Fibrous pericardium

The fibrous pericardium is roughly conical and clothes the heart. Superiorly, it is continuous exteriorly with the adventitia of the great vessels; inferiorly it is attached to the central tendon of the diaphragm and a small muscular area of its left half. Above, the fibrous pericardium not only blends externally with the great vessels, but is continuous with the pretracheal fascia. Anteriorly, it is also attached to the posterior surface of the sternum by superior and inferior sternopericardial ligaments, although the extent of these ‘ligaments’ is extremely variable, and the superior one is often undetectable. The pericardium is securely anchored by these connections and maintains the general thoracic position of the heart, serving as the ‘cardiac seat belt’.

Anteriorly, the fibrous pericardium is separated from the thoracic wall by the lungs and the pleural coverings. However, in a small area behind the lower left half of the body of the sternum and the sternal ends of left fourth and fifth costal cartilages, the pericardium is in direct contact with the thoracic wall. Until it regresses, the lower end of the thymus is also anterior to the upper pericardium. The principal bronchi, oesophagus, oesophageal plexus, descending thoracic aorta and posterior parts of the mediastinal surface of both lungs are posterior relations. Laterally are the pleural coverings of the mediastinal surface of the lungs. The phrenic nerve, with its accompanying vessels, descends between the fibrous pericardium and mediastinal pleura on each side. Inferiorly, the pericardium is separated by the diaphragm from the liver and fundus of the stomach.

The aorta, superior vena cava, right and left pulmonary arteries and the four pulmonary veins all receive extensions of the fibrous pericardium. The inferior vena cava, which traverses the central tendon, has no such covering.

Serosal pericardium

The serosal pericardium is a closed sac within the fibrous pericardium, and has a visceral and a parietal layer. The visceral layer, or epicardium, covers the heart and great vessels and is reflected into the parietal layer, which lines the internal surface of the fibrous pericardium. The reflections of the serosal layer are arranged as two complex ‘tubes’: the aorta and pulmonary trunk are enclosed in one, and the superior and inferior venae cavae and the four pulmonary veins in the other. The tube surrounding the veins has the shape of an inverted J. The cul-de-sac within its curve is behind the left atrium and is termed the oblique sinus. The transverse sinus is a passage between the two pericardial ‘tubes’ (Fig. 56.1). It has the aorta and pulmonary trunk in front and the atria and great veins behind (see Fig. 56.2B,D). The arrangement of the oblique and transverse sinuses, along with that of the main ‘principal’ cavity, is further affected by the development of complex three-dimensional pericardial recesses between adjacent structures. These recesses can be grouped according to the siting of their orifices or ‘mouths’. From the principal pericardial cavity, the postcaval recess projects towards the left behind the atrial termination of the superior vena cava. It is limited above by the right pulmonary artery and below by the upper right pulmonary vein. Its mouth opens superolaterally to the right. The right and left pulmonary venous recesses each project medially and upwards on the back of the left atrium between the superior and inferior pulmonary veins on each side, indenting the side walls of the oblique sinus. The superior aortic recess extends from the transverse sinus. From its mouth, located inferiorly, it ascends posterior to, then to the right of, the ascending aorta and ends at the level of the sternal angle. The inferior aortic recess, also extending from the transverse sinus, is a diverticulum descending from a superiorly located mouth to run between the lower ascending part of the aorta and the right atrium. The left pulmonary recess, with its mouth under the fold of the left vena cava, passes to the left between the inferior aspect of the left pulmonary artery and the upper border of the superior left pulmonary vein. The right pulmonary recess lies between the lower surface of the proximal part of the right pulmonary artery and the upper border of the left atrium.

Fig. 56.1 Interior of the serosal pericardial sac after section of the large vessels at their cardiac origin and removal of the heart (seen from the front). See text for additional named recesses of the general serosal pericardial cavity and its transverse sinus.

(From Sobotta 2006.)

Fig. 56.2 The heart and great vessels: A, Anterior and B, posterior views with corresponding three-dimensional reconstructions from multislice CT scanning (C, D). (Main figures from Sobotta 2006.)

A triangular fold of serosal pericardium is reflected from the left pulmonary artery to the subjacent upper left pulmonary vein as the fold of the left superior vena cava. It contains a fibrous ligament, a remnant of the obliterated left common cardinal vein (left duct of Cuvier). This ligament descends anterior to the left pulmonary hilum from the upper part of the left superior intercostal vein to the back of the left atrium, where it is continuous with the oblique vein of the left atrium. The left common cardinal vein may persist as a left superior vena cava which then replaces the oblique vein of the left atrium and empties into the coronary sinus. When both common cardinal veins persist as right and left superior venae cavae, the transverse anastomosis between them, which normally forms the left brachiocephalic vein, may be small or absent. When there is a left superior vena cava, it is joined by the left superior intercostal vein.

