Anatomy, Dimensions, and Terminology

Right Atrium

The right atrium ( Fig. 1-2 ) is the heart chamber that normally receives systemic venous drainage from inferior and superior venae cavae. It also normally receives the major portion of coronary venous drainage from the coronary sinus. Morphologic characteristics important for identifying the right atrium are presence of the limbus of the fossa ovalis, which surrounds the valve of the fossa ovalis (septum primum) superiorly, anteriorly, and posteriorly; a wide-based, blunt-ended, right-sided atrial appendage (auricle); eustachian valve at the orifice of the inferior vena cava and thebesian valve at the orifice of the coronary sinus; and crista terminalis, which separates trabeculated from nontrabeculated (venous) portions of the atrium ( Fig. 1-3 ).

The normal structures are sometimes expressed in an excessive or unusual manner. These are not themselves functionally important abnormalities but are usually associated with cardiac malformations. Thus, the eustachian and thebesian valves may be sufficiently prominent to appear to divide the right atrium into two parts, a common finding in tricuspid atresia. The right atrial appendage may be juxtaposed leftward, and the left atrial appendage is less frequently juxtaposed rightward. Juxtaposition of the atrial appendages is usually associated with cardiac malformations.

Radiologically, the definitive morphologic features of the right atrium may be difficult to recognize. Occasionally, the atrial septum is seen well enough in angiographic profile to delineate the limbus of the fossa ovalis, and sometimes the right atrial appendage is outlined sufficiently to differentiate its shape from that of the left atrial appendage. The fact that the hepatic portion of the inferior vena cava usually drains into the right atrium often makes it possible to determine the location of the right atrium by passage of a catheter from the inferior vena cava to the heart. Cardiovascular magnetic resonance imaging (MRI) and three-dimensional (3D) echocardiographic computed tomography are increasingly able to identify even complex morphologic features of this and other cardiac chambers and their connections.

The atria are not in normal position in some patients; in these cases, the wide-based, blunt-ended right atrial appendage is the most secure indicator that an atrium is morphologically a right atrium. Other indicators of the morphology of the atria, and of atrial situs, include venous drainage and the situs indicated by the pulmonary artery and bronchial anatomy.

Left Atrium

The left atrium ( Fig. 1-4 ) is the cardiac chamber that normally receives pulmonary venous drainage from the four pulmonary veins. Its septal surface is characterized by the flap valve of the fossa ovalis (septum primum), in contrast to the limbus of the fossa ovalis present on the right atrioseptal surface. The left atrial appendage (auricle) is long and narrow, in contrast to bluntness of the right atrial appendage, and is the best indicator that the atrium is morphologically a left atrium. There is no crista terminalis at the base of the left atrial appendage, the only trabeculated structure in the left atrium.

In general, at cardiac catheterization, the location of the left atrium is determined by exclusion after identifying the position of the right atrium as described earlier. With normal pulmonary venous connection, the left atrium may be well opacified after a right ventricular or pulmonary artery injection.

Right Ventricle

Topographically, the right ventricle has a large sinus portion that surrounds and supports a tricuspid atrioventricular (AV) valve (inlet portion) and includes the apex and a smaller infundibulum (outlet portion) that supports a semilunar valve. The inlet and outlet valves of the right ventricle are thus widely separated. The entire sinus portion of the right ventricle and most of the infundibulum (both free wall and septum) are coarsely trabeculated.

The septal surface of the right ventricle is divided into an inlet portion, a trabecular portion (sometimes called the apical trabecular portion ), and an outlet portion ( Fig. 1-5 ). Alternatively, the septal surface of the right ventricle may be divided into posterior (basal), middle, apical (anterior), and infundibular (conal) portions ( Fig. 1-6 ). The inlet portion of the ventricular septum surrounds and supports the tricuspid valve. ¹ The trabecular portion is that portion with the coarse trabecular pattern typical of the right ventricle (see Fig. 1-6 ). The outlet portion of the right ventricular aspect of the ventricular septum is smooth but complex and has three components. The largest is the infundibular (conal) septum, which separates the pulmonary from the aortic and tricuspid valves. Only part of the infundibular septum is interventricular (see Fig. 1-5 ), and in some malformations (e.g., double outlet right ventricle), none of it may be. It must be emphasized that the most distal cephalad portion of the infundibular septum is not, strictly speaking, part of the ventricular septum, because in the normal heart, the pulmonary valve arises from the apex of a cone of muscle and does not have a septal attachment. A second part of the outlet portion of the septum is the anterior (superior) extension, or division, of the trabecula septomarginalis (septal band). A third small, very anterior portion is a narrow extension superior to the trabecular septum.

