, although this is far less frequent than LAA thrombus, potentially because of a wider mouth, more musculature, and absence of multilobularity in the RAA. The complex arrangement of pectinate muscles in the RAA in a minority of individuals may predispose to catheter entrapment and perforation. Accessory pathways that connect between the RAA and the ventricle have been reported.
Superior Vena Cava
Superior vena cava (SVC) has a close relation to the right superior pulmonary vein (PV) posteriorly and the ascending aorta medially (see Fig. 5.4 ). The RA myocardium has extensions into the SVC, although typically muscle is absent in the IVC. Muscle sleeves are seen in three-quarters of SVCs, extending a mean distance of 4 mm (3.8 ± 9.4 mm) above the orifice. Two-fifths of extensions into the SVC have a symmetric circumferential sleeve of muscle rather than an isolated projection on one side. Isolating the SVC might thus require circumferential ablation. The azygos vein drains into the posterior SVC for an average distance of 2.3 cm from the SVC–RA junction. Approximately 6% the myocardial sleeves extend all the way into the azygos vein.
Clinical Correlation . A study suggested that the SVC sleeve length may be associated with risk of inducible SVC fibrillation. Catheter mapping in the SVC may demonstrate the signals of nearby structures. Similarly, far field SVC signals can be recorded when mapping the anterior and superior aspects of the right superior PV (see Fig. 5.4 ). The substrate for atrial arrhythmias or atrial fibrillation may reside in the azygos veins as well.
Superior Vena Cava–Right Atrium Junction
The anterior aspect of the SVC and RA junction is complex with variable thickness and direction of the muscle fibers. The sagittal bundle, composed of one or two prominent pectinates, extends anteriorly from the superior part of the crista terminalis and branches into the RAA, potentially ending in a ring-like formation at the tip of the appendage. In addition to the crista terminalis and the sagittal bundle, an arcuate ridge can be present, extending from the crista terminalis to the superior limbus of the fossa ovalis on the interatrial septum (see Fig. 5.3 ).
Clinical Correlation . The sagittal bundle is a pathway for preferential conduction from the sinus node into the RAA. Giving the complexity of the conduction system, the signal recorded in this area can be complex. Along with the actuate ridge, it can be the source of automatic tachycardias, and variability in thickness of these bundles may predispose to conduction abnormalities within the RA that are important for reentrant arrhythmias. Successful ablation at the arcuate ridge in a case of inappropriate sinus tachycardia has been described previously.
Crista Terminalis
The crista terminalis, also referred to as the terminal crest, is a C-shaped muscular ridge on the lateral endocardial aspect of the RA (see Fig. 5.3 ). It separates the smooth venous component of the RA, formed by the sinus of vena cava (sinus venarum) posteriorly, from the rough pectinate musculature of the RAA anteriorly. Corresponding to the crista terminalis on the lateral epicardial aspect of the RA is a superoinferiorly oriented groove filled with adipose tissue, the sulcus terminalis. The sulcus terminalis overlies the location of the sinoatrial (SA) node in the musculature of the crista terminalis near the orifice of the SVC. The crista terminalis originates at the interatrial grove posterior to the ascending aorta where its fibers coalesce with those of the Bachmann bundle. It courses laterally and inferiorly anterior to the SVC orifice and eventually branches out as trabeculations into the cavotricuspid isthmus (CTI) anterior to the IVC orifice.
Clinical Correlation . Myocytes are oriented along the long axis of the crista, thereby facilitating preferential conduction in the longitudinal direction. However, the interlacing bundles of the crista terminalis trabeculations predispose to conduction delay and block transversely across the crista, which can set up the conditions for intraatrial reentry (focal atrial tachycardia). It also serves as an anatomic barrier to conduction traversing across the lateral RA wall during most typical atrial flutters.
Cavotricuspid Isthmus and Adjacent Structures
The CTI is a complex region that comprises of important landmarks for ablation of CTI-dependent atrial flutter ( Fig 5.5 ). There is the Eustachian ridge, which separates the IVC and the smooth-walled sinus venarum posteriorly and the CS ostium anteriorly. Anteriorly, there is the inferior aspect of the tricuspid annulus. Cabrera and coworkers divided the CTI into three separate sections (lateral, central, and paraseptal), with the myocardium being thinnest at the central region and thickest at the paraseptal isthmus. Its mid portion is also the narrowest part, serving as an optimal location targeted for ablation. In contrast, the lateral section, which is a continuation of pectinate muscles branching out from the crista terminalis, is rich of trabeculation that may impede satisfactory ablation. In half the cases, the RCA course is within 4 mm of the lateral isthmus.
