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 ).

Interior of normal right atrium, viewed from right side at operation. AA, Atrial appendage; AnMV, position of mitral valve anulus on other side of septum, indicated by dotted line; AnTV, anulus of tricuspid valve, indicated by dotted line; ASCTV, anteroseptal commissure of tricuspid valve; CS, coronary sinus orifice; CT, crista terminalis (inside of sulcus terminalis); EuV, eustachian valve; FO, fossa ovalis (sometimes called septum primum ); IVC, inferior vena cava orifice; LFO, limbus of fossa ovalis (C-shaped, extending anteriorly and posteriorly to enclose fossa ovalis); SLTV, septal leaflet of tricuspid valve; SVC, superior vena cava orifice; ThV, thebesian valve; TT, tendon of Todaro; X, muscular portion of atrioventricular septum; Z, membranous portion of atrioventricular septum.

Interior of right atrium, oriented as at operation. Right atrium receives superior and inferior venae cavae. Its trabeculated portion is separated from its smooth portion by the crista terminalis. Fossa ovalis is located in center of atrial septum, surrounded on its superior, anterior, and posterior aspects by the limbus. Coronary sinus is positioned inferiorly. Coronary sinus, along with tendon of Todaro and anulus of septal leaflet of tricuspid valve, form the boundaries of triangle of Koch. Atrioventricular node and proximal portions of bundle of His, portions of the specialized conduction system, lie within the triangle of Koch. Right coronary artery lies in atrioventricular groove, the anatomic point of separation of right atrium and right ventricle.

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, including four-dimensional MRI, three-dimensional [3D] echocardiography, computed tomography [CT], 3D printing, and virtual reality) is 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 atria and atrial situs morphology include venous drainage and the situs indicated by the pulmonary artery and bronchial anatomy. The morphologic identification of each atrium becomes of particular importance in certain conditions such as heterotaxy (see Chapter 53 ).

The left atrium ( Fig. 1.4 ) is the cardiac chamber normally receiving 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 with a windsock appearance, in contrast to the 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.

Interior of normal left atrium viewed from right side at operation as, for example, during mitral valve operations. Stippled area indicates position of right trigone, which contains bundle of His. Crosshatching marks left trigone, the area of greatest risk to aortic valve during mitral valve replacement. AA, Base of atrial appendage; ALMV, anterior leaflet of mitral valve; LPV, orifices of left pulmonary veins; PLMV, posterior leaflet of mitral valve; RIPV, orifice of right inferior pulmonary vein; RSPV, orifice of right superior pulmonary vein; SP, septum primum.

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

Topographically, the right ventricle has a large sinus portion that surrounds and supports a tricuspid 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. * The trabecular portion is the 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 none of it may be interventricular in some malformations (e.g., double outlet right ventricle). 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 (TSM) (septal band). A third small, very anterior portion is a narrow extension superior to the trabecular septum.

Interior of normal right ventricle, particularly trabecular and outlet portions, oriented as at operation. Infundibular (conal) septum separates pulmonary valve from tricuspid valve, and only its rightward portion and inferior part of its central portion form part of the interventricular septum (see also Fig. 1.6 ). Entire outlet portion of septum of infundibulum is composed of the septal extension of infundibular septum, anterior limb of trabecula septomarginalis (TSM) (septal band), and in front of that, a heavily trabeculated portion of septum. AL, Anterior (superior) limb of TSM; AP, anterior papillary muscle; InfS, infundibular (conal) septum; MB, moderator band; MS, position of membranous septum; PE, parietal extension of infundibular septum (parietal band); PL, posterior limb of TSM, giving origin to medial papillary muscle; SE, septal extension of infundibular septum; TS, trabeculated portion of septum, part of which lies in infundibulum and remainder in sinus portion of ventricle; TSM, trabecula septomarginalis (septal band); VIF, ventriculoinfundibular fold.