1 The phrase inlet septum is in some ways undesirable because the term has developmental implications, and the large inlet septum on the right ventricular side is not duplicated on the left side. Use of the term trabecular to describe a portion of the sinus septum is also undesirable in some ways because part of the infundibular septum is also trabeculated (see Fig. 1-5 ).

Laterally to the right, the infundibular septum imperceptibly merges with the free right ventricular wall immediately beyond its attachment to the membranous septum; at that point, it can be called the parietal extension of the infundibular septum ( Fig. 1-7 ). The parietal band lies anterior to the right aortic sinus (see Fig. 1-7 ), partially overlying that portion of the free wall of the right ventricle termed the ventriculoinfundibular fold . Many surgeons call the infundibular septum and the parietal band the crista superventricularis . Medially and to the left, the infundibular septum merges with the trabecular portion of the septum between the limbs of the particularly prominent smooth, Y-shaped muscle bundle called the trabecula septomarginalis ( Fig. 1-8 ). The trabecula septomarginalis extends apically to become continuous with the moderator band, a prominent trabeculation running from septum to free wall.

The junction between outlet and sinus (trabecular) portions of the right ventricle is clearly demarcated only along the lower margin of the outlet portion of the septum. The incomplete muscular ridge formed by the outlet septum (here, specifically, the infundibular septum) and the parietal band, together with the septal and moderator bands, forms a natural line of division between the posteroinferior sinus portion and the anterosuperior outlet portion of the ventricle. It is in this area that ventricular septal defects (VSDs) most commonly occur; the morphology of the area gives the name “junctional,” or “conoventricular,” to these defects.

The papillary muscle arrangement supporting the three leaflets of the tricuspid valve is different from that of the mitral valve in the left ventricle. In the case of the tricuspid valve, in addition to a single large anterior papillary muscle attached to the anterior free wall that fuses with the moderator band, there are multiple smaller posterior papillary muscles attached partly to the posterior (inferior) free wall and partly to the septum, and a group of small septal papillary muscles. The lowermost of these small septal muscles attaches posterior to the trabecula septomarginalis (see Fig. 1-5 ) and the uppermost, called the medial (conal) papillary muscle (muscle of Lancisi or muscle of Luschka), to the posterior limb of the septal band ( Fig. 1-9 ).

Left Ventricle

The left ventricle consists of a larger sinus portion, which supports a bicuspid AV valve and includes the apex, and a much smaller outlet (outflow) portion beneath a semilunar valve. The inlet and outlet valves of the left ventricle lie juxtaposed within its base, and inflow and outflow portions are separated by the anterior mitral leaflet ² ( Fig. 1-10 ).

2 In this text, cusps of atrioventricular valves are termed leaflets, although current anatomy texts use the term cusp for both semilunar and atrioventricular valves.

The entire free wall of the left ventricle and apical half to two thirds of the septum are trabeculated ( Fig. 1-11 ; see also Fig. 1-10 ), but the trabeculations are characteristically fine compared with those in the right ventricle. The septal surface of the left ventricle may be considered to have a sinus portion, most of which is trabeculated, and a smooth outlet (outflow) portion (see Fig. 1-11 ). The part of the sinus portion of the septum immediately beneath the mitral valve may be termed the inlet septum, and the rest of the sinus portion, the trabecular septum ( Fig. 1-12 ). The outlet (outflow) portion lies in front and to the right of the anterior mitral leaflet, corresponding to the inlet portion on the right ventricular side of the septum, and includes the AV septum ( Fig. 1-13 ). In contrast to the right ventricular side, where the septal tricuspid leaflet is the only valvar attachment to the septum, on the left ventricular side, the rightward half of the anterior mitral valve leaflet attaches to the septum posteriorly, and the right and part of the noncoronary aortic cusps attach to it anteriorly (see Fig. 1-12 ). The leftward half of the anterior mitral leaflet is in fibrous continuity with the aortic valve in an area termed the aortic–mitral anulus ( Fig. 1-14 ; see also Figs. 1-12 and 1-13 ). The anteriorly placed right ventricular infundibular (conal) septum lies opposite the aortic valve ( Fig. 1-15 ). It may occasionally be displaced into the left ventricular outflow beneath the aortic valve; occasionally, muscle may also extend between the aortic and mitral valves, forming a true infundibulum to the left ventricle (see Fig. 1-10 ). The papillary muscles are called anterolateral (or simply anterior ) and posteromedial (posterior). No papillary muscles attach to the left side of the ventricular septum.