The paraseptal isthmus region is bounded by the CS ostium laterally. Just lateral to the CS ostium there is a pouch-like depression called the sub-Eustachian pouch, between the Eustachian ridge and the tricuspid annulus. There is a lot of variability in the depth of the sub-Eustachian pouch that sometimes can be 10 mm deep or more ( Fig. 5.6 ). This may lead to unsatisfactory catheter placement and suboptimal ablation with failure to get bidirectional CTI block. In addition, the distal RCA and its posterolateral branch may approximate the paraseptal isthmus vestibular endocardium by less than 3 mm, an important relation to remember to avoid arterial damage while ablating in this area. Furthermore, in 10% of cases the inferior AV nodal extensions reach the paraseptal isthmus region.
Clinical Correlation. The CTI has anisotropic conduction properties with the muscle fibers oriented parallel to the tricuspid annulus. It is a natural location for ablation to achieve conduction block and interrupt the atrial flutter circuit. For routine typical atrial flutter cases, the central isthmus may be the best site for linear ablation between the tricuspid annulus and the IVC because of its shorter dimension, thinner wall, and greater distance from the RCA and the AV node.
Eustachian Valve
During fetal life, blood is oxygenated by the placenta and returns to the heart by the IVC. The Eustachian valve at the orifice of the IVC directs this oxygenated blood across the foramen ovale to the left atrium (LA) and the systemic circulation. The Eustachian valve gradually degenerates completely or remains as a small vestigial remnant at the Eustachian ridge on the anterior lip of the IVC ostium.
Clinical Correlation . Occasionally, the Eustachian valve persists as a web-like Chiari network, which may be indistinguishable from an intracardiac thrombus. Extreme form of its remnant is known as cor triatriatum, which can be an impediment to passage and manipulation of catheters.
Thebesian Valve
A crescent-shaped thebesian valve, which covers the posterior aspect of the CS ostium just left and anterior of the Eustachian ridge, is visible in over 60% of autopsy hearts. (see Fig. 5.5 ). The thebesian valve is variable in size and orientation, although it preferentially covers the posterior aspect of the CS ostium just left and anterior of the Eustachian ridge.
Clinical Correlation . Rarely, the thebesian valve can be circumferential around the ostium of the CS causing obstruction and complicating cannulation of the CS.
Triangle of Koch
The tendon of Todaro is a superiorly and slightly anteriorly directed continuation of the Eustachian ridge across the interatrial septum that extends to the central fibrous body ( Fig. 5.7 ). As the actual tendon is difficult to identify, a surrogate line connecting the Eustachian ridge/valve and the central fibrous body may be used to identify its location. The triangle of Koch is an important anatomic landmark located on the anterior aspect of the septal RA wall. The three borders of the triangle of Koch are formed by (1) the base of the septal leaflet of the tricuspid valve (anterior and leftward), (2) the tendon of Todaro (posterior and rightward), and (3) the superior lip of the CS ostium. The triangle of Koch is the site of the compact AV node and its continuation with the bundle of His, which penetrates the membranous septum anterior to the central fibrous body. The AV node consists of slow and fats-pathway extensions.
Clinical Correlation . The inferior AV nodal extension or the slow pathway lies inferiorly in the triangle of Koch and is frequently targeted by ablation in this region at next to the CS ostium for the treatment of AV nodal reentry tachycardia and a nodofascicular accessory pathway. To reduce risk of AV block, caution should be exercised when mapping or ablating near the fast pathway, which is behind (posterior and to the right of) the tendon of Todaro and possibly also on the left atrial septum ( Fig. 5.8 ).
Interatrial Septum
The interatrial septum is anterosuperior to the aortic root and posterior to the epicardial invagination by adipose and fibrous tissues between the sinus of the venae cava in the RA and the right-sided PVs entering the LA (See Fig. 5.6 ). Thus the true interatrial septum is in fact small and comprises of the area of fossa ovalis, which serves as a good site for transseptal puncture. Immediately anterior to the fossa ovalis is the ascending aorta ( Fig. 5.9 ). In a minority of patients (34% aged <30, 25% aged 31–80, and 20% aged >80), a patent foramen ovale persists. Although a patent foramen ovale (PFO) can serve as a natural site for the transseptal approach, its orientation may make the sheath or the catheter too anterior to the desirable position. Making a transseptal approach is generally preferred.