Right ventricular side of septum after right atrium, right ventricle, and pulmonary trunk have been exposed by removing their anterior walls and rightward portion of aorta and parietal band (or parietal extension of infundibular septum). Entire right ventricular septum is displayed, together with relationship of infundibular septum to aortic root. In this heart, infundibular septum is less prominent than in some. Dashed line defines AV portion of membranous septum. Dotted lines define arbitrary division of sinus septum into posterior (beneath septal tricuspid leaflet), middle, and apical portions. Specimen corresponds to a right anterior oblique projection in cineangiography. A, Left anterior division septal band; Ao, aorta; CoS, coronary sinus; FO, fossa ovalis; IS, cut end of infundibular septum; NC, noncoronary aortic sinus; P, right posterior division of septal band; PT, pulmonary trunk; R, right coronary aortic sinus; SLTV, septal tricuspid leaflet; TSM, trabecula septomarginalis (septal band).

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 supraventricularis . 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 TSM ^, ( Fig. 1.8 ). The TSM extends apically to become continuous with the moderator band, a prominent trabeculation running from septum to free wall.

Demonstration of interrelationships between right ventricular aspect of ventricular septum and other structures in a longitudinal coronal section through heart. AA, Ascending aorta; LC, left coronary cusp; LPC, left pulmonary cusp; MPM, medial papillary muscle; NC, noncoronary cusp; RC, right coronary cusp; TSM, trabecula septomarginalis (septal band); TV, tricuspid valve.

Interior of right ventricle after it has been opened close to anterior septal margin and along its acute margin inferiorly, and the anterior wall hinged to the right. Specimen is oriented anatomically, with aorta and pulmonary trunk at top. Pulmonary trunk also has been opened. Attachments of trabecula septomarginalis (septal band) are clearly demonstrated. A, Left anterior division of TSM; APM, anterior papillary muscle; InfS, infundibular (conal) septum; MB, moderator band; P, right posterior division of septal band giving origin to medial papillary muscle; PPM, posterior papillary muscle; PV, pulmonary valve; TSM, trabecula septomarginalis (septal band).

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, form 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 cusps 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 posteriorly to the TSM (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 ).

Interior of right ventricle, oriented as at operation. Right ventricle is divided into three portions: inlet, containing tricuspid valve and surrounding ventricular septum; sinus, or coarse trabecular portion; and outlet, or conus portion, containing infundibular septum and pulmonary valve. Right lateral extension of infundibular septum merges with right ventricle as ventriculoinfundibular fold (parietal band). Medially and to left, infundibular septum merges with right ventricle to form Y-shaped muscle bundle called TSM (septal band). Trabecula septomarginalis extends to apex as the moderator band. An important landmark is the medial (conal) papillary muscle of the tricuspid valve.

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 ^† ( Fig. 1.10 ).

Interior of left ventricle after anterior ventricular and aortic walls have been excised, leaving obtuse margin, posterior free wall, and septum intact. Specimen is oriented anatomically. Posterior (mural) mitral valve leaflet lies against posterior free wall, whereas anterior mitral leaflet hinges in part from fibrous subaortic curtain and in part from septum and separates outflow portion of ventricle from remaining sinus portion. Arrow indicates direction of left ventricular outflow. Papillary muscles and chordae tendineae support mitral valve. APM, Anterior papillary muscle; ALMV, anterior leaflet of mitral valve; P, posterior free wall; PLMV, posterior leaflet of mitral valve; PPM, posterior papillary muscle; S, septal surface.

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 is 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 aorticmitral 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, and 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.

Interior of left ventricle after the free wall, including mitral valve apparatus, has been displaced to observer’s right and away from the septal surface by a fish-mouth incision into the left ventricle and aorta to demonstrate sinus and outlet portions of the septum. ALMV, Anterior leaflet of mitral valve; MS, membranous septum; NC, noncoronary aortic cusp; O, outlet septum; R, right coronary aortic leaflet; S, sinus septum.

Interior of normal left ventricle, viewed from a slightly different perspective than in Fig. 1.11 to demonstrate inlet, outlet, trabecular, and membranous portions of septum. ALPM, Anterolateral papillary muscle; AoM, aortic-mitral anulus (continuity); InS, inlet septum; MS, membranous septum; OS, outlet septum; PMPM, posteromedial papillary muscle; TS, trabecular septum.