Myoarchitecture of the Ventricles

The adult ventricular mass is made up of a three-dimensional network of myocardial cells. This network is highly structured and arranged in layers in which the myocardial cells have a preferred orientation. In all hearts, the ventricular wall is arranged in three layers: superficial (subepicardial), middle, and deep (subendocardial). Superficial and deep layers are present in both right and left ventricles, whereas the middle layer is only present in the left ventricle. The superficial and deep layers are anchored at the ventricular orifices to fibrous structures of the central fibrous skeleton of the heart. This suggests that myocardial contraction plays an active role in cardiac valve function. The middle layer, unique to the left ventricle, shows a circumferential pattern. No planes of fibrous septation are present between the three layers. Instead, the distinction between one layer and the next is made by a change in muscle fiber direction. This is particularly evident in the ventricular septum, where the superficial layer of the right ventricle invaginates at the interventricular sulcus to form a thin muscular layer that forms the right side of the ventricular septum, covering the circumferentially arranged muscle fibers of the middle layer of the left ventricle. There are age-related changes in direction of the muscle fibers in the superficial layer. With advancing fetal and infant age, muscle fiber arrangement progresses from a horizontal to an oblique orientation. This change is especially evident in the right ventricle and probably reflects the changing pressure gradient between right and left ventricles.

The anatomy of the muscular subpulmonary infundibulum was studied by Merrick and colleagues. They point out a freestanding sleeve of myocardium supporting the pulmonary valve that is separate from the underlying anatomic ventricular septum, and Van Praagh argues that this subsemilunar infundibulum “belongs” to the great arteries, not the ventricles. It may be identified by changing directions of myocardial muscle fibers, referred to by surgeons as “layers” of the septum. This anatomic feature makes possible the safe separation of the pulmonary trunk from the right ventricular outflow tract for use as a valve substitute (autograft).

Great Arteries

The aorta is the great artery arising from the base of the heart that normally gives rise to the systemic and coronary arteries. Identity of the aorta is established by recognizing it as the vessel of origin of the brachiocephalic arteries, which never arise from the pulmonary artery. It is not so definitively the vessel of origin of the coronary arteries; occasionally one, or rarely both, coronary arteries may arise from the pulmonary artery ( Appendix 1A ).

The pulmonary trunk (main pulmonary artery) is the great artery that normally gives rise to the pulmonary arterial system. The pulmonary trunk characteristically has no brachiocephalic vessels arising from it. At angiography, differentiation between pulmonary trunk and aorta may require careful study, as the brachiocephalic vessels may opacify with the pulmonary trunk by filling through a patent ductus arteriosus. The pulmonary valve is normally anterior, and the aortic valve posterior and to the right, in individuals with visceral and atrial situs solitus.

Muscular Septum

See “ Right Ventricle ” and “ Left Ventricle .”

Membranous Septum

The membranous septum (pars membranacea) is the fibrous part of the cardiac septum separating the left ventricular outflow tract from, in part, the right ventricle and, in part, the right atrium. The line of division between these components is determined by attachment of the tricuspid valve anulus to the septum (see Fig. 1-12 ). On the right ventricular side of this attachment is the interventricular component. On the right atrial side, it forms the membranous portion of the AV septum.

Atrioventricular Septum

The AV septum is the portion of the cardiac septum that lies between the right atrium and left ventricle. It consists of a superior membranous portion and an inferior muscular portion. The AV septum is apparent because the septal attachment of the tricuspid valve is more apical than the septal attachment of the anterior leaflet of the mitral valve ( Fig. 1-17 ). Viewed from the left ventricular side, the muscular component forms part of the outlet septum (see Fig. 1-13 ). The AV node lies in the atrial septum adjacent to the junction between membranous and muscular portions of the AV septum, and the bundle of His passes toward the right trigone between these two components ( Fig. 1-18 ).

Sinus Node

The sinus (sinoatrial) node is located along the anterolateral aspect of the junction between the superior vena cava and the right atrial appendage ( Fig. 1-19 ). In rare cases, it extends medially across the crest of the caval–atrial junction. The node is superficial, lying just beneath the epicardial surface in the sulcus terminalis, and is approximately 15 × 5 × 1.5 mm. It is pierced by the relatively large sinus node artery. (For details of the blood supply, see “ Coronary Arteries ”.)

