Cardiac Anatomy and Pathology

Cardiac Anatomy and Pathology

1  Cardiac Anatomy and Pathology SIEW YEN HO I t has been 500 years since the birth of Andreas Vesalius, the father of anatomy. In his most importa...

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Cardiac Anatomy and Pathology SIEW YEN HO

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t has been 500 years since the birth of Andreas Vesalius, the father of anatomy. In his most important and influential work, “De Humani Corporis Fabrica Libri Septem,” published in seven volumes between 1543 and 1555, Vesalius demonstrated the importance of dissection of cadavers for detailed knowledge of human anatomy. Historically, anatomy is a fundamental part of the medical curriculum. These days, to greater or lesser degrees, traditional dissection has been replaced by computer-based interactive learning, problembased learning, plastinated specimens, and so on,1,2 leading to concerns in some departments.3,4 Nevertheless, the basic anatomy of the heart has remained unchanged over the centuries. It is as relevant to the understanding of cardiac diseases for diagnosis and therapy then as now. However, the perspectives from which clinicians can view and approach the heart have evolved, especially over the past few decades, alongside changes in therapeutic strategies and rapid advances in imaging and interventional technologies. The importance of revisiting cardiac anatomy is particularly relevant to operators who need to access the heart chambers and great vessels. This chapter on cardiac anatomy aims to provide an overview of the fundamentals of the normally structured heart with emphasis on components relevant to device implantation. Much of the information is basic revision for the experienced operator but, it is hoped, will help the beginner to build upon the modern day anatomic curriculum. Because the heart structures are not transparent, the figures accompanying the chapter are orientated mostly in anatomic fashion instead of attempting to simulate precisely the views as portrayed in fluoroscopic projections.

THE HEART IN THE CHEST The topographic anatomy of the heart is crucial to practitioners, as highlighted by Walmsley5 in 1958 when he commented that descriptions of cardiac anatomy disregard the cardinal principle of using terms in relation to anatomic position. He noted that many textual descriptions and innumerable figures throughout the medical literature view the heart as if it would be held in the hand, with the atria above the ventricles and the left and right hearts lying alongside each other in a sagittal plane providing basic and false concepts that have caused untold confusion in the past. For instance, by standing the heart on its apex, it is easy to see how the anterior and posterior descending coronary arteries acquired their names. It was MacAlpine6 who, in emphasizing the importance of describing the heart in its anatomic location for appropriate clinical correlations, termed the orientation of the heart seen in its living condition as attitudinal. The names of the chambers, however, remain unaltered, although right heart chambers are not strictly to the right or left heart chambers strictly to the left. Viewed from the anteroposterior perspective, the cardiac silhouette is generally taken to be trapezoidal in shape. The rib cage provides good markers for charting the cardiac silhouette. The normal position of the cardiac apex is generally taken to be in the fifth intercostal space in the midclavicular line. The lower border is a nearly horizontal line in the area of the left sixth rib to the right sixth costal cartilage. The upper border is hidden behind the sternum at the level of the second and third cartilages. The right margin of the heart peeps out behind the right border of the sternum between the right third and sixth cartilages. In the infant, the upper part of the cardiac shadow is broad due to the prominence of the overlying thymus gland.

Inferior to the thymus, a fibrous pericardial sac encloses the mass of the heart. The sac has cuff-like attachments around the adventitia of the great arteries and veins as they enter or emerge from the heart. The pericardial cavity is contained between the double-layered serous pericardium. The parietal pericardium is adherent to the fibrous pericardium, whereas the visceral layer is densely adherent to the cardiac surface forming the epicardium. Because of the contours of the heart and great arteries, there exist two recesses within the pericardial cavity. These are the transverse and oblique sinuses. The transverse sinus occupies the inner heart curvature and lies between the posterior surface of the great arteries and the anterior surface of the atrial chambers. The reflection of the serous pericardium around the four pulmonary veins and the inferior caval vein forms the oblique sinus. It is important to note that the right phrenic nerve descends along the lateral aspect of the superior caval vein to pass in front of the hilum of the right lung and then along the fibrous pericardium lateral to the right atrium to reach the diaphragm (Fig. 1-1A). It is closely related to the anterior wall of the right upper pulmonary vein.7 The left phrenic nerve descends on the left side close to the aortic arch and onto the fibrous pericardium over the left atrial appendage and the left ventricle, taking variable courses along the anterior and lateral aspects in close relationship to the left coronary veins and great cardiac vein.8 When the pericardium is removed, the major part of the heart visible from the front is the ventricular mass. Here, the morphologically right ventricle occupies the greater part (Fig. 1-1B). The left ventricle appears only as a narrow slip along the left cardiac border. The shape of the heart is generally likened to a pyramid with a base and an apex. The apex points downward, forward, and to the left, whereas the base faces posteriorly and to the right. The cardiac apex is usually represented by the vortex of the left ventricle, but the cardiac base is less well defined. The anatomic base is formed mainly by the left atrium receiving the pulmonary veins and to a small extent by the posterior part of the right atrium. The base in clinical practice, however, refers to the portion of the heart near the parasternal parts of the second intercostal spaces. The cardiac surfaces are described as the sternocostal, diaphragmatic, left, and right. The sternocostal surface is covered anteriorly by the sternum and pleurae. The diaphragmatic surface is horizontally orientated. The sharp angle formed mainly by the right ventricle and occupying the lower heart border is the acute margin of the heart. The rounded obtuse margin of the heart is formed mainly by the left ventricle to the left of the sternocostal surface.

RELATIONSHIPS OF CARDIAC CHAMBERS The relative positions of the cardiac chambers and great vessels are readily displayed with an endocast (Fig. 1-2). The so-called right heart chambers are anterior and to the right (see Fig. 1-2A, B). From the frontal aspect, the right border of the cardiac silhouette is formed exclusively by the right atrium. The superior and inferior caval veins join the upper and lower margins of the venous component of the right atrium. The inferior cardiac border lying nearly horizontally on the diaphragm is marked by the right ventricle. The sloping left border is made up of the left ventricle, but it merges with the pulmonary trunk near the upper border. Apart from the tip of its appendage curling around the edge of the pulmonary trunk, the left atrium is not visible

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SECTION 1  Basic Principles

Aorta Pulm trunk Left phrenic nerve

RA

use Ob t

Right phrenic nerve

gin ma r

RV

LV

Ac

ute

ma

rg

A

in

B

Figure 1-1  A, Diagram showing frontal view of the heart covered by its fibrous pericardium (gray shading) and the courses of the right and left phrenic nerves. B, Frontal view of heart specimen demonstrating that little of the so-called left heart chambers is visible. LV, Left ventricle; RA, right atrium; RV, right ventricle.