Cardiac size, shape and external features

The heart is a hollow, fibromuscular organ of a somewhat conical or pyramidal form, with a base, apex and a series of surfaces and ‘borders’. Enclosed in the pericardium, it occupies the middle mediastinum between the lungs and their pleural coverings (Fig. 56.1). It is placed obliquely behind the body of the sternum and the adjoining costal cartilages and ribs. Approximately one-third of the mass lies to the right of the midline.

An average adult heart is 12 cm from base to apex, 8–9 cm at its broadest transverse diameter and 6 cm anteroposteriorly. Its weight varies from 280 to 340 g (average 300 g) in males and from 230 to 280 g (average 250 g) in females. Cardiac weight is 0.45% of body weight in males and 0.40% in females. Adult weight is achieved between the ages of 17 and 20 years. The oblique position of the heart may be emphasized by comparing it to a rather deformed pyramid, with the base facing posteriorly and to the right, and the apex anteriorly and to the left. A line from the apex to the approximate centre of the base, projected posterolaterally, emerges near the right midscapular line. Some surfaces of the cardiac ‘pyramid’ are flat, others more or less convex, these aspects merging along rather ill-defined ‘borders’. Precise definition of surfaces and intervening ‘borders’ is, therefore, difficult. In the account that follows, official nomenclature (Terminologia Anatomica 1998) and more generally used terms from clinical practice are given as alternatives. The heart is described as having a base and apex, its surfaces being designated as sternocostal (anterior), diaphragmatic (inferior) and right and left (pulmonary). Its borders are termed upper, inferior (‘acute’ margin or border) and left (‘obtuse’ margin or border). Some name the right surface a ‘border’, despite its extent. One avoidable source of confusion is the use of ‘posterior’, which can be replaced with the unambiguous term ‘diaphragmatic’. If posterior is to be used for a cardiac surface, it should be reserved for the base. (However, compounding this difficulty, there are a number of different usages of the term ‘cardiac base’.)

The heart is placed obliquely in the thorax. The atrial and ventricular septal structures are virtually in line, but inclined forwards and to the left at 45° to a sagittal plane. The planes of the mitral and tricuspid valves, although vertical and not precisely co-planar, are broadly at right angles to the septal plane. The right atrium, therefore, is not only to the right, but also anterior and inferior to the left atrium. It is also partly anterior to the left ventricle, an important atrioventricular septum intervening. The right ventricle forms most of the anterior aspect of the ventricular mass (Fig. 56.3), only its inferior end is to the right of the left ventricle, its upper left extremity (pulmonary orifice) is to the left and superior relative to the aortic valve. The left atrium forms most of the posterior aspect of the heart, whereas the left ventricle is only prominent inferiorly, running along the left margin to reach the apex. The atria are essentially right of and posterior to their respective ventricles. These general dispositions are of the greatest importance in planning or interpreting radiographs, scans, angiocardiograms and echocardiograms.

Fig. 56.3 Dissection that displays the heart, the great vessels and the lungs in situ: the manubrium sterni has been retracted cranially and the thymus has been removed totally. The pericardium has been partially removed and the hilum of the lung has been dissected to expose the tracheobronchial lymph nodes.

(From Sobotta 2006.)

Grooves on the cardiac surface

The division of the heart into four chambers produces boundaries that are visible externally as grooves (sulci). Some are deep and obvious and contain prominent structures. Others are less distinct, even barely perceptible, and are sometimes obscured, in part, by the major structures that cross them. The interatrial groove is a shallow groove separating the two atria. The lateral limits are defined by the borders of the atria. The atrioventricular (coronary) groove (or sulcus) separates the atria from the ventricles. This groove, containing the main trunks of the coronary arteries, is oblique. It descends to the right on the sternocostal surface, separating the right atrium (and its auricle or appendage) from the oblique right margin of the right ventricle and its infundibulum. Its upper left part is obliterated where it is crossed by the pulmonary trunk and, behind this, the aorta, from which the coronary arteries originate. Continuing to the left, the groove curves around the ‘obtuse’ margin and descends to the right, separating the atrial base from the diaphragmatic surface of the ventricles (Fig. 56.2A,D). This diaphragmatic part of the atrioventricular groove then curves around the ‘acute’ margin at its lower right end to become confluent with the sternocostal part. Thus the groove passes from high on the left to low on the right, with the diaphragmatic part being a little to the left of the sternocostal. A section that includes the atrioventricular groove is at 45° to the sagittal plane and at a greater but variable angle to the transverse and coronal planes. It approximately traverses the lines of attachment of the atrioventricular valves and (even less precisely) those of the aortic and pulmonary valves. A line at right angles to the centre of this plane will descend forwards and leftwards to the cardiac apex.