Ventricular septum from its left ventricular side, displaying relationship between its outflow portion and aortic and mitral valves. Pins protrude along line of attachment of tricuspid septal leaflet to right ventricular side of septum. Septal tissue inferior to this and dashed line correspond to right ventricular inflow, and septal tissue superior to it corresponds to AV septum. Arrow indicates a nodulus Arantii. (In this specimen, also shown in Fig. 1.11 , right coronary artery ostium is located eccentrically near right noncoronary commissure.) ALMV, Anterior leaflet of mitral valve; AV, atrioventricular septum (muscular portion); L, left aortic cusp; MS, membranous septum, with AV portion superior to dashed line and interventricular portion inferior to it; NC, noncoronary aortic cusp; R, right aortic cusp.

Interior of left ventricle, lateral view. Trabecular and outflow portions of ventricular septum are demonstrated. Inflow portion is beneath and behind mitral valve. Anterior leaflet of mitral valve is in fibrous continuity with aortic valve. Passageway below aortic valve, bounded by outflow portion of ventricular septum and anterior leaflet of mitral valve, is called the left ventricular outflow tract . Mitral valve is supported by two papillary muscles, anterior and posterior, arising from free wall of left ventricle.

Transverse section of heart at level of medial papillary muscle of tricuspid valve, showing curvature of septum that results in right ventricular infundibulum lying superior and anterior to aortic valve. Curvature of papillary muscles of the mitral valve is also shown. (Specimen is from a 9-month-old infant with a patent ductus arteriosus and pulmonary hypertension.) AV, Aortic valve; LV, left ventricle; MV, mitral valve; P, posterior division of trabecula septomarginalis (septal band) and medial papillary muscle; PB, parietal band (parietal extension of infundibular septum); PV, pulmonary valve; RV, right ventricle; TV, tricuspid valve; VS, ventricular septum.

The adult ventricular mass is made up of a 3D 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 present only 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 orientation. 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 the direction of 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.

Merrick and colleagues studied the anatomy of the muscular subpulmonary infundibulum. 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 this subsemilunar infundibulum “belongs” to the great arteries, not the ventricles. It may be identified by changing directions of myocardial muscle fibers, which surgeons call “layers” of the septum. This anatomic feature makes safe separation of the pulmonary trunk from the right ventricular outflow tract possible for use as a valve substitute (autograft) in the Ross operation (see Chapter 12 ).

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 connect to the pulmonary trunk (see Chapter 38 ).

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 connecting to 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.

See “Right Ventricle” and “Left Ventricle.”

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.

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 ).

Cardiac septation. AV septum lies between left ventricle and right atrium. Septal attachment of tricuspid valve is more toward apex of heart than is attachment of mitral valve. Left ventricular outflow tract is angulated to the right.

Diagram of right heart, aortic root, and conduction tissue at approximately 65-degree right anterior oblique projection. Plane of mitral valve attachment (dashed line) corresponds to atrial edge of muscular AV septum and inferior edge of membranous septum, but differs from plane of tricuspid valve (solid line) . Muscular portion of AV septum is frequently smaller than depicted. Ao, Ascending aorta; AVN, atrioventricular node extending into bundle of His and right bundle branch; AVS, muscular atrioventricular septum; CS, coronary sinus; FO, fossa ovalis; IVC, inferior vena cava; M, moderator band; MS, membranous septum, crossed by attachment of tricuspid valve; MV, mitral valve anulus; RC, right coronary artery; S, portion of trabecula septomarginalis (septal band); SVC, superior vena cava; TV, tricuspid valve.

(Modified from McAlpine WA. Heart and Coronary Arteries . Berlin: Springer-Verlag; 1975.)

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 .”)

Cardiac conduction system. (A) Sinus node is located on anterolateral aspect of junction between superior vena cava and right atrial appendage. Internodal pathways are not well defined anatomically and are presented here as proposed pathways. AV node lies in triangle of Koch. Right and left bundle branches spread out on subendocardial surfaces of right and left aspects of ventricular septum. (B) This section of the heart, taken more posterior than that shown in A, demonstrates location of AV node in triangle of Koch, bounded by coronary sinus, anulus of tricuspid valve septal leaflet, and tendon of Todaro. The common AV bundle (bundle of His) continues from AV node to penetrate central fibrous body and reach posteroinferior margin of membranous septum. Left bundle branch spreads over left ventricular aspect of ventricular septum. Right bundle branch continues on right ventricular surface of ventricular septum into moderator band.