from the frontal aspect. Being the most posterior of the cardiac chambers, the left atrium lies directly in front of the esophagus. A key feature of the anatomy of the normally structured heart is the central location of the aortic root (see Fig. 1-2A, B). It springs from the left ventricle to be located anterior to both atrial chambers and the atrial septum, and posterior to the right ventricular outflow tract. Any procedures carried out in the aortic root can potentially impact upon all the other cardiac chambers and valves, the major coronary arteries, and the atrioventricular conduction system. Viewing the endocast from the right lateral aspect shows the location of the right atrium posterior and to the right of the right ventricle (see Fig. 1-2C). The plane of the right atrioventricular junction containing the annular insertion of the tricuspid valve is orientated nearly vertically. The right ventricle sweeps from posterior to anterior and passes cephalad such that its outflow tract lies superior to that of the left ventricle (see Fig. 1-2A). This crossover relationship between right and left ventricular outflow tracts is due to the left outflow tract being directed rightward and cephalad in a projection toward the right shoulder. The plane of the pulmonary valve is nearly horizontal and located well cephalad, making the pulmonary valve the most superiorly situated of the cardiac valves. The aortic valve is adjacent but located to the right and posterior in relation to the pulmonary valve. The two sets of semilunar valves are not at the same level. Instead, the plane of the aortic valve tilts inferiorly at an angle to the pulmonary valve, and its orifice is directed not only upward but also rightward at an angle of at least 45 degrees to the median plane.9,10 From the left lateral aspect, the left ventricle can be seen projecting forward and leftward with the apex directed inferiorly (see Fig. 1-2D). The finger-like left atrial appendage usually points anterosuperiorly from the left anterior side of the body of the left atrium. Usually, there are four pulmonary veins entering the posterior aspect of the left atrium, but variations are not uncommon. The great cardiac vein and its continuation into the coronary sinus pass along the epicardial side of the inferior atrial wall. This venous channel is a good guide to the left parietal border of the heart in left anterior oblique view. Like the tricuspid valve, the plane of the mitral orifice is not horizontal. In right anterior oblique view the plane of the tricuspid orifice forms an angle of 25 degrees with that of the mitral orifice.10 Although the anterosuperior parts of these planes merge, they diverge and are

separate inferoposteriorly. Viewed from the anteroposterior perspective, the orifices of the four cardiac valves are like a cascade of plates. The pulmonary valve is situated most superiorly in a nearly horizontal plane, whereas the aortic valve slopes rightward and posteriorly from the pulmonary valve. The aortic valve is sandwiched by the D-shaped mitral orifice, which lies leftward and posteriorly, and the tricuspid orifice, which is located rightward and anteriorly. THE RIGHT ATRIUM Anatomically, the right atrium can be considered as having four components: a venous component, an appendage, a vestibule, and the right atrial aspect of the atrial septum (see Fig. 1-2C).11 However, other than the terminal crest (crista terminalis) clearly marking the border between the appendage and the venous component, the other borders are indistinct. The right atrium is situated more anteriorly relative to the left atrium. Hence in the transverse bodily plane, the plane of the atrial septum extends obliquely rightward, from anterior to posterior, normally at an angle of approximately 60 degrees to the sagittal plane. Right Atrial Appendage, Terminal Crest, and Sinus Node From the epicardial aspect, it can be seen that the right atrium is dominated by its large and triangular-shaped appendage, which extends anteriorly and laterally (Figs. 1-2C and 1-3A). It is a misconception to consider only the tip portion of this structure as the appendage.12 Nevertheless, in frontal fluoroscopic view the tip points superiorly and anteriorly and lies above the anterosuperior part of the right atrioventricular junction (for orientation see Fig. 1-2A). A “windscreen wiper” movement then assures the interventional physician that the catheter/lead is positioned, as intended, in the tip of the atrial appendage. In right anterior oblique (RAO) projection the tip points to the right of the screen, whereas in left anterior oblique (LAO) projection it is on the left. Thus in frontal projection, when an interventionist has maneuvered a catheter/lead into the tip of the atrial appendage of a patient, seeing a “windscreen wiper” movement can assure him or her the positioning is as intended. Usually, a fat-filled groove (sulcus terminali) corresponding internally to the terminal crest (crista terminalis) can be seen along the lateral wall demarcating the junction between appendage and venous



CHAPTER 1  Cardiac Anatomy and Pathology

Anterior view Right Left

Antero-superior view left Right

Pulm SCV Aorta trunk LAA

Pulm trunk

SCV Aorta

RA RA LV

LV

RV

RV

B

A

SCV

Aorta Aorta

Pulm trunk Pulm trunk

ICV

Vest

ibule

RAA RV RV

LPA LAA LA

LSPV LIPV

LV cs

C

Right lateral view Posterior Anterior

D

Left lateral view Anterior Posterior

Figure 1-2  Endocast of a Normal Heart Displayed in Different Views to Highlight the Spatial Relationships Between “Right Heart” (Blue) and “Left Heart” (Red) Structures. A, The crossover relationship between right and left ventricular outflow tracts is represented by the arrows. Note the imprints of the semilunar leaflets of the aortic and pulmonary valves demonstrate their locations are not at the same level or in the same plane. B, This tilted view shows the central location of the aortic valve and its relationship to the cardiac chambers. C, The right atrial appendage (RAA) dominates the free wall of the right atrium. D, The right ventricular outflow tract passes anterosuperiorly relative to the left ventricular outflow tract. The coronary sinus (cs) runs along the inferior wall of the left atrium (LA). The left atrial appendage (LAA) is finger-like. White dotted line represents the location of the annulus of the tricuspid valve. ICV, Inferior caval vein; LIPV, left inferior pulmonary vein; LPA, left pulmonary artery; LSPV, left superior pulmonary vein; LV, left ventricle; RA, right atrium; RV, right ventricle; SCV, superior caval vein.

components (see Fig. 1-3A). The sinus node is located in this groove, close to the superior cavoatrial junction.13,14 In the majority of individuals, the sinus node is tadpole-shaped, approximately 1.5 cm long in the adult heart. Its “head” portion extends subepicardially from near the crest of the appendage to pass laterally and inferiorly deep into the musculature of the terminal crest where its “tail” portion is embedded.14 The specialized myocytes composing the sinus node are in a fibrous tissue matrix, usually surrounding a prominent nodal artery. Multiple prongs of nodal tissue extend into ordinary working atrial myocytes that make up the terminal crest, enabling transmission of the nodal impulse. Because the myocytes in the crest are mainly aligned longitudinally along its length, the crest is an important bundle for preferential conduction.