Clinical Correlation . The relationships and anatomy of the interatrial septum have many electrophysiologic implications. The area of fossa ovalis is the desirable site for transseptal puncture. During the transseptal puncture, the system consisting of needle and sheath is being pulled back from the SVC, where the first drop is at the level of SVC–RA junction and the second drop is from the ascending aorta/ the right sinus of Valsalva (RSOV) to fossa ovalis. Puncture through other regions of the interatrial septum, such as across the superior limbus, will actually exit the heart before reentering the LA.
Bachmann Bundle
The Bachmann bundle or the interatrial subepicardial tract is a bundle of parallel myocardial strands connecting the RAA and LAA. The bundle runs through the superior aspect of the atria in between the SVC, superior to the RAA and the ascending aorta, subepicardially across the interatrial groove, and superior to the LA roof before reaching the neck of the LAA. In the vicinity of the LA, the superior part traverses the left lateral ridge in front of the orifices of the left superior PV.
Clinical Correlation . Disruption of the bundle’s structure causes interatrial conduction block, which can be an arrhythmogenesis site for various atrial tachyarrhythmias and related to electromechanical dysfunction of the LA.
Left Atrium
The LA is positioned posterior and forms most of the base of the heart. In situ in the chest, the LA is posterior, leftward, and slightly superior in relation to the RA. The aortic root runs along the anterior aspect of the interatrial septum (see Fig. 5.9 ). Left of the aortic root, the main pulmonary artery trunk is anterosuperior to the LA with the pulmonary artery bifurcation and the proximal right pulmonary artery forming the roof of the LA. The tracheal bifurcation is posterosuperior to the LA with the main right and left bronchi superior to the ipsilateral PVs going to the hila of the lungs. Below the tracheal bifurcation, the esophagus has an immediate relationship with the posterior wall of the LA and the PV. The esophagus descends along the fibrous pericardium and is separated from the LA posterior wall only by the epicardial space (specifically the oblique sinus), which contains fibrous, adipose tissues, esophageal arteries lymph nodes, and the vagal nerve. The anterior aspect of the descending thoracic aorta contacts three-quarters of the fibrous pericardium, the LA posterior wall, or left inferior PVs as it dives inferiorly in the chest.
Compared with its right-sided counterpart, the LA is structurally less complex and has a smooth wall. Overall, the pectinates are much less prominent in the LA than in the RA and are located almost exclusively in the LA appendage. The vestibule of the LA is larger than that of the RA, and it encompasses the atrial wall between the venous parts posteriorly and the mitral valve annulus anteriorly. The vestibule may often have pits and crevices including the openings of small cardiac veins (foramina venarum minimarum), which can occasionally be large enough to trap the ablation catheter. The LA posterior wall tends to be thinner in those with atrial fibrillation, being the thinnest in the middle, between the inferior PV ostia. This is most important when ablating in this area because of the proximity of adjacent vital structures such as the esophagus and vagal nerve plexus. The roof of the LA is the thinnest region of the LA , with various morphologies including flat, concave, or convex types. Just above the LA roof is a transverse sinus that separates the LA and the pulmonary artery. Literally, the LA roof forms the floor of the transverse sinus (see the epicardial section below). This sinus contains the Bachman bundle, and the sinoatrial nodal artery branched from the left circumflex artery (LCX; in ∼27% of general population).
Clinical Correlation . Ablation of the posterior wall of the LA and PV isolation carry a small risk of injuries to the adjacent structures. Esophageal injury and LA-esophageal fistula can be found in both radiofrequency ablation and cryoablation techniques. Air from the esophagus can reach the oblique sinus, evident as a pneumopericardium on a chest x-ray, and is rarely seen in the LA on CT. Because of many variations of the esophageal position and movement, it is critical to mark and monitor the position of the esophagus during the procedure ( Fig. 5.10 ). A rarer complication, atriobrachial fistula after atrial fibrillation has been reported. There are dense vagal nerve fibers and plexuses that cross the esophagus in regions relevant to ablation in the LA. Injury of the left vagus in this region could result in upper gut dysmotility.
