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Cardiac Anatomy for Catheter Mapping and Ablation of Arrhythmias
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 ).
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