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On the endocardial aspect, the terminal crest usually appears as a distinct C-shaped muscular ridge in most hearts but is flatter and less obvious in others. Nevertheless, it is the endocardial border that marks the anatomic division between the rough walled appendage and the smooth intercaval venous area of the right atrium (Fig. 1-3B). It arises medially from the septal aspect to pass anteriorly and laterally in relation to the orifice of the superior caval vein and then it descends in a posteroinferior direction toward the orifice of the inferior caval vein, where it ramifies into multiple bundles that extend into the area anterior to the caval vein orifice and the tricuspid valve orifice that is described as the cavotricuspid isthmus (see below). Characteristically, an array of pectinate muscles arises from the anterior and rightward margin of the crest spreading to the anterior, lateral, and inferior walls of the atrium. Pectinate muscles do not reach the tricuspid orifice. Pectinate muscles always terminate at the smooth walled atrial vestibule surrounding the tricuspid orifice (see Fig. 1-2A-C). In right lateral view, the sector of the vestibule receiving the pectinate muscles will be located anteriorly and orientated nearly vertically; hence it is described by McAlpine as the anterior vertical muscle bundle.15 Clearly visible on the endocardial surface, a variable number of pectinate muscles take off almost perpendicularly from the crest. Of variable thicknesses, they branch into thinner and thinner muscle bundles that line the translucent parts of the appendage wall. In some hearts there may be one or more broad pectinate muscles that branch near to their origin from the crest in palm-leaf fashion. The branching and criss-cross arrangement of the pectinate muscles may facilitate the tips of leads to be lodged. However, the operator should be aware that the atrial wall in between the pectinate muscles is very thin, being almost parchment-like (see Fig. 1-3B), and could be vulnerable to active-fixation leads.12 At its anterosuperior part, the crest usually gives origin to a larger pectinate muscle recognized as the sagittal bundle which marks the border between the anterolateral and posteromedial components of the appendage (Fig. 1-4).12,16 In view of its location, it is likely that the sagittal bundle serves as a pathway for preferential conduction to the anterolateral part of the appendage. Moreover, the area where the crest arises from the septal aspect is continuous with the rightward extensions of the Bachmann bundle, which run in the subepicardium.12 The juncture facilitates transmission of the sinus impulse into the left atrial wall. Eustachian Valve, Vestibule, Triangle of Koch, and Atrioventricular Node The eustachian valve guards the anterior and anterolateral quadrants of the entrance of the inferior caval vein. In fetal life it directs blood from the inferior caval vein toward the oval fossa. In the adult heart it is usually a crescent-shaped flap of fibrous or fibromuscular tissue with a cord-like free margin that can be traced medially into the muscle of the eustachian ridge (sinus septum) as the tendon of Todaro (see Fig. 1-4).17 In some cases the valve has considerable height, like a hurdle, when advancing the catheter anteriorly from the inferior caval vein toward the tricuspid valve or into the right ventricle. Occasionally, there are perforations in the eustachian valve, or the valve may be a filigreed mesh (Chiari network) that may be so extensive as to stretch across the atrium to attach near the orifice of the superior caval vein. Catheters passing through such a valve may become entangled and deflected from the intended course. The smooth-walled vestibule surrounds the outlet of the atrium. In the area of the cavotricuspid isthmus, the vestibule occupies the anterior portion of the isthmus. Posterior to the smooth vestibule, the isthmus wall is irregular in thickness, comprising the terminal ramifications of the terminal crest and pectinate muscles and thinner fibrofatty areas in between the muscle bundles. In about 10% of hearts, the wall in the posterior portion of the isthmus, proximal to the vestibule, forms a pouch (see Fig. 1-4). When prominent, the pouch can resemble an appendix; hence it is termed the appendix posterior of His, inferior medial atrial fossa,15 or subeustachian sinus. Not infrequently, the appendage has another pouch in its anterolateral part, relating to the

SECTION 1  Basic Principles

Posterior

Anterior Aorta

Figure 1-3  A, A heart specimen viewed from the right lateral perspective. The dotted shape represents the location of the sinus node in the terminal groove (sulcus). The broken line along the free wall to the appendage tip marks the incision line made to display the inside of the right atrium shown in B. B, The free wall has been deflected posteriorly to reveal the terminal crest (crista), pectinate muscles, and the sagittal bundle (*). The double lines mark the intercaval area. The septal aspect seen en face gives a misleading impression that the septum is an extensive area. The inset shows a piece of the appendage wall accompanied by its histologic section (upper panel) highlighting the thinness of the atrial wall in between  the pectinate muscles. Histologic section in trichrome stain: red, myocardium; green, fibrous tissue. ICV, Inferior caval vein; SCV, superior caval vein; TV, tricuspid valve.

Tip Aorta Tip

sta

Fossa

ICV

B

Sup Ant

C

ris

ta

Inf

SCV

ER

ICV

*

Vestib

CS EV

ule

TV

Pouch

Figure 1-4  The Compact Atrioventricular Node (Red Filled Shape) Is Located at the Apex of the Triangle of Koch, and Its Extensions Bifurcate Inferiorly Toward the Coronary Sinus (CS). The triangle is marked posteriorly by the tendon of Todaro (broken line) buried within the eustachian ridge (ER) and anteriorly by the hinge line (dot and dash line) of the tricuspid valve (TV). The blue shape represents the site of the membranous septum. The open arrow indicates the commissure between the septal leaflet and the anterosuperior leaflet. Note the regular triangular-shaped eustachian valve  (EV) and the smaller thebesian valve (*). ICV, Inferior caval vein; SCV, superior caval vein.