Left Atrial Appendage
The LAA is a small multilobular structure with a finger-like extension from the anterior part of the LA to the AV sulcus and LV free wall. The body of the LAA is anterior to the LA and parallel to the left PVs. The tip is in close relation to the pulmonary artery, right ventricular (RV) outflow tract, and LV free wall ( Fig. 5.11 ). Inferior to the LAA and the ostium, there is the LCX, and the great cardiac vein (GCV) courses along the AV groove and the mitral valve. The left phrenic nerve also courses posterolaterally. There is the Bachmann bundle, which ends near the LAA neck. Even though the LAA is trabeculated with comb-like folds of pectinate muscles, its wall is actually thin (∼1 mm). Slightly medial to the LAA ostium and neck there is the ligament of Marshall (LOM), or a remnant of the left SVC, which is an epicardial landmark between the left lateral aspect and left superior PV (see Fig. 5.11 ). On the endocardial aspect of the LOM, the invagination of the ligament causes a prominent landmark called the Q-tip, or Coumadin ridge, or left atrial endocardial ridge. This structure lies between the left-sided PVs posteriorly and the LAA anteriorly. The left lateral ridge differs from the Eustachian ridge as it is primarily myocardial without significant fibro-fatty components. In general, there are many variations on the LAA morphology, size, shape, and number of lobes.
Clinical Correlation . The LAA is the most common source of thrombus in patients with atrial fibrillation, which is likely as a result of its tubular anatomy, a small mouth, and the presence of multiple lobes that promote stasis. There has been an increased interest in potential interventions of the LAA for stroke prevention. The LA appendage can be arrhythmogenic for atrial tachycardia or a maintenance circuit for atrial fibrillation. There is a recent study proposing LAA isolation to improve success rate for atrial fibrillation ablation. The vein of Marshall (VOM) is of importance to cardiac electrophysiologists because muscle tissue in the vein and the surrounding autonomic network may be the trigger for atrial fibrillation and can serve as a passageway for macroreentrant atrial flutter circuits following linear ablation of the mitral isthmus or surgical maze. In such cases, ablation of the VOM may be necessary. As a result of dense cardiac autonomic ganglion located at the epicardial surface of the VOM, ablation at this site or at the left lateral ridge may cause an autonomic disturbance such as transient bradycardia or high grade AV block during the procedure.
Pulmonary Veins
Commonly there are four PV ostia; two each draining the right and left lungs. However, variations in PV anatomy are present in over 25% patients, more frequently involving the right-sided PVs and up to six PVs have been reported. A number of PV variants exist, including left common PV, additional middle PVs, and in some instances, a vein may enter directly into the roof of the RA (right top PV). The left-sided PV ostia are usually slightly superior to the right-sided ostia. The superior PV ostia are also anterior to the inferior PV ostia. Normally the right superior PV courses close to the RA–SVC junction anteriorly. The right inferior PV passes posteriorly to the intercaval area of the RA. The orifice to the left superior PV is located immediately posterior to the LAA, and the anterior wall of the vein close to the ostium juxtaposes the posterior wall of the appendage (see Fig. 5.11 ).
Both the presence and extent of myocardial extensions into the PVs have been well documented. Myocardial sleeves are thicker and extend farther in the superior PVs compared with the inferior PVs. These sleeves usually extend farthest in the left superior PV, followed in order of decreasing length by the right superior, left inferior, and right inferior veins. In each PV, the myocardial sleeves are thickest proximal to the heart at the venoatrial junction, and become thinner more distally along the vein.
Clinical Correlation . These PV myocardial extensions result in predisposition for arrhythmogenesis from these areas. Because of the interindividual variability in PV anatomy, awareness of variable anatomic patterns of the PVs in different individuals and carefully examining preprocedural MRI/CT, and ICE are important to maximize the effectiveness of catheter ablation of atrial fibrillation and to avoid the complication of PV stenosis as a result of ablation in the veins.