Internally, the ventricles are separated by the septum. The mural margins of the septum correspond to the anterior and inferior (diaphragmatic) interventricular grooves. The anterior groove, seen on the sternocostal cardiac surface, is near and almost parallel to the left ventricular obtuse margin. On the diaphragmatic surface, the groove is closer to the midpoint of the ventricular mass. The interventricular grooves extend from the atrioventricular groove to the apical notch on the acute margin, which is a little to the right of the true cardiac apex.

Cardiac base, apex, surfaces and borders

Right atrium

Interior surface

The interior surface of the right atrium can be divided into three regions: a smooth-walled venous component posteriorly that leads, anteriorly, to the vestibule of the tricuspid valve and the auricle (Fig. 56.4A,B). The wall of the vestibule is smooth, but its junction with the auricle is ridged all around the atrioventricular junction. The smooth-walled part receives the opening of the venae cavae and the coronary sinus. It represents the venous component (sinus venosus) of the developing heart. The wall of the vestibule has a ridged surface and that of the auricle is trabeculated. Both are derived from the embryonic atrium proper.

Fig. 56.4 A, The interior of the heart, revealed by incising it along its right and part of the lower surface. The rest of the heart has been turned over to the left. B, The triangle of Koch, which is defined by the tendon of Todaro, orifice of the coronary sinus and the septal cusp of the tricuspid valve.

(A from Sobotta 2006.)

The superior and inferior venae cavae open into the venous component. The superior vena cava returns blood from head, neck and upper limb through an orifice that faces inferoanteriorly and has no valve, and also receives blood from the chest wall and the oesophagus via the azygos system. The inferior vena cava is larger than its superior counterpart: it drains blood from all structures below and including the diaphragm into the lowest part of the atrium near the septum. Anterior to its orifice is a flap-like valve, the Eustachian valve or valve of the inferior vena cava (Fig. 56.4A,B). Of varying size, this valve is found along the lateral, or right, margin of the vein. When traced inferiorly, it runs into the sinus septum (see below), where it is contiguous with the valve of the coronary sinus. The lateral part of the Eustachian valve becomes continuous with the lower end of the terminal crest. The valve is a fold of endocardium enclosing a few muscular fibres. It is large during fetal life, when it serves to direct richly oxygenated blood from the placenta through the foramen ovale of the atrial septum into the left atrium. The valve varies markedly in size in postnatal life; it is sometimes cribriform or filamentous but often is absent. A particularly prominent recess, behind the Eustachian valve, is seen posteroinferiorly relative to the ostium of the coronary sinus.

The coronary sinus opens into the venous atrial component between the orifice of the inferior vena cava, the fossa ovale and the vestibule of the atrioventricular opening (Fig. 56.4A,B). The coronary sinus is often guarded by a thin, semicircular valve that covers the lower part of the orifice (Thebesius’ valve, also known as the Thebesian valve). The upper limb of this valve joins the Eustachian valve; a tendinous structure, the tendon of Todaro, runs from this commissure into the sinus septum, which is the septum between the coronary sinus and the fossa ovale. The tendon of Todaro runs forwards to insert into the central fibrous body and is one of the landmarks of the triangle of Koch (see below). The ostium of the coronary sinus forms a prominent landmark in the right atrium. The sinus itself lies within the left atrioventricular groove (Fig. 56.2B,D), and is the conduit for return of most of the venous blood from the heart, although some atrial veins drain directly to the right or left atrial chambers. The coronary sinus begins at the point where the oblique vein of the left atrium joins the great cardiac vein. The sinus receives the middle and small cardiac veins close to its junction with the right atrium.

Several small venous ostia, draining the minimal atrial veins, are found scattered around the atrial walls. They return a small fraction of blood from the heart, and are most numerous on the septal aspect. The anterior cardiac veins and, sometimes, the right marginal vein may enter the atrium through larger ostia.

The atrium proper and the auricle are separated from the venous sinus by the crista terminalis. This smooth, muscular ridge begins on the upper part of the septal surface and, passing anterior to the orifice of the superior vena cava, skirts its right margin to reach the right side of the orifice of the inferior vena cava. It marks the site of the right venous valve of the embryonic heart, and corresponds externally to the terminal groove. The sinu-atrial node is located within the superior part of the groove, lateral to and extending below the orifice of the superior vena cava (Fig. 56.4A,B).