Vestibule

Su lcu s

Appendage

A

Post

*

Crista SCV

SCV

Cri

6

TV

ICV

region of the acute cardiac margin, which McAlpine termed the inferolateral fossa or appendage.18 The myocardial thickness of the vestibule is approximately 2 mm, tapering to <1 mm toward the tricuspid valve orifice where its musculature minimally overlaps the atrial surface of the valvar leaflets. Epicardially, the vestibule is covered extensively by the fatty tissues of the atrioventricular groove through which the right coronary artery passes. The distance of the coronary artery from the endocardial surface of the vestibule decreases down to <3 mm when the artery is traced anticlockwise around the atrioventricular groove from the superoanterior position to inferior position as seen in a left anterior oblique view.12 The triangle of Koch, the anatomic landmark for the location of the atrioventricular node, occupies the septal and inferior paraseptal quadrant of the vestibule (see Fig. 1-4). The tendon of Todaro running in an anterosuperior direction inside the eustachian ridge is the marker for the posterior border of the triangle. It is a fine, fibrous strand, only revealed by dissecting into the ridge or on histological preparations. The anterior border is formed by the hinge line (annular insertion) of the septal leaflet of the tricuspid valve, whereas the inferior border is the orifice of the coronary sinus together with the vestibule anterior to it. The tendon of Todaro inserts into the central fibrous body, which meets with the membranous part of the cardiac septum, which in turn, is crossed by the hinge line of the septal leaflet. The apex of the triangle of Koch at the convergence of these two borders is the putative location of the atrioventricular node. In 45 degree RAO view the triangle is more or less in the same plane as the screen. The apex seen at fluoroscopic RAO is judged to be at the anterosuperior limit of the septal leaflet of the tricuspid valve. The anatomic apex, however, is a few millimeters posteroinferior to that. It is here that the bundle of His would penetrate into the central fibrous body and its continuation, the common atrioventricular bundle, passes anteriorly and leftward, sandwiched between the membranous septum and the muscular ventricular septum. On histology the body of the compact atrioventricular node can be identified as a knob-shaped structure approximately 5 mm long and wide in the adult heart, overlying the central fibrous body, whereas its two inferior prongs extend toward the mitral and



CHAPTER 1  Cardiac Anatomy and Pathology

tricuspid valves (see Fig. 1-4). The rightward inferior prong can extend some distance even into the vestibular musculature relating to the coronary sinus orifice. It has been implicated as the substrate for slowpathway conduction in atrioventricular nodal reentrant tachycardia.19 A zone of histologically discrete transitional cells surrounding the compact node serves as the atrial input from the walls of both atria and from the lower portions of the atrial septum. Importantly, only a thin layer of vestibular myocardium covers the right side of the transitional cell zone and the compact atrioventricular node. The size of the triangle of Koch is variable.20,21 It has been noted to be smaller in hearts with an enlarged coronary sinus, for example, in association with a persistent left superior caval vein, resulting in the compact atrioventricular node being closer to the coronary sinus orifice than is usual.22

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The orifice is viewed en face from the RAO perspective. It is usually guarded by a flimsy crescent-shaped thebesian valve that is attached to its inferior and posterior margins and covering half or more of the aperture, but there are variations. The valve can be membranous, fibromuscular, or muscular.24 It can be fenestrated, band-like, strand-like, or even absent, leaving the orifice unguarded. Occasionally, the valve can be so extensive as to overlap the orifice entirely. In such cases, the direction of catheter access might need to be modified to allow the catheter to be inserted. Congenital atresia of the coronary sinus orifice may preclude right atrial access to the coronary venous system.25 In such cases the coronary venous return may be channeled via a persistent left superior caval vein through which catheter access could be gained. If present, a cross connecting vein between both right and left superior caval veins could allow access from the right. Normally, the coronary sinus orifice is approximately 6 to 8 mm in diameter, but there are two situations where the orifice is much larger. In approximately 0.3% to 0.5% of the population this is due to persistence of the left superior caval vein draining into the right atrium via the coronary sinus. Rarely, the large orifice is associated with partially or totally anomalous connections of the pulmonary veins to the coronary sinus allowing drainage into the right atrium. It should be noted that the atrial walls surrounding the coronary sinus orifice are not septal. Instead, this part of the atrium overlies the crux region of the heart, which is known to cardiac surgeons as the inferior pyramidal space.26,27 The space is filled with epicardial fatty tissues of the atrioventricular groove and contains the dominant coronary artery and the branch supplying the atrioventricular node. On the ventricular side of the space lie the right and left ventricular walls as they converge to form the septum.

The Venous Component, Orifices of Caval Veins, and Coronary Sinus On the endocardial surface, there is no distinct border between the venous component and the septal component (see Fig. 1-3B). Nevertheless, the venous component occupies the posterior part, the posteroinferior part, and also extends medially toward the septal part of the chamber. In humans, the axis of the superior and inferior caval veins entering the atrium makes an obtuse angle, giving the posterior wall a slight indentation, whereas in other mammals such as the sheep or pig, the angle is sharper and the distinct fold is known as the tubercle of Lower. In humans, the orifice of the superior caval vein is anterolateral to the upper part of the atrial septum. As discussed previously, the anterior and anterolateral margins of the orifice is marked by the terminal crest (see Fig. 1-4). In right lateral perspective, the superior caval vein enters the atrium more anteriorly than the inferior caval vein. The posterior atrial wall is confluent with the posterior walls of both caval veins. The right atrial myocardium can extend 1 cm or more over the wall of the superior caval vein, but myocardium is usually deficient around the inferior caval vein, especially posteriorly.11,23 The orifice of the coronary sinus is located medially, anteriorly, and superiorly relative to the orifice of the inferior caval vein (see Fig. 1-4).

The Anterior Wall and the Atrial Septum On the endocardial surface of the right atrium, there is no anatomic demarcation between the septum and the anterior wall. Indeed, at first sight the septum appears more extensive than it actually is (Figs. 1-4, 1-5A). This is also the impression when viewed from the fluoroscopic RAO perspective. Viewed from the right atrial aspect, the true atrial septum is confined only to the thinnest area, which is the floor of the

RSPV

SCV

Aorta

SCV

L

RIPV

N R F

F cs

ICV

A

ICV

RV

EV

LA

TV

RA

MV

B

Figure 1-5  A, Window dissection of a heart in near-frontal view. The oval foss (F) has a well circumscribed raised rim. The atrial wall bulging into the cavity is the aortic mound (triangle). The aortic sinuses (L, left coronary; N, noncoronary; R, right coronary) are labeled to demonstrate their relationship to the atrial septum. The small arrow points to a small pit. This is one of several thebesian vein orifices commonly seen on the septal aspect. When particularly large, it can trick the operator into thinking the catheter has dropped onto the floor of the fossa. B, Longitudinal cut through the atria simulating a four-chamber echocardiographic plane shows the atrial septum in profile. The floor of the fossa (F) is the thinnest part and when perforated allows direct communication between right and left atria. The superior margin of the muscular rim shown here is a deep infold filled with epicardial fat (shape). The arrow points to the artery supplying the sinus node. cs, Coronary sinus; EV, eustachian valve; ICV, inferior caval vein; LA, left atrium; MV, mitral valve; RA, right atrium; RIPV, right inferior pulmonary vein; RSPV, right superior pulmonary vein; RV, right ventricle; SCV, superior caval vein; TV, tricuspid valve.