Internodal Pathways

The spread of activation between sinus node and AV node occurs preferentially through the muscle bundles delimited by orifices of the right atrium (see Fig. 1-19 ). Considerable histologic and electrophysiologic investigation has been carried out to determine whether pathways of specialized conduction tissue exist within these broad muscle bundles and connect the sinoatrial (SA) and AV nodes. Investigators have not found discrete internodal tracts composed of homogeneous cells or fibers, although some have identified Purkinje-like cells in the major muscle bundles of adult hearts. Controversy continues as to whether these pale cells seen in the atrial myocardium are Purkinje-type cells and whether they form preferential conduction pathways.

Atrioventricular Node

The AV node lies directly on the right atrial side of the central fibrous body (right trigone) in the muscular portion of the AV septum, just anterosuperior to the ostium of the coronary sinus. At times, its posterior margin has been found to lie directly against the coronary sinus ostium. It has a flattened oblong shape and an average dimension in adults of 1 × 3 × 6 mm. Its left surface lies against the mitral anulus. Viewed from the right atrium, the AV node can be localized within a triangle—described by Koch ( Fig. 1-20 )—formed by the tricuspid anulus, tendon of Todaro (continuation of the eustachian valve that runs to the central fibrous body), and coronary sinus ostium. The opening of the coronary sinus is usually a good landmark for the nodal triangle, but in hearts with an abnormal coronary sinus orifice, the nodal triangle is variable in relation to the coronary sinus. Examples of variability are when the coronary sinus opens to the left of the ventricular septum, when there is malalignment between atrial and ventricular septal structures, and when the atrial septum is absent.

Bundle of His and Bundle Branches

The common AV bundle (bundle of His) is a direct continuation of the AV node. The bundle passes through the rightward part of the right trigone of the central fibrous body to reach the posteroinferior margin of the membranous ventricular septum. This area is just inferior to the commissure between the tricuspid valve's septal and anterior leaflets (see Fig. 1-19, B ). Its diameter in the region of the central fibrous body is about 1 mm. The bundle courses along the posteroinferior border of the membranous septum and crest of the muscular ventricular septum, giving off fibers that form the left bundle branch. This branching occurs beneath the commissure between the right and noncoronary cusps in close proximity to the aortic valve, over a distance of 6.5 to 20 mm, after which the remaining fibers form the right bundle branch ( Fig. 1-21 ). The bundle of His lies on the left side of the ventricular septal crest in about 75% to 80% of human hearts and on the right side of the crest in the remainder. In the latter situation, the His bundle connects to the left bundle by a relatively narrow stem.

The left bundle branch fans out over the left ventricular septal surface, gradually forming two or three main radiations. It is not uncommon for the anterior and posterior subdivisions to be accompanied by a central, third radiation that originates from the His bundle or from both of the former subdivisions. The anterior radiation travels toward the base of the anterolateral papillary muscle of the left ventricle. The wider posterior subdivision courses toward the base of the posteromedial papillary muscle. Multiple peripheral anastomoses occur among the subdivisions of the left bundle branch system as it distributes to the left ventricle.

The right bundle branch originates from the bundle of His in the region of the anteroinferior margin of the membranous septum and courses along the right ventricular septal surface, passing just below the medial papillary muscle and along the inferior margin of the septal band and the moderator band to the base of the anterior papillary muscle. The fibers then fan out to supply the walls of the right ventricle. Proximally, the right bundle averages about 1 mm in diameter. It is usually subendocardial in its proximal portion, intramyocardial in its middle portion, and again subendocardial near the base of the anterior papillary muscle. VSDs associated with malalignment of portions of the ventricular septum affect these relationships to some extent.

Mitral Valve

The AV valve of the left ventricle, the mitral valve, is bicuspid, with an anterior (aortic, or septal) leaflet and a posterior (mural, or ventricular) leaflet ( Fig. 1-23 ). Tissue that could be called commissural leaflets is usually present at the commissures between these two leaflets. The combined area of the two mitral leaflets is twice that of the mitral orifice, resulting in a large area of coaptation. When this large area is lost because of malalignment of the leaflets, undue stress is placed on the chordae tendineae, and they may rupture. Although there has been some controversy as to the definition of commissural areas, particularly in regard to clefts in the posterior leaflet, Silver and colleagues describe chordae tendineae that define the limits of the septal (anterior) and posterior leaflets. Rusted and colleagues found the depth of commissures in the normal mitral valve averaged 0.7 to 0.8 cm and never exceeded 1.3 cm in the 50 hearts they studied.