The pectinate muscles (musculi pectinati), almost parallel muscular ridges, extend anterolaterally from the terminal crest and reach into the auricle, where they form several trabeculations.

The septal wall presents the fossa ovale, an oval depression above and to the left of the orifice of the inferior vena cava. Its floor is the primary atrial septum, the septum primum. The rim of the fossa is prominent and, although often said to represent the edge of the so-called septum secundum, in reality it is merely the infolded walls of the atrial chambers. It is most distinct above and in front of the fossa, and is usually deficient inferiorly. A small slit is sometimes found at the upper margin of the fossa, ascending beneath the rim to communicate with the left atrium. This represents failure of obliteration of the fetal foramen ovale, which remains patent in up to one-third of all normal hearts.

Anteroinferior in the right atrium is the large, oval vestibule leading to the orifice of the tricuspid valve. A triangular zone, the triangle of Koch, is defined between the attachment of the septal cusp of the tricuspid valve, the anteromedial margin of the ostium of the coronary sinus, and the round, collagenous, palpable, subendocardial tendon of Todaro (Fig. 56.4A,B). The triangle is a landmark of particular surgical importance, indicating the site of the atrioventricular node and its atrial connections. Anterosuperior to the insertion of the tendon of Todaro, the septal wall is formed by the atrioventricular component of the membranous septum, intervening between the right atrium and subaortic outlet of the left ventricle (Fig. 56.5). The atrial wall bulges anterosuperiorly above the membranous septum. This area is the aortic mound (torus aorticus) and marks the location of the non-coronary sinus of the aorta with its enclosed valvular cusp.

Fig. 56.5 A dissection opening the ventricles, viewed from the front.

(From Sobotta 2006.)

Right ventricle

The right ventricle extends from the right atrioventricular (tricuspid) orifice nearly to the cardiac apex. It then ascends to the left to become the infundibulum, or conus arteriosus, reaching the pulmonary orifice and supporting the cusps of the pulmonary valve. Topographically, the ventricle possesses an inlet component which supports and surrounds the tricuspid valve; a coarsely trabeculated apical component; and a muscular outlet or infundibulum, which surrounds the attachments of the cusps of the pulmonary valve.

Tricuspid valve

The atrioventricular valvular complex, in both right and left ventricles, consists of the orifice and its associated anulus, the cusps, the supporting chordae tendineae of various types and the papillary muscles.

Harmonious interplay of all these, together with the atrial and ventricular myocardial masses, depends on the conduction tissues and the mechanical cohesion provided by the fibroelastic cardiac skeleton. All parts change substantially in position, shape, angulation and dimensions during a single cardiac cycle.

Tricuspid valvular orifice

The tricuspid valve orifice is best seen from the atrial aspect and measures on average 11.4 cm in circumference in males and 10.8 cm in females. It has a clear line of transition from the atrial wall or septum to the lines of attachment of the valvular cusps. Its margins are not precisely in a single plane. It is almost vertical, but at 45° to the sagittal plane and slightly inclined to the vertical, such that it ‘faces’ (on its ventricular aspect) anterolaterally to the left and somewhat inferiorly (Fig. 56.6). Roughly triangular, its margins are described as anterosuperior, inferior and septal, corresponding to the lines of attachment of the valvular cusps.

Fig. 56.6 Relation of the sternocostal surface and valves of the heart to the thoracic cage. The right heart is blue, the arrow denotes the inflow and outflow channels of the right ventricle; the left heart is treated similarly in red. The positions, planes and relative sizes of the cardiac valves are shown. The position of the letters A, P, T and M indicate respectively the aortic, pulmonary, tricuspid and mitral auscultation areas of clinical practice. Note that, for the purpose of illustration, the orifices of the aortic, mitral and tricuspid valves are shown with some separation between them. In reality, the cusps of the three valves are in fibrous continuity (see Fig. 56.10).

The connective tissues around the orifice of the atrioventricular valves separate the atrial and ventricular myocardial masses completely, except at the point of penetration of the atrioventricular bundle, and vary in density and disposition around the valvular circumference. Extending from the right fibrous trigone component of the central fibrous body are a pair of curved, tapered, subendocardial tendons, or ‘prongs’ (fila coronaria) that partly encircle the circumference. The latter is completed by more tenuous, deformable fibroblastic sulcal areolar tissue. Although the extent of fibrous tissue varies with sex and age, the tissue within the atrioventricular junction around the tricuspid orifice is always less robust than similar elements found at the attachments of the mitral valve. The topographical ‘attachment’ of the free valvular cusps in the tricuspid valve does not wholly correspond to the internal level of attachment of the fibrous core of the valve to the junctional atrioventricular connective tissue: the line of attachment of the cusp is best appreciated in the heart when examined grossly, and this feature is also more readily discerned clinically.