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SECTION 1  Basic Principles

oval fossa and the immediate surrounding rim known as the limbus.28 The rim can be well defined especially in its superior and anterior margins, whereas its posteroinferior margin is less distinct. The fossa floor is usually composed of fibromuscular tissues and is approximately 1 to 1.5 mm thick but it is thinner if the floor is aneurysmal. In fetal life the floor acts like a valve allowing flow from the right atrium to enter the left atrium. Thus the flap-like fossa valve is larger on the left atrial side so that after birth it can close off the communication when pushed against the fossa rim by increase in left atrial pressure. The valve then becomes adherent to the rim, but in approximately 25% to 30% of heart specimens the anterosuperior margin is not sealed down, leaving a passage for passing a probe from the right atrium to the left atrium through the fossa, described as a patent foramen ovale (PFO). The size and location of the fossa varies from heart to heart and so does the extent of its rims in different quadrants. Some hearts have fossae with well-defined rims giving good contours on the septal profile, whereas in some the septal surface is fairly featureless making it difficult to be certain where the true septum is when carrying out transseptal procedures. The rim, being an infolding of the embryonic atrial wall, is filled with epicardial fatty tissues and often contains the artery that supplies the sinus node (Fig. 1-5B). This interatrial groove, recognized by cardiac surgeons as the Waterston or Sondergaard groove, allows surgical access into the left atrium without going through the right atrium. In hearts with a small fossa the fold tends to be deeper. Therefore crossing the septum through the peripheral parts of the rim could run the risk of causing a tamponade because of transgression through epicardial tissues, albeit the deep ingress of fatty tissues theoretically could withhold any leakage from a pin prick. Another consideration is that a catheter lodged in the thickness of the rim could be less maneuverable than one that has passed through the thin fossa floor. This could be a particular issue in cases described as having lipomatous hypertrophy of the atrial septum where the excess fat filling the infolded rim can make its thickness 2 cm or more. The atrial wall anterior to the fossa bulges into the chamber to accommodate the aortic root on the epicardial side. This part of the anterior wall, termed the aortic mound, highlights the hazard of exiting the atrial chamber here (see Fig. 1-5A). Muscular continuity between the atrial chambers peripheral to the septum is frequently found as bridges in the subepicardium.28,29 The most prominent interatrial bridge is the Bachmann bundle. This is a broad muscular band that runs epicardially from close to the anterior wall of the superior caval vein into the anterior walls of both atriums. The muscular fibres in the Bachmann bundle, as in the terminal crest, are well aligned. Smaller bridges across the interatrial groove are often present superiorly and posteriorly. Inferior bridges of variable widths join the intercaval area of the right atrium to the left atrium. Further muscular bridges from the left atrial wall often overlie and run into the wall of the coronary sinus.30 THE RIGHT VENTRICLE Anatomically speaking, the right ventricle has three portions: inlet, apical trabecular, and outlet portions (Fig. 1-6). However, there are no clear anatomic borders for these portions. The inlet portion is taken to be the portion that contains the tricuspid valve and its papillary muscles whereas the outlet portion is the funnel-like part leading to the pulmonary valve, and the apical portion lies in between the inlet and outlet portions. As shown on the endocast, the right ventricle has a complex shape and, due to the curvature of the ventricular septum, it has an overlapping relationship with the left ventricle (see Fig. 1-6). The inlet projects from posteroinferiorly to anteriorly and leftward, whereas the outlet curves superiorly and posteriorly. The muscular separation between inlet and outlet parts is the supraventricular crest which is a fold of the right ventricular wall and is attached to the most superior part of the ventricular septum (see Fig. 1-6). Inside the right ventricle, the inlet portion extends from the hinge line (annulus) of the tricuspid valve to the papillary muscles. At the

Pulm trunk

Pulm valve

Aorta Outlet SMT

RCA mb Inlet

Apical

A Pulm trunk

RA

B

RA

C

Figure 1-6  A, Window dissection showing the right ventricle from right lateral perspective. The broken lines attempt to mark the borders between the three portions of the chamber. Note the thin wall at the apex. Trabeculations criss-cross the apical portion. The moderator band (mb) connects the septum to the free wall (removed from specimen). The supraventricular crest (double-headed arrow) clasped between the limbs of the septomarginal trabeculation (SMT) separates the pulmonary valve from the tricuspid valve. B, The endocast in approximately right anterior oblique (RAO) 30-degree view demonstrates that behind the supraventricular crest sits the aortic root. The crest had occupied the space of the inner heart curvature between the right heart valves which are marked by dotted lines. C, The endocast of the right heart in right lateral view reveals the scoop of the inner curvature. The path of the coronary sinus and great cardiac vein (arrow) in this orientation is seen behind. RA, Right atrium; RCA, right coronary artery.

nearly circular hinge line there is a mild constriction between the cavities of the right atrium and the right ventricle. The leaflets of the tricuspid valve can be distinguished as septal, anterosuperior, and inferior or mural, and the commissures are designated anteroseptal, anteroinferior, and inferior. The septal leaflet with its cords inserting directly to the ventricular septum is characteristic of the tricuspid valve. The medial papillary muscle is usually a small papillary muscle arising from the superior part of the septum, beneath the attachment of the supraventricular crest (Fig. 1-7A). It anchors the fan-shaped cord that supports the commissure between the septal and anterosuperior leaflets. It is also an important landmark for the right bundle branch, which emerges from the septal musculature to become subendocardial at the base of the papillary muscle. The septal leaflet has cordal attachments directly to the ventricular septum, but it also receives cords from the medial papillary muscle or muscle complex. The annular attachment of the septal leaflet crosses the membranous septum dividing the membrane into two components: a supratricuspid atrioventricular portion (between right atrium and left ventricle) and an infratricuspid interventricular portion (between both ventricles). Often there is a gap of leaflet tissue at the membranous area giving the false impression of this being a commissure (Fig. 1-7B, C). The anterosuperior leaflet is



CHAPTER 1  Cardiac Anatomy and Pathology

Pulm valve Aorta RCA

SMT mp

RA

mb

S

A Antero-septal commissure ap

A-S mp mp

S

ip I

B

ip

C

Figure 1-7  A, A window dissection of a heart viewed from right anterolateral perspective. Cutting across the supraventricular crest (double-headed arrows) reveals it is a muscular fold with the right coronary artery (RCA) coursing outside. There is a complete muscular infundibulum underneath the pulmonary valve. The right bundle branch (red dots) emerges from the septum at the insertion of the medial papillary muscle (mp) to descend in the subendocardium of the septomarginal trabeculation (SMT). From there it branches and a fascicle travels via the moderator band (mb) to the parietal wall. B, Heart specimen in similar orientation as above but with the right atrial and ventricular walls incised, allowing the parietal wall to be deflected superiorly to display the anterosuperior (A-S), septal (S), and inferior (I) leaflets of the tricuspid valve. C, A magnified view of B showing the area of the membranous septum (blue dots) and the tricuspid annulus (broken line). ap, Anterior papillary muscle; ip, posterior papillary muscle; RA, right atrium.