The larger anterior (septal, aortic, anteromedial) leaflet is roughly triangular in shape, with the base of the triangle inserting on about one third of the anulus. It has a relatively smooth free margin with few or no indentations. A distinct ridge separates the region of closure (rough zone) from the remaining leaflet (clear zone). The clear zone is devoid of direct chordal insertions. The anterior leaflet is in fibrous continuity with the aortic valve through the aortic–mitral anulus and forms a boundary of the left ventricular outflow tract. This region of continuity occupies about one fourth of the mitral anulus and corresponds to the region beneath half the left coronary cusp and half the noncoronary cusp of the aortic valve. The limits of this attachment are demarcated by the right and left fibrous trigones ( Fig. 1-24 ). The commissure between the left and noncoronary sinuses of the aortic valve is located directly over the middle of the anterior leaflet of the mitral valve ( Fig. 1-25 ; see also Fig. 1-24 ). These points do not correspond to the commissures of the mitral valve (see Fig. 1-4 ). The AV node and bundle of His are at risk of surgical damage adjacent to the right trigone.

The smaller posterior (mural, ventricular, posterolateral) leaflet inserts into about two thirds of the anulus and typically has a scalloped appearance. Ranganathan and colleagues found the posterior leaflet to be divided into three segments in 46 of the 50 normal mitral valves they studied. The posterior leaflet has rough and clear zones corresponding to those of the anterior leaflet, as well as a basal zone close to the anulus, which receives chordae directly from left ventricular trabeculae.

The mitral valve leaflets may be described using a segmental classification. The valve leaflets are segmented into six sections, A1 to A3 for the anterior and P1 to P3 for the posterior (see Fig. 1-25, C ). Sections A1 and P1 represent the anterolateral sections, A2 and P2 the middle sections, and A3 and P3 the posteromedial sections. This segmental classification has been useful in describing morphology observed at operation, multiplane 2D transesophageal echocardiography, and 3D echocardiography.

The majority of chordae tendineae to the mitral valve originate from the two large papillary muscles of the left ventricle: anterolateral and posteromedial. Each leaflet receives chordae from both papillary muscles, and the majority insert on the free leaflet edge. Papillary muscles are often thought of as fingerlike structures protruding into the left ventricular cavity from the ventricular wall, possibly because these muscles are frequently visualized in two dimensions by angiography or echocardiography. Actually, the papillary muscles have a somewhat crescent shape that conforms to the curvature of the free wall of the left ventricle. This is reasonable because the papillary muscles and chordae tendineae are derived embryologically by undermining of the left ventricular myocardium.

Victor and Nayak examined 100 normal human hearts at autopsy, evaluating and characterizing the papillary muscles and arrangement of the chordae. The anterolateral papillary muscle is attached by chordae tendineae to the left half of the anterior and posterior mitral leaflets (as viewed by a surgeon through the usual right-side approach to the mitral valve), whereas the posteromedial papillary muscle is attached by chordae tendineae to the right-sided half of both anterior and posterior leaflets. Papillary muscles are considered an anterolateral “group” and a posteromedial “group” because there is often more than a single papillary muscle “belly.” There are patterns of mostly single or two muscle bellies, but occasionally three, four, or even five bellies are observed. When there are three muscle bellies, the papillary muscle supporting the chordae to the commissure arises separately from the ventricular wall. Commissural chordae are shorter than the others and usually originate from the highest tip of the papillary muscle. Victor and Nayak also described variations of the chordal attachments. There are usually 4 to 12 chordae originating from each papillary muscle group (range, 2 to 22). Chordal branching results in a number of chordae inserting to the mitral valve leaflet, ranging from 12 to 80.

Acar and colleagues proposed a clinical morphologic classification of the papillary muscles. A single undivided papillary muscle is referred to as type I . Type II refers to papillary muscles cleaved in a sagittal plane into two heads that separately support the anterior and posterior leaflets of the mitral valve. Type III papillary muscles are cleaved in a coronal plane, forming an individual head that supports the commissural chordae. Type IV refers to papillary muscles divided into multiple heads, with a separate papillary muscle originating as a separate muscular band close to the mitral anulus, which supports short chordae to the commissure.

Tandler defined three orders of chordae. Those of the first order insert on the free margin of the leaflet, those of the second order insert a few to several millimeters back from the free edge, and those of the third order insert at the base of the leaflet (applicable only to the posterior leaflet).

Lam and colleagues reclassified chordae into rough zone (including strut chordae), cleft, basal, and commissural chordae . These investigators suggest that this classification provides a clear definition of mitral valve leaflets and should be useful in studying mitral valve function.