Opening of the tricuspid valve

Despite its name, the tricuspid valve acts more like a bicuspid valve, because the septal cusp, the smallest of the three cusps, is fixed between the right and left fibrous trigones and the atrial and ventricular septa. The remainder of the tricuspid anulus is muscular. During diastole the right ventricle relaxes, the anulus dilates and the large anterior and posterior cusps move away from the plane of the anulus into the right ventricle. During systole the anulus constricts as the right ventricle contracts, and the two major cusps move like sails to abut a relatively immobile septal cusp and the septum itself (Fig. 56.7).

Fig. 56.7 Summary of some of the principal events that occur in the cardiac cycle and which are mentioned at various points throughout the chapter. Systole begins at the onset of the first heart sound (see phonocardiogram) and ends at the onset of the second heart sound, when diastole begins and this cycle repeats itself.

Pulmonary valve

The pulmonary valve, guarding the outflow from the right ventricle, surmounts the infundibulum and is situated at some distance from the other three cardiac valves (Figs 56.8, 56.9, 56.10). Its general plane faces superiorly to the left and slightly posteriorly. It has three semilunar cusps attached by convex edges partly to the infundibular wall of the right ventricle and partly to the origin of the pulmonary trunk. The line of attachments is curved, rising at the periphery of each cusp near their zones of apposition (the commissures) and reaching the sinutubular ridge of the pulmonary trunk (Fig. 56.11A–C). Removal of the cusps shows that the fibrous semilunar attachments enclose three crescents of infundibular musculature within the pulmonary sinuses, whereas three roughly triangular segments of arterial wall are incorporated within the ventricular outflow tract beneath the apex of each commissural attachment. There is, thus, no proper circular ‘anulus’ supporting the cusps of the valve, and the fibrous semilunar attachment is an essential requisite for snug closure of the nodules and lunules of the cusps (see below) during ventricular diastole. It is difficult to name the cusps and corresponding sinuses of the pulmonary valve and trunk precisely according to the coordinates of the body, because the valvular orifice is obliquely positioned. The official nomenclature (Terminologia Anatomica 1998) refers to an anterior, a posterior and a septal cusp, based on their position in the fetus. The position changes with development and in the adult there is one anterior semilunar cusp, and right and left semilunar cusps.

Fig. 56.8 The anterior or sternocostal surface of the heart.

(From Sobotta 2006.)

Fig. 56.9 The base of the ventricles, after removal of the atria and the pericardium. Coronary arteries and cardiac veins can be seen. Contrast the planes and positions of the aortic and pulmonary valves. Contrast with Fig. 56.10.

(From Sobotta 2006.)

Fig. 56.10 Principal elements of the fibrous skeleton of the heart. For clarity, the view is from the right posterosuperior aspect. Perspective causes the pulmonary anulus to appear smaller than the aortic anulus, whereas in fact the reverse is the case. Consult text for an extended discussion.

(Copyright from The Royal College of Surgeons of England. Reproduced with permission.)

Fig. 56.11 The structure of the aortic root is best conceptualized in terms of a three-pronged coronet (A): there are at least three rings within this coronet, but none supports the entirety of the attachments of the valvular cusps (compare with C). In B, the cusps have been resected at their attachment to the aortic wall. Note the relationship of the cusp insertions and the ventriculo-arterial junction. In C, the root of the aorta has been cut open and distended, in order to show the insertion of the semilunar cusps. Note the zone of fibrous continuity between the cusps of the aortic and mitral valves and their relationship to the fibrous trigones, and the semilunar attachment of the cusps (compare with B).

(Redrawn by courtesy of Professor RH Anderson, Institute of Child Health, University College, London.)

Each cusp is a fold of endocardium, with an intervening, and variably developed, fibrous core. The core is substantial along both the free edge and the semilunar attached border; the latter is particularly thickened at the deepest central part (nadir) of the base of each cusp (thus never forming a simple complete fibrous ring). The free margin of each cusp contains a central localized thickening of collagen, the nodule of the semilunar ‘cusp’ (nodule of Arantius). Perforations within the cusps close to the free margin and near the commissures are frequently present, but are of no functional significance. Each semilunar cusp is contained within one of the three sinuses of the pulmonary trunk.