suspended like a curtain from its annular attachment and is the most extensive of the three leaflets. A larger papillary muscle, the anterior papillary muscle, usually supports the body of the anterosuperior leaflet and its junction with the inferior leaflet. Smaller anterior papillary muscles arising in the vicinity provide additional support. The commissure between septal and inferior leaflets is supported by a group of small inferior papillary muscles. The cavity of the apical portion is criss-crossed by many interconnecting muscle bundles (trabeculations) running between the septum and the parietal wall (see Figs 1-6A, 1-7A). They are characteristically coarse when compared with the finer trabeculations within the left ventricle. Due to the trabeculations overlying the compact

9

myocardium of the ventricular wall, the endocardial surface is uneven and of irregular thickness. At the very apical part, the musculature of the wall thins to approximately 1.5 mm. The right ventricle has a distinctive muscle bundle termed the moderator band (see Fig. 1-7A). This bridges the ventricular cavity between the septum and the parietal wall, giving rise along the way to the anterior papillary muscle of the tricuspid valve. It arises from the body of the septomarginal trabeculation at about a third to half the distance from the apex. Within the moderator band runs a major subbranch of the right bundle branch. The septomarginal trabeculation itself is a Y-shaped muscular band that is adherent to the septal surface (see Fig. 1-7A). In between its limbs lies the supraventricular crest, which is an infolding of the heart wall forming the ventricular roof. This crest separates the two right heart valves and is an integral part of the outlet, continuous superiorly with the free-standing subpulmonary muscular infundibulum that is a tube-like structure supporting the pulmonary valve. Although it is common to refer to septal and free wall portions of the right ventricular outflow tract, it should be noted that the infundibulum does not have a septal component. It stands proud above the ventricular septum. Due to this feature, the entire pulmonary valve can be removed surgically, for example, for use as an autograft in the Ross procedure,31 without leaving a hole in the ventricular septum. The septal component of the right ventricular outlet is only in its most proximal part, where the supraventricular crest inserts into the septum between the limbs of the septomarginal trabeculation. Furthermore, the right ventricular outlet curves to pass anterior and cephalad to the left ventricular outlet (see Fig. 1-6B, C). Any perforation in the “septal” part is more likely to go outside the heart than into the left ventricle. Two of the pulmonary sinuses are adjacent to two aortic sinuses, the right and left coronary sinuses, although the planes of the aortic and pulmonary valves are at an angle to one another (see Fig. 1-2A). The main coronary arteries are in proximity to the so-called septal part. The infundibular wall tapers from approximately 3 to 5 mm proximally to 1.5 mm at the anatomic junction between the ventricular musculature and arterial wall of the pulmonary trunk. The crescentic hinge lines of the pulmonary leaflets cross the anatomic junction, enclosing within the depths of the sinuses small semilunar areas of infundibular myocardium.32 THE LEFT ATRIUM As with the right atrium, the left atrium has three components and shares its septum. The atrial appendage is characteristically a small finger-like cul-de-sac in human hearts where thrombi may form.33 In most hearts the appendage extends from between the anterior and lateral walls of the left atrium and its tip is directed anterosuperiorly, overlapping the left border of the right ventricular outflow tract or the pulmonary trunk and the main stem of the left coronary or the circumflex artery (Fig. 1-8A, B). It is not uncommon to find the tip of the appendage directed laterally and backward, although in a few hearts the tip portion passes behind the arterial pedicle to sit in the transverse pericardial sinus. The external appearance of the left atrial appendage is that of a slightly flattened tube with crenellations, often with one or more bends, and often terminates in a pointed tip. Due to its slightly flattened shape, the lower surface usually overlies the left ventricle, whereas the upper surface is beneath the fibrous pericardium. There is no terminal crest. The pectinate muscles are frond-like muscle bundles mostly confined to the endocardial surface of the atrial appendage. The junction of the appendage with the body of the left atrium, also described as the os or orifice, is usually oval-shaped. When viewed from within the left atrial chamber, the os is situated anterior to the orifices of the left pulmonary veins but is separated from them by a ridge-like structure (Fig. 1-8B, C). In reality, the lateral ridge is a slight infolding of the left atrial wall that is filled with epicardial tissues including the remnant of the vein of Marshall, nerve bundles, and in some hearts, the sinus node artery. In contrast to the right atrium, the body of the atrium including the septal component is fairly smooth-walled. The venous component

10

SECTION 1  Basic Principles

LSPV

Pulm trunk RSPV LAA

SCV

LAA

LSPV Aorta Pulm valve

LIPV RCA

A

RV

B LPV

Pulm valve

LAA

RSPV

LAA

R

Ve

sti

bu

le

RIPV SCV

LCA

L

MV

N gcv

MV

CS

C

TV

CS

D

Figure 1-8  A, Left-lateral view showing the relationship of the left atrial appendage to the pulmonary trunk and the left pulmonary veins. B, A tilted anterosuperior view showing the central location of the aortic valve and its relationship to the right ventricular tract anteriorly and the atrial chambers posteriorly. In this heart the left atrial appendage (LAA) overlaps the left coronary artery (LCA). Note the infolding of the atrial wall between the LAA and the left superior pulmonary vein (LSPV). C, This four-chamber cut reveals that the ridge seen on the endocardial surface is a fold (arrow) between the left pulmonary vein (LPV) and the LAA. There is no demarcation between the smooth walled venous component of the atrium and the vestibule. D, The posterior parts of the atria have been removed to allow a view onto the orifices of the mitral and tricuspid valves (MV, TV). The course of the great cardiac vein (gcv) and coronary sinus (CS) is along the left atrial wall. L, Left coronary aortic sinus; LIPV, left inferior pulmonary vein; LSPV, left superior pulmonary vein; N, noncoronary aortic sinus; R, right coronary aortic sinus; RCA, right coronary artery; RIPV, right inferior pulmonary vein; RSPV, right superior pulmonary vein; RV, right ventricle; SCV, superior caval vein.

receives the pulmonary veins and the vestibular component surrounds the mitral orifice, but there are no anatomic landmarks that mark the border between the two, although frequently a few pits or crevices are seen in the inferior wall at the border zone. Although mainly smooth on the endocardial surface, the atrial walls are composed of differently aligned myocardial bundles with marked regional variations in thickness.11,28,34 There are usually four pulmonary veins entering the left atrium, but there are also considerable variations.35 The transition between atrial wall and venous wall is smooth. When the veins are funnel-shaped as they enter the atrium, it is difficult to define the orifices precisely. It is common to find extensions of atrial muscle on the adventitial side of the veins, especially around the superior veins. The orifices of the right pulmonary veins are alongside the plane of the atrial septum with the orifice of the right upper vein lying behind the entrance of the superior caval vein into the right atrium (see Fig. 1-8C).