The design of the mitral valve offers the largest possible orifice during the diastolic phase of ventricular filling and limits the slightest obstruction to flow at low pressures in the left atrium and left ventricle. The valve opens as the anterior leaflet swings anteriorly away from the posterior leaflet. Orifice dimensions are enhanced by flexion of the anterior leaflet (see Fig. 1-25, B ). During systole, the mitral valve closes under the full load of left ventricular contraction. The anterior leaflet straightens and extends toward the posterior leaflet. The posterior leaflet functions like a shelf to stop the movement of the anterior leaflet as the leaflets appose.

Tricuspid Valve

The tricuspid valve, the AV valve of the right ventricle, has three leaflets: anterior, posterior, and septal ( Fig. 1-26 ). Its orifice is roughly triangular and larger than the mitral orifice. The anulus is relatively indistinct, especially in the septal region. The leaflets and chordae tendineae are thinner than those of the mitral valve. Its orientation is nearly vertical.

The anterior (anterosuperior) leaflet is the largest of the three leaflets and may have notches creating subdivisions. Silver and colleagues found a notch close to the anteroseptal commissure in 47 of the 50 anterior leaflets they examined. This notch was occasionally as deep as a commissure, but could be differentiated from a true commissure by the type of chordal attachments. The chordae attaching to this leaflet arise from anterior and medial papillary muscles. The anterior papillary muscle is the larger of the two, its base arising from the right ventricular free wall and trabecula septomarginalis.

The posterior (inferior) leaflet is usually the smallest and is commonly scalloped. Its chordae originate from the posterior and anterior papillary muscles. It is attached wholly to the ventricular free wall.

The septal leaflet is usually slightly larger than the posterior leaflet. Its chordae arise from the posterior and septal papillary muscles. Most of this leaflet and its chordae attach to the membranous and muscular portions of the ventricular septum, although part may attach to the posterior wall of the right ventricle. The transition between the attachments to the posterior wall and septum is associated with a fold in the leaflet.

Of major surgical importance is proximity of the conduction system to the septal leaflet and its anteroseptal commissure. The membranous septum usually lies beneath the septal leaflet inferior to the anteroseptal commissure, but attachments at the septal and anterior leaflets are variable, so parts of either may attach to the membranous septum. The bundle of His penetrates the right trigone beneath the interventricular component of the membranous septum (usually about 5 mm inferior to the commissure) and runs along the crest of the muscular septum (see Conduction System ). That portion of the septal leaflet between the membranous septum and the commissure extends around the tricuspid anulus, away from the septum, to the right ventricular free wall (see Fig. 1-2 ). This portion of the tricuspid valve may form a flap over some VSDs.

Aortic Valve

The aortic valve is normally tricuspid and composed of delicate cusps and sinuses of Valsalva. These components form three cuplike structures that constitute the entire valve mechanism; the valve is in fibrous continuity with the anterior leaflet of the mitral valve and the membranous septum (see Fig. 1-25 ).

The free edge of each cusp is of tougher consistency than the remainder of the cusp. At the midpoint of each free edge is a fibrous nodulus Arantii . On either side of each nodulus is an extremely thin, crescent-shaped portion of the cusp termed the lunula (see Fig. 1-13 ). The lunulae are occasionally fenestrated near the commissures. These regions form the area of coaptation during valve closure.

The aortic sinuses (sinuses of Valsalva) are dilated pockets of the aortic root that form the outer component of the three cuplike closing structures of the aortic valve (see Fig. 1-25 ). The coronary arteries arise from two of the aortic sinuses. The walls of the sinuses are considerably thinner than the wall of the aorta proper, an important consideration when designing proximal aortotomies.

The crown-shaped anulus, fibrous trigones, aortic cusps, aortic sinuses, and sinutubular junction share a dynamic coordinated action to provide unidirectional transmission of large volumes of blood pumped intermittently through the channel while maintaining laminar flow, minimal resistance, optimal coronary artery flow, and least damage to blood elements during widely variable and frequently changing conditions.

The origins of the coronary arteries are the basis of a nomenclature for the sinuses and cusps. The ostia of the right and left coronary arteries identify the right and left sinuses and cusps. The sinus and cusp without an associated coronary artery are termed noncoronary . Several other nomenclatures for the cusps and sinuses have been described (see Morphology in Chapter 52 ).