Left atrium

Although smaller in volume than the right, the left atrium has thicker walls (3 mm on average). Its cavity and walls are formed largely by the proximal parts of the pulmonary veins, which are incorporated into the atrium during development. The only clear derivative of the left part of the embryonic atrium is the auricle, together with the vestibule of the mitral valve. The left atrium is roughly cuboidal and extends behind the right atrium, separated from it by the obliquely positioned septum, thus the right atrium is in front and anterolateral to the right part of the left atrium. The left part is concealed anteriorly by the initial segments of the pulmonary trunk and aorta: part of the transverse pericardial sinus lies between it and these arterial trunks. Anteroinferiorly, and to the left, it adjoins the base of the left ventricle at the orifice of the mitral valve. Its posterior aspect forms most of the anatomical base of the heart and is approximately quadrangular, receiving the terminations of (usually) two pulmonary veins from each lung. It forms the anterior wall of the oblique pericardial sinus (Fig. 56.1). This surface ends at the shallow vertical interatrial groove, which descends to the cardiac crux. The left atrial auricle is constricted at its atrial junction and all the pectinate muscles of the left atrium are contained within it. It is characteristically longer, narrower and more hooked than the right auricle, its margins being more deeply indented. It turns forwards to the left of the pulmonary trunk, overlapping its origin (Fig. 56.8). Interiorly, the four pulmonary veins open into the upper posterolateral surfaces of the left atrium, two on each side. Their orifices are smooth and oval, the left pair frequently opening via a common channel. Some minimal cardiac veins return blood directly from the myocardium to the cavity of the left atrium. The left atrial aspect of the septum has a characteristically rough appearance, bounded by a crescentic ridge, concave upwards, which marks the site of the foramen ovale.

Left ventricle

General and external features

The left ventricle is constructed in accordance with its role as a powerful pump that sustains pulsatile flow in the high-pressured systemic arteries. Variously described as half-ellipsoid or cone-shaped, it is longer and narrower than the right ventricle, extending from its base in the plane of the atrioventricular groove to the cardiac apex. Its long axis descends forwards and to the left. In transverse section, at right angles to the axis, its cavity is oval or nearly circular, with walls about three times thicker (8–12 mm) than those of the right ventricle (Fig. 56.12). It forms part of the sternocostal, left and inferior (diaphragmatic) cardiac surfaces. Except where obscured by the aorta and pulmonary trunk, the base of the ventricular cone is superficially separated from the left atrium and atrial auricle by part of the atrioventricular groove; the coronary sinus runs in the posterior aspect of the groove to reach the right atrium (Fig. 56.2B,D). The anterior and posterior interventricular grooves indicate the lines of mural attachment of the ventricular septum and the limits of the left and right ventricular territories. The sternocostal surface of the ventricle curves bluntly into its left surface at the obtuse margin.

Fig. 56.12 Left and right ventricles: cross section perpendicular to axis of the heart; superior aspect.

(From Sobotta 2006.)

Internal features

The left ventricle has an inlet region, guarded by the mitral valve (ostium venosum), an outlet region, guarded by the aortic valve (ostium arteriosum), and an apical trabecular component. The left atrioventricular orifice admits atrial blood during diastole, flow being towards the cardiac apex. After closure of the mitral cusps, and throughout the ejection phase of systole, blood is expelled from the apex through the aortic orifice. In contrast to the orifices within the right ventricle, those of the left ventricle are in close contact, with fibrous continuity between the cusps of the aortic and mitral valves (the subaortic curtain; Fig. 56.13). The inlet and outlet turn sharply round this fibrous curtain (Fig. 56.9).

Fig. 56.13 The aortic orifice opened from the front to show the cusps of the aortic valves, their nodules, lunules, commissures and the triple-scalloped line of anular attachment. Also shown is the continuity of the subaortic curtain with the mitral anterior cusp (i.e. ‘aortic baffle’) and the coronary ostia, and the spatial relationship of the aortic orifice to the pulmonary orifice and to the left ventricle.

(From Sobotta 2006.)

The anterolateral wall is the concavo-convex ventricular septum, a muscular wall the convexity of which is the posteromedial profile of the right ventricle as seen in section. It thus completes the circular outline of the left ventricle (Fig. 56.12). Towards the aortic orifice, the septum becomes the thin, collagenous interventricular component of the membranous septum, an oval or round area below and confluent with the fibrous triangle separating the right and the non-coronary cusps of the aortic valve.

Between the lower limits of the free margins of the cusps of the mitral valve and the apex of the ventricle, the muscular walls are deeply trabeculated. These trabeculae carneae are finer and more intricate than those of the right ventricle, but similar in structure. Trabeculation is characteristically well developed near the apex, whereas the upper reaches of the septal surface are smooth (Fig. 56.13).