The course of the coronary sinus is related to the inferior aspect of the left atrial wall on its epicardial surface, but the sinus is not located immediately adjacent to the hinge line of the leaflets of the mitral valve (Fig. 1-8D). It has a variable relationship with the mitral annulus and with the circumflex artery. It runs along the epicardial aspect of the vestibular portion of the atrial wall but at varying distances from the mitral annulus along its course.36 THE LEFT VENTRICLE When the heart is viewed from the front, most of the left ventricle is behind the right ventricle, and due to its approximately conical shape, its outlet overlaps its inlet. Unlike the right ventricle, which has muscular separation between its inlet and outlet valves, the normal human heart almost always has fibrous continuity between the aortic and mitral valves (Fig. 1-9). The left ventricular wall is normally 12 to



CHAPTER 1  Cardiac Anatomy and Pathology

Aorta R



N

AL

pmp alp

Apex

Figure 1-9  The Left Ventricle Has Been Opened Through an Incision in Its Anterior Wall and Into the Aortic Valve. The display is viewed from left lateral perspective. Note the area of fibrous continuity (open arrow) between the aortic valve and the anterior leaflet (AL) of the mitral valve. The membranous septum is marked with an asterisk. The anterolateral and posteromedial papillary muscles (alp, pmp) are close together. Note the false tendons (small arrows), fine apical trabeculations, and the thin wall at the apex. N, Noncoronary aortic sinus; R, right coronary aortic sinus.

15 mm thick, but it tapers considerably to approximately 1 to 2 mm at the cardiac apex. Trabeculations occupying the endocardial surface of the apical portion of the ventricle are finer than those seen in the right ventricle. On the septal side, the trabeculations extend up to about two thirds from the apex, leaving the upper third of the septum beneath the aortic valve smooth. Although more common in animal hearts, it is not unusual to find in human hearts fine muscular strands or so-called false tendons connecting the septum with the papillary muscles or the parietal wall.37 Many of these carry the distal ramifications of the left bundle branch. The inlet portion is guarded by the orifice of the mitral valve. The hinge line (annulus) of the mitral leaflets has a very limited attachment to septal structures. The larger portion of the leaflets is hinged to the parietal atrioventricular junction, whereas a third is the span of fibrous continuity with the aortic valve. The two leaflets of the mitral valve are disproportionate in size. Although dubbed the “anterior” and “posterior” leaflets they are not precisely in these locations. Nevertheless, the “anterior” leaflet, which is in fibrous continuity with the aortic valve, is deep and hangs like a curtain between the inflow and outflow tracts. By contrast, the mural (or “posterior”) leaflet is shallow but has a longer hinge that accounts for nearly two thirds of the annulus to form the parietal atrioventricular junction. The free margin of the mural leaflet usually has three or more scallops. The mitral leaflets are attached via tendinous cords to two groups of papillary muscles that arise from the parietal wall of the ventricle at about midventricular level. There is usually a single papillary muscle in the anterolateral position and a cluster of several papillary muscles in the posteromedial position. These two groups of papillary muscles are in close proximity, and any apparent separation is an artifact of postmortem dissection (see Fig. 1-9). Cords arising from the papillary muscles include the commissural cords that mark the locations of the mitral commissures,

11

cleft cords that support the gaps between the scallops, and cords that insert to the underside of the leaflets. Further short cords termed basal cords arise directly from the ventricular wall to support the mural leaflet near its annular attachment. Notably, the mitral valve does not have cordal attachments to the ventricular septum by virtue of the ventricular outflow tract occupying the space between the septum and the “anterior” mitral leaflet. Due to the curvature of the ventricular septum, the outlet is bordered anterosuperiorly by muscle and posteroinferiorly by leaflet tissue. Two of the three sinuses of the aortic valve have muscular support, these being the ones adjacent to, or facing, the pulmonary valve (see Fig. 1-8D). The facing aortic sinuses give rise to the right and left coronary arteries and contain within their bases small segments of ventricular myocardium, but that in the left sinus is much less.38 The third sinus, the noncoronary sinus, and part of the left sinus adjoining it do not have muscular support; this being the region of aortic-mitral fibrous continuity. At the septal end of valvar fibrous continuity is the right fibrous trigone and at the parietal end is the left fibrous trigone. The right trigone in continuity with the membranous septum forms the central fibrous body through which the atrioventricular conduction bundle penetrates. The common atrioventricular conduction bundle emerges from the central fibrous body to pass leftward between the membranous septum and the crest of the muscular ventricular septum. 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 leaflets of the aortic valve (see Fig. 1-9). 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.

THE CARDIAC VEINS The venous return from the myocardium is channeled via two routes. One route of drainage occurs via small thebesian veins that open directly into the cardiac chambers, and the thebesian orifices appear like small pits on the endocardial surface. The greater portion of drainage (approximately 85%), however, is collected by the greater coronary venous system that mainly routes via the coronary sinus.39,40 The main coronary veins in the greater system are the anterior interventricular vein joining with the great cardiac vein, the middle, and the small cardiac veins (Fig. 1-10). The anterior interventricular and middle veins run alongside the anterior descending and posterior descending coronary arteries, respectively, and drain into the coronary sinus. Proximally, the coronary sinus begins at its os in the right atrium. From its os, the sinus curves, following the inferior wall of the left atrium, and it passes leftward toward its junction with the great cardiac vein. Reportedly, the length of the coronary sinus is 4 cm in anatomic series and 7 cm in clinical series. The discrepancy is probably related to the definition of the channel that is designated as the sinus.41-43 The shape of the coronary sinus is usually like a long funnel with the widest part at the orifice. The sinus is enlarged when it receives a persistent left superior caval vein or, rarely, an anomalous pulmonary vein that drains into the right atrium. In such cases, catheter or lead access from the right atrium to the coronary venous system may require more manipulation to avoid going into the persistent left caval vein or anomalous pulmonary vein. Instead, the left superior caval vein could be used to gain access more easily. In the majority of hearts there is a small valve known as the thebesian valve guarding the entrance. It is often crescent-shaped, thin, fibromuscular in composition, hinged at the posteroinferior margin, and covers a third to half of the orifice.24,43 Variations in morphology include fenestrations in the valve, fibrous bands or strands, and fishnetlike when it is part of the Chiari network of the eustachian valve. A study of heart specimens found an unguarded orifice in 15%.42 Occasionally, the orifice may be completely occluded by a membrane or the sinus itself is atretic. In these cases, coronary venous drainage is via the thebesian veins or takes an alternative route through collateral veins,