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 regarding whether these pale cells seen in the atrial myocardium are Purkinje-type cells and whether they form preferential conduction pathways.

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 of 1 × 3 × 6 mm in adults. 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.

Anatomic and surgical aspects of cardiac conduction system. (A) Diagram of triangle of Koch within right atrium; triangle is defined by tendon of Todaro, orifice of coronary sinus, and tricuspid anulus. (B) Diagram showing relationship of AV node and AV bundle (bundle of His) to triangle of Koch. AV node lies within the triangle, and AV bundle is located at apex of triangle. CS, Coronary sinus; FO, fossa ovalis; IVC, inferior vena cava; RAA, right atrial appendage; Rvv, right venous valve; SVC, superior vena cava.

(From Anderson and colleagues. ).

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 close 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.

Diagram of AV conduction system and its relationship to membranous septum and aortic valve, viewed from left ventricular side. AV node is in the AV septum on right atrial side of the right trigone, which is beneath the nadir of the posterior (noncoronary) cusp of aortic valve. Common AV bundle courses along posteroinferior margin of membranous septum, giving off fibers that form the left bundle branch. This region lies beneath the commissure of right and noncoronary (posterior) aortic cusps. Right bundle branch originates from common AV bundle in the region of anteroinferior border of membranous septum. approx, Approximately; AVN, atrioventricular node; CB, branching portion of common atrioventricular bundle; fasc, fascicle; LBB, left bundle branch; MS, membranous septum; RBB, right bundle branch.

(From Titus JL. Normal anatomy of the human cardiac conduction system. Mayo Clin Proc . 1973;48:24.)

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 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.

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 about the definition of commissural areas, particularly regarding 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.

Normal mitral valve viewed from an anterolateral left ventriculotomy, with anterolateral left ventricular wall held forward and to observer’s right. ALMV, Anterior leaflet of mitral valve; ALPM, anterolateral papillary muscle; LVOT, left ventricular outflow tract; PLMV, posterior leaflet; PMPM, posteromedial papillary muscle.

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 His bundle are at risk of surgical damage adjacent to the right trigone.

Diagram of the crownlike area of attachment of aortic valve. (A) View from direction of mitral valve. This area is in continuity with the membranous septum and anterior (septal) mitral valve leaflet. Subaortic curtain is the aortic–mitral anulus. Posterior commissure of aortic valve is directly over midpoint of anterior leaflet of mitral valve. (B-C) Aortic wall flattened out, illustrating U-shaped attachment of aortic valve.

(From Zimmerman J. The functional and surgical anatomy of the aortic valve. Isr J Med Sci . 1969;5:862.)

Anatomic relationships of mitral and aortic valves. (A) Commissure between left and noncoronary sinuses of aortic valve is located directly above midpoint of anterior leaflet of mitral valve. Dashed line represents the midpoint of both anterior and posterior leaflets of mitral valve, separating the chordal attachments to anterior and posterior papillary muscles. (B) Three-cusp semilunar aortic valve opens widely during systole without touching aortic wall, owing to sinuses of Valsalva. During diastole, mitral valve opens not only as anterior leaflet swings away from posterior leaflet but also as it flexes, to create the widest possible orifice. This is illustrated as a series of flexion lines. (C) Segmental classification used to describe mitral valve morphology at surgery and 2-dimensional and 3-dimensional echocardiography.

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 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 and 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.

Most 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. Usually, 4 to 12 chordae originate from each papillary muscle group (range, 2 to 22). Chordal branching results in several chordae inserting into 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 clearly defines 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.

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.

Normal open tricuspid valve, viewed from in front through right ventricular cavity. ALTV, Anterior leaflet; APM, anterior papillary muscle; ASCTV, anteroseptal commissure; MPM, medial papillary muscle; PLTV, posterior leaflet; PPM, posterior papillary muscle; SLTV, septal leaflet.

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 TSM.

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.