Mitral valve

The general comments already made in respect to the tricuspid valve apply equally to the mitral. The valve has an orifice with its supporting anulus, cusps and a variety of chordae tendineae and papillary muscles.

Aortic valve

The smooth left ventricular outflow tract, or aortic vestibule, ends at the cusps of the aortic valve. Although stronger in construction, the aortic valve resembles the pulmonary (Figs 56.9, 56.11, 56.13) in possessing three semilunar cusps, supported within the three aortic sinuses of Valsalva. Although the aortic valve, like the pulmonary valve, is often described as possessing an anulus in continuity with the fibrous skeleton, there is no complete collagenous ring supporting the attachments of the cusps. As with the pulmonary valve, the anatomy of the aortic valve is dominated by the fibrous semilunar attachment of the cusps (Fig. 56.11C).

Echocardiography

The gross anatomy of the heart can be evaluated by two-dimensional echocardiography in the para-sternal, apical, suprasternal and subcostal positions (Fig. 56.14). The standardized planes used are long axis, short axis and four-chamber. Echocardiography allows a detailed assessment of the functional anatomy of the heart. The long-axis view is obtained by placing the ultrasound transducer in the left apicosternal position and provides detailed images of the left ventricle, aorta, left atrium, and mitral and aortic valves (Fig. 56.14C). Angling the beam towards the right also allows assessment of the right atrium, right ventricle and tricuspid valves. Rotating the transducer by 90° in the clockwise direction produces the short-axis view, which allows assessment of the left ventricle, papillary muscles, chordae tendineae and mitral valves (Fig. 56.14B). The four-chamber view demonstrates the ventricles, atria, and mitral and tricuspid valves (Fig. 56.14A). Rotation of the transducer allows two-chamber views of the heart and more detailed assessment of the aorta and aortic valves. Cardiac magnetic resonance and computer tomography provide similar information on cardiac structure and function (Fig. 56.14D–F), together with complementary information on great vessels and other extracardiac intrathoracic stuctures.

Fig. 56.14 Cardiac anatomy shown by transthoracic echocardiography and computed tomography. A, Four-chamber view. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. B, Short-axis view at aortic valve level. Note three aortic cusps R, right; L, left; and NC, non-coronary; and central position of the aorta; LA, left atrium; PV; pulmonary valve, RA, right atrium; RV; right ventricle; TV, tricuspid valve. C, Parasternal long-axis view; AV, aortic valve; LA, left atrium; LV, left ventricle; RV, right ventricle; MV; mitral valve. D, E and F, Corresponding images by computed tomography. Note different orientation compared with transthoracic echocardiography (looked at from below, foot-to-head). R, right; L, left; A, anterior; P, posterior; H, head; F, foot.

(Images courtesy of Dr Konstantinos Dimopoulos, Royal Brompton and Chelsea and Westminster Hospitals, London.)

Abnormalities of cardiac position

The most severe abnormality of position is an extrathoracic heart, so-called ectopia cordis. The heart usually projects to the surface through the lower thoracic and upper abdominal wall, remaining covered, in most instances, by the fibrous pericardium. There is usually additional herniation of the abdominal contents. Another abnormality of cardiac position is a mirror-image i.e. reversal in shape and position of the heart: the heart is predominantly positioned in the right hemithorax (dextroposition) with its apex directed to the right instead of the left (dextrocardia). This arrangement may be part of ‘situs inversus’, where the heart, great vessels and abdominal organs occupy mirror-image positions. Dextrocardia may be associated with Kartagener’s syndrome (a subgroup of ciliary dyskinesias). More usually, an abnormal location of the heart occurs in cases of isomerism, in which both sides of the thorax, including the main bronchi and the atrial appendages, retain features of either morphological rightness or leftness. Isomerism is also commonly associated with anomalous arrangement of the abdominal organs, right isomerism is associated with ‘absence’ of the spleen (so called asplenia) and left isomerism with multiple spleens (polysplenia). The intracardiac anatomy in cases of isomerism is almost universally abnormal, and there is a range of heart defects (the defects are usually more simple in left atrial isomerism, and more complex in right atrial isomerism). These intrathoracic abnormal arrangements, with or without abdominal abnormalities, may first manifest themselves following ‘routine’ chest radiography. Reversal of normal anatomy has obvious clinical implications, a typical example being a patient with mirror-image arrangements and appendicitis, who presents with an acute left lower quadrant pain.