12

SECTION 1  Basic Principles

gcv gcv aiv

iv

cs

ov

ov

cs

mv

iv

mv

rv

aiv

A

B

gcv

gcv lom

ov

lom cs

iv

cs

mv

C

D

Figure 1-10  A, Representation of the greater coronary venous system in anterior projection. B, The coronary sinus venous system as seen from left anterior oblique perspective. C, This representation is a view from left inferior perspective. The entrance of the vein/ligament of Marshall (lom) marks the junction between the great cardiac vein (gcv) and the coronary sinus (cs). D, Dissection of a heart specimen orientated in similar fashion as C shows a flimsy crescentic valve at the junction between gcv and lom (open arrow). The orifice of the middle cardiac vein is partly obstructed by a valve (arrow). aiv, Anterior interventricular vein; iv, left inferior or inferolateral vein; mv, middle cardiac vein; ov, obtuse marginal vein; rv, right coronary vein.

for example, retrogradely up a persistent left superior caval vein, which in turn connects to the right superior caval vein. The entrance of the vein of Marshall, or oblique left atrial vein, marks the junction of the great cardiac vein with the venous end of the tube-shaped coronary sinus. The Marshall vein is a fibrous ligament in most individuals. When widely patent, it is the persistent left superior caval vein. In the majority, when a lumen is present, it is narrow, rarely exceeding 2 cm in length before tapering to a blind end. In the absence of the vein of Marshall, or its remnant, Vieussens valve is taken as the anatomic landmark for the junction. Found in 80% to 90% of hearts, this very flimsy valve has one to three leaflets that can provide slight resistance to the catheter (see Fig. 1-10).44 Once past Vieussens valve, a sharp bend in the great cardiac vein can cause a further obstacle in 20% of patients.39 Another marker for the junction between vein and coronary sinus is the end of the muscular sleeve around the sinus, but in some cases, the sleeve may extend to 1 cm or more over the great vein.45 Bundles from the sleeve run into the

left atrial wall and also cover the outer walls of adjacent coronary arteries. From the cardiac apex, the anterior interventricular vein ascends superiorly and turns leftward and posteriorly to continue as the great cardiac vein, which then enters the left atrioventricular groove. The branches of the anterior interventricular vein drain the anterior left ventricular wall, parts of the anterior right ventricular wall, and the ventricular septum. Near the apex, anastomosis of branches of the anterior interventricular vein with branches of the middle and posterior veins are not uncommon. The anterior interventricular vein ascends to the left of the anterior descending coronary artery to the anterolateral wall of the left ventricle (the summit), where it becomes known as the great cardiac vein. Here it passes close to the first division of the left coronary artery and crosses over the initial course of the circumflex artery, under the cover of the left atrial appendage. The great vein marks the inferior border of the arterovenous triangle of Brocq and Mouchet, the other two borders being the anterior descending and



CHAPTER 1  Cardiac Anatomy and Pathology

the circumflex arteries (see Fig. 1-10). Usually, the vessels in the triangle are covered over by epicardial fat pads. The great vein then curves, following the left atrioventricular groove, and is joined by tributaries from the left ventricular obtuse margin and the inferior wall, as well as veins from the left atrium as it approaches the coronary sinus. The number, distribution, courses, and calibers of the left ventricular veins joining the great cardiac vein vary from individual to individual. In the majority of hearts there are one or more prominent veins that ascend the left ventricular wall that lies between the anterior interventricular vein and the middle cardiac vein (Fig. 1-11). Usually, these are the obtuse marginal vein and the left inferior or inferolateral veins, and they may course obliquely instead of directly apex to base. Smaller tributaries are common on the parietal (lateral) wall. Often there are small valves at the entrances of the branches into the great vein or coronary sinus (see Fig. 1-10). Curves and bends in these veins can help provide stability for implant of leads. When utilizing the left ventricular or interventricular veins for lead implants, it is worth noting that the left phrenic nerve running in the pericardium may pass across the anterior interventricular vein or the obtuse marginal vein.7,8 Notably, the wall on the epicardial side of these veins is thin, not being covered by muscle or fat pads, rendering it “unprotected.” Furthermore, although coronary veins are usually superficial to arteries, cross-overs between arteries and veins are not uncommon.43 The middle cardiac vein (or posterior interventricular vein) ascends the diaphragmatic surface of the heart from the apex and opens into the coronary sinus just within the sinus os. It receives branches from the ventricular septum and the inferior walls of both ventricles. Close to its entry into the sinus it sometimes receives a branch that ascends obliquely from the left lateral and inferior wall. The latter, described as the posterior vein, could be mistaken for the middle cardiac vein. Because of anastomoses of branches of the middle cardiac vein at the apex with those of the anterior interventricular vein, a guidewire passed into the anterior vein could then travel via the middle cardiac vein to enter the coronary sinus. Occasionally, the middle vein enters the right atrium directly and opens adjacent to the os of the coronary sinus. Indeed, the close proximity of the two venous orifices may result in the coronary sinus catheter being inserted into the middle cardiac vein unintentionally. On the epicardial aspect the middle cardiac vein passes just superficial to the right coronary artery at the cardiac crux, where the vessels are usually covered by fat pads. Running along the inferior surface of the heart, the middle cardiac vein is not related to the phrenic nerves but leads placed in or toward the apical portion may cause direct capture of the left hemidiaphragm. The small cardiac vein (right coronary 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 (see Fig. 1-11). When joined by the acute marginal vein (the vein of Galen) and other veins from the anterior wall of the right ventricle, the small vein becomes larger. More commonly the acute marginal and anterior veins drain directly into the right atrium. In some hearts, the anterior veins merge into a venous lake in the right atrial wall.

13

Intermediate LCA

LAD

aiv Cx gcv LAA Deflected posteriorly gcv Cx

A

Obtuse margin

gcv CS art

ICV art

mv

RCA

B Figure 1-11  A, Left lateral view of a heart with the left atrial appendage (LAA) pulled backward to display the arterovenous triangle formed by the left coronary arteries and the great cardiac vein. B, This view of the diaphragmatic aspect shows the variable relationship and distribution of the left inferior venous branches (arrows) in relation to the branches of the coronary arteries (art). Note the angulation of the veins as they enter the coronary sinus (CS). The right coronary vein is marked with arrowheads. aiv, Anterior interventricular vein; Cx, circumflex artery; gcv, great cardiac vein; ICV, inferior caval vein; LAD, left anterior descending coronary artery; LCA, left coronary artery; mv, middle cardiac vein; RCA, right coronary artery.

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14

SECTION 1  Basic Principles

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