Pulmonary Valve

Pulmonary valve structure is similar to that of the aortic valve. The pulmonary valve normally has three cusps, with a nodule at the midpoint of each free edge, and lunulae and thin, crescent-shaped coaptive surfaces on both sides of the nodules. The pocket behind each cusp is the sinus. Major differences from the aortic valve are (1) lighter construction of pulmonary valve cusps, (2) normal absence of coronary artery origins, and (3) normal lack of fibrous continuity with the anterior tricuspid valve leaflet. Pulmonary valve cusps are supported entirely by freestanding musculature, having no direct relationship with the ventricular septum. The pulmonary valve is lifted away from the ventricular septum by the subpulmonary infundibulum. The first septal branch of the left anterior descending coronary artery pierces the ventricular septum below the shortest part of the subpulmonary infundibulum. The artery is protected by the subpulmonary infundibulum.

Pulmonary valve cusps have been described by several terminologies, usually named by their relationships to the aortic valve: right, left, and anterior (nonseptal). Kerr and Goss found that a commissure of the pulmonary valve was adjacent to a commissure of the aortic valve in 199 of 200 specimens they studied. These investigators suggested that the cusps of each arterial valve should be termed right adjacent, left adjacent, and opposite (or, as suggested by Anderson, right facing, left facing, and nonfacing ) in relationship to the adjacent commissure of each valve.

Left Main Coronary Artery

The left main coronary artery extends from the ostium in the left sinus of Valsalva to its bifurcation into the left anterior descending and left circumflex branches. Its usual length is 10 to 20 mm, with a range of 0 to 40 mm. It normally courses between the pulmonary trunk and the left atrial appendage to reach the left AV groove. Occasionally, additional vessels originate from the left main coronary artery and course parallel to the diagonal branches of the left anterior descending branch. Such an additional artery (formerly called a ramus intermedius ) is termed the first diagonal branch of the left anterior descending artery. Rarely (in 1% of persons), the left main coronary artery is absent, the left anterior descending and left circumflex coronary arteries originating directly from the aorta via separate ostia.

Left Anterior Descending Coronary Artery

Beginning as a continuation of the left main coronary artery, the left anterior descending coronary artery courses along the anterior interventricular sulcus to the apex of the heart. Part of it may be buried in muscle. In most cases, this artery extends around the apex into the posterior interventricular sulcus, supplying the apical portion of both right and left ventricles. This vessel supplies branches to the right ventricular free wall (usually small), septum, and left ventricular free wall. One or more branches to the right ventricle connect with infundibular branches from the proximal right coronary artery. This important route for collateral flow is the loop of Vieussens . The septal arteries arise almost perpendicularly from the left anterior descending coronary artery, a characteristic sometimes helpful in angiographic identification of the anterior descending artery. A variable number of diagonal arteries course obliquely between the anterior descending and left circumflex arteries and supply the left ventricular free wall anteriorly and laterally.

Variations in the left anterior descending artery are infrequent, although in about 4% of hearts, it exists as two parallel vessels of about equal size. It may terminate before the apex or extend as far as the posterior AV groove.

Left Circumflex Coronary Artery

The left circumflex coronary artery originates from the left main coronary artery at about a 90-degree angle, with its initial few centimeters lying medial to the base of the left atrial appendage. The sinus node artery occasionally originates from the first few millimeters of the left circumflex artery. Rarely, the circumflex artery terminates before the obtuse margin. A large branch originating from the proximal left circumflex artery and coursing around the left atrium near the AV groove is termed the atrial circumflex artery . The ventricular branches of the circumflex artery, the obtuse marginal arteries, supply the obtuse margin of the heart and may be embedded in muscle. Often their position can then be identified at operation by the altered color (reddish or light tan) of the overlying thin muscle layer compared with that of the remainder of the ventricular wall. Those branches supplying the inferior surface of the left ventricle in a heart with a left dominant system (or in one with a co-dominant system in which the right coronary artery gives rise only to a posterior descending artery) are termed left posterolateral (marginal) arteries . In hearts with a left dominant system, the left circumflex coronary artery gives rise to the posterior descending artery at or usually before the crux. Variations in the origin and length of the left circumflex artery, and in the number and size of its marginal branches, are common.

Right Coronary Artery

The right coronary artery is usually a single large artery and courses down the right AV groove. Branches supplying the anterior right ventricular free wall exit from the AV sulcus in a looping fashion because of the depth of the right coronary artery in the sulcus. In this same area, the anterior right atrial artery arises, and this branch often gives origin to the sinus node artery. More distally, a lateral right atrial artery usually arises (this artery is frequently severed when an oblique right atriotomy is made). In the region of the acute margin of the heart, a relatively constant long branch of the right coronary artery arises, the acute marginal artery, which courses most of the way to the apex of the heart. The right coronary artery in most hearts crosses the crux, where it takes a characteristic deep U-turn, giving off the atrioventricular node artery at the apex of the turn. The right coronary artery then terminates by bifurcating into the right posterior descending coronary artery and the right posterolateral segment artery . The posterior descending coronary artery descends in the posterior interventricular sulcus for a variable distance, giving rise to septal, right ventricular, and left ventricular branches. Variations in its anatomy are numerous, and it frequently arises before the crux. The right posterolateral segment of the right coronary artery gives origin to marginal branches to the inferior surface of the left ventricle in most hearts with a right dominant system.