Mitral Isthmus
The mitral isthmus is an area of the posteroinferior left atrial wall spanning between the left inferior PV ostium and the mitral annulus ( Fig. 5.12 ) that is often a target of linear ablation for atrial fibrillation and left atrial flutter. The thickness of the atrial myocardium progressively decreases down the isthmus closer to the mitral annulus. When the LA dilates, the isthmus can be more prominent. A distinct pouch (more commonly multiple pseudo diverticula) containing myocardial tissue pectinate muscles on its floor, similar to a sub-Eustachian pouch, may be found in up to 20% of patients. In this condition, the mitral valve leaflets insert onto the atrial myocardium. On the epicardial surface of the heart, the GCV and the distal LCX course near the mitral annulus.
Clinical Correlation . Blood flow in the nearby vessels can cause convective cooling, or a heat sink, preventing adequate lesion delivery with radiofrequency ablation during mitral isthmus ablation. When epicardial ablation through the GCV in addition to endocardial ablation is required to overcome the heat sink, care to avoid injury to the LCX is required. The relationship between the GCV and LCX is variable and coronary angiogram is often required. Mitral valve disjunction is a condition where the mitral valve leaflets insert onto the atrial myocardium ( 
). Its prevalence is thought to be more common and often underrecognized. To achieve mitral isthmus line ablation with this condition, it may be necessary to advance the catheter into the left ventricle and retroflex to reach the mitral valve leaflet.
Structural Anatomy of the Atrioventricular Junction and the Central Fibrous Body
There is a lack of myocardial continuity at the AV junction. The fibrous tricuspid and mitral annuli (AV rings) are a part of the fibrous skeleton of the heart and electrically insulate atria and ventricles from each other. The AV junctions have been defined to include the complex septal and paraseptal areas containing the AV conduction structures and the AV rings. The mitral annulus is slightly smaller and lies posterior and slightly superior to the tricuspid annulus. Epicardially, the tricuspid groove is deeper than the mitral groove.
The central fibrous body or the right fibrous trigone is an area of fibrous cardiac skeleton where the aortic, mitral, and tricuspid valves meet (see Figs 5.7 and 5.13 ). It is the confluence of the commissures between the septal and anterior tricuspid leaflets, the right and noncoronary aortic cusps, and the mitral leaflets. The tricuspid and mitral AV junctions are confluent at the area of the AV septum. Because of the slight apical displacement of the tricuspid annulus compared with the mitral annulus, the AV septum separates the RA from the LV. Some authors have divided the septal annular region as anterior, mid, and posterior (or attitudinally correct superior, mid, and inferior), whereas others have restricted the term septal to the true membranous septum and referred to other areas as paraseptal. Just slightly anterior to the central fibrous body, there is the membranous septum where the bundle of His penetrates the AV junction at this septum. On the right, the superoseptal AV junction is in the region of the confluence of the supraventricular crest of the RV with the membranous septum. In the left superior paraseptal region, there is fibrous tissue between the noncoronary aortic cusp and the anterior mitral leaflet (aorto-mitral continuity [AMC] or intervalvular fibrosa) without any atrial or ventricular muscular structure.
Clinical Correlation . Accessory AV pathways exist all around the AV junction (see Fig. 5.13 ). The left anteroseptal region between the noncoronary aortic cusp and the anterior mitral leaflet (AMC or intervalvular fibrosa) is without any atrial or ventricular muscular structure and extremely rarely described as having accessory AV connections. To minimize risk of AV block, the anteroseptal pathways in the peri-Hisian region can be approached from the left paraseptum or noncoronary cusp. The posteroseptal pathways are off the true septum and are accessible from the CS and its branches, especially when on the left posteroseptal region.
Inferior Pyramidal Space
The inferior pyramidal space, or crux of the heart, is the extracardiac, fibroadipose, opened space located at the posterior septal region between the four cardiac chambers. This is a complex small region bounded anteriorly by the inferomedial RA and the posterosuperior LV, and the LA and the RA walls posteriorly ( Fig. 5.14 ). The CS courses through the base of the inferior pyramidal space on the left side. The space contains the AV nodal artery running near the RA wall within 0.5 to 5 mm of the CS orifice, which is located 2.3 ± 0.4 cm superior and medial to the space. The central fibrous body lies above the pyramidal space. MCV, also known as the posterior interventricular vein, courses with the posterior ascending artery and enters the the CS on the right side of the pyramidal space. Near the junction of the MCV and CS, there are dense cardiac autonomic plexuses and ganglion.