Variations in the right coronary artery are common. It may have a dual origin from the right sinus of Valsalva. In about 10% of hearts, it bifurcates within a few millimeters of the aortic ostium, forming two diverging trunks of equal size. In half the cases, the artery supplying the right ventricular infundibulum arises separately from the aortic sinus and is then termed the conus artery . The sinoatrial node (sinus node) artery originates from the second or third centimeter of the right coronary artery in many hearts (see “ Coronary Arterial Supply to Specialized Areas of the Heart ”). The acute marginal artery crosses the diaphragmatic surface of the right ventricle in 10% to 20% of hearts and reaches the anterior aspect of the diaphragmatic portion of the ventricular septum, to which it gives branches.

Coronary Arterial Supply to Specialized Areas of the Heart

The predominant blood supply to the ventricular septum is from the left anterior descending coronary artery via four to six large septal arteries 70 to 80 mm in length. In contrast, the septal arteries from the posterior descending coronary artery (except for the AV node artery) are rarely more than 15 mm in length ( Fig. 1-30 ). They supply only a small zone of the ventricular septum near the posterior interventricular sulcus and in the region of the AV node. The septal arteries from the posterior descending artery may, however, serve as an important source of collateral circulation. Until their final terminations, the septal arteries from both anterior and posterior descending arteries course along the right ventricular side of the septum, where pressure is lower than on the left side. In the 10% of hearts with a left dominant circulation, the entire blood supply is from the left coronary artery.

The sinus node artery is a single artery in 89% and double in 11% of hearts. Its origin is from the right coronary artery in 55% to 65% of cases and from the left circumflex or left main coronary artery in the remainder. When it arises from the right coronary artery, it courses posteriorly and superiorly over the anterior wall of the right atrium beneath the right atrial appendage to the base of the superior vena cava ( Fig. 1-31 ). The sinus node artery may penetrate the interatrial septum in its course to the superior vena cava. It then encircles the cava clockwise or counterclockwise, or bifurcates and encircles it in both directions. If the sinus node artery arises from the left circumflex artery, it courses over the left atrial wall, variably penetrates the interatrial septum, and ascends to the base of the superior vena cava, encircling that vessel as when it originates from the right coronary artery.

The AV node artery arises from the characteristic U-turn of the right coronary artery as it crosses the crux of the heart. The AV node is usually supplied by the dominant coronary artery. The AV node artery courses superiorly and anteriorly and terminates with a distinctive angulation. An important accessory blood supply to the AV node is Kugel artery, which originates from the proximal segment of either the right coronary artery or the left circumflex artery and courses through the interatrial septum to the crux of the heart to anastomose with the AV node artery. In the atrial septum, Kugel artery anastomoses with branches of the sinus node artery. Kugel artery is the source of blood supply to the AV node in 40% of normal hearts. The right superior descending artery supplied the AV node in 70% of hearts studied by Abuin and Nieponice. This atrial vessel has its origin within the first centimeter of the right coronary artery, giving branches to the ventriculoinfundibular fold of the right ventricle, penetrating the right atrium, and continuing along the anterior border of the fossa ovalis. The artery goes through the central fibrous body and supplies the bundle of His and the area within the triangle of Koch, including the AV node. Kugel artery and the right superior descending artery are at risk of injury in operations requiring dissection around the right coronary artery (aortic root replacement, Ross procedure) and in the AV groove (extended transseptal approaches to the mitral valve).

The bundle of His and proximal few millimeters of the main bundle branches are supplied by the AV node artery. The remainder of the bundle branches and the Purkinje arborization within the septum are supplied by septal arteries originating from the left anterior descending artery.

The anterolateral papillary muscle of the right ventricle, located near the junction of apical septum and free wall, is supplied by branches from the left anterior descending coronary artery. The anterolateral papillary muscle of the left ventricle is supplied primarily by one or more branches from the left anterior descending coronary artery, but it may also be supplied by circumflex marginal branches. The arterial supply of the posteromedial papillary muscle of the left ventricle is from terminal branches of the right or circumflex arteries, depending on the distribution of these arteries to the inferior surface of the left ventricle.