Most congenital heart abnormalities can be detected during antenatal ultrasound screening. Neonates with severe congenital heart disease may present with tachypnoea, difficulty in feeding, cyanosis and/or cardiovascular collapse. There may be audible murmurs on auscultation. The vast majority of abnormalities may be detected by postnatal echocardiography; in a very few cases, cardiac catheterization and direct measurements of pressure, oxygen saturations and angiography may be required.

Acyanotic cardiac defects

Acyanotic heart defects are the result of either left to right cardiac shunting through heart defects (intra- or extra-cardiac), or of obstruction. Left to right shunting leads to an increased workload and stress on the heart and the lungs as a consequence of increased pulmonary blood flow and, in turn, increased pulmonary venous return. Depending on the location of the shunt and the magnitude of left to right shunting, patients are at risk of developing pulmonary arterial hypertension, unless timely heart surgery is undertaken. Examples of defects leading to left to right shunting are simple septal defects such as atrial or ventricular septal defects or patent ductus arteriosus, or more complex cardiac defects, including atrioventricular septal defects, and/or a combination of any of these defects with abnormal atrioventricular or ventriculoarterial connections (e.g. a double outlet right ventricle, where more than 50% of both the aorta and the pulmonary artery originate from the right ventricle) (Ch. 59). Obstructive lesions may involve the atrioventricular or semilunar valves (mitral, aortic or pulmonary valve stenosis), or narrow vessels (e.g. coarctation of the aorta or interrupted aortic arch). Depending on the level and severity of the obstruction, patients may have a range of clinical presentations from an asymptomatic heart murmur (common) to cardiovascular collapse (uncommon). Treatment in all cases is directed towards normalizing heart workload, systemic and pulmonary blood flow, and cardiac output, and may be surgical and/or catheter-based.

Cyanotic cardiac defects

Cyanotic cardiac defects may be attributable to a right to left intra- or extra- cardiac shunt or to a severe reduction in pulmonary blood flow. They may be caused by simple lesions such as severe pulmonary stenosis with an atrial septal defect and right to left shunting; moderate lesions including Fallot’s tetralogy, in which there is a ventricular septal defect, right ventricular outflow obstruction, right ventricular hypertrophy and an over-riding aorta; or more complex lesions including transposition of the great arteries (Fig. 56.15A,B) and tricuspid and pulmonary atresia (often in the setting of hypoplastic right ventricle and ‘single’ ventricular physiology). Neonates with severe cyanotic congenital heart defects may be dependent on the patency of the ductus arteriosus: early diagnosis, treatment with intravenous prostaglandin infusion and timely transfer to a tertiary centre for more definitive therapy, is key to survival and good long-term outcomes.

Fig. 56.15 A, Cardiac magnetic resonance angiography in a patient with a normal heart. Note the criss-cross relationship of the great arteries; the aorta arises from the left ventricle and runs to the right and posteriorly to the pulmonary trunk (at its origin) and towards the head. The pulmonary artery arises from the right and anterior right ventricle, crosses over and then runs to the left of the aorta and towards the back (where it bifurcates into the right and left pulmonary arteries at the underside of the aortic arch). B, Cardiac magnetic resonance angiogram from a patient with transposition of the great arteries. Note the parallel or side by side relationship of the great vessels and loss of their normal criss-cross relationship. The aorta runs anteriorly and comes off the right ventricle. The hypertrophied right ventricle, supports the systemic circulation, and the ventricular septum bows from right to left. Ao, aorta; PT, pulmonary trunk; RV, right ventricle; LV, left ventricle.

(By courtesy of Dr Philip Kilner, Royal Brompton Hospital, London.)

In recent years, major advances in the diagnosis and management of infants with congenital cardiac malformations have resulted in an increased number of adolescents and adults with palliated or repaired, but not cured, congenital heart disease. This is an area that poses multiple challenges to both cardiovascular and other medical disciplines.

Overview of the conduction system

Of all the cells in the heart, those of the sinu-atrial node generate the most rapid rhythm, and therefore function as the pacemaker of the heart. The impulse, believed to be generated in the P cells, is transmitted over preferentially conducting pathways to right and left atria and to the atrioventricular node. At the atrioventricular node, the impulse is delayed by 40 ms. This delay allows the atria to eject their contents fully before contraction of the ventricles begins. It also places an upper limit on the frequency of signals that can be transmitted to the ventricles. Slender transitional cells, closer in morphology to normal cardiac cells, extend from the node into the stem and principal branches of the atrioventricular bundle (of His). Here, they become continuous with cells of more distinctive appearance, the Purkinje fibres. Conduction of the impulse is rapid in the bundle and its branches (2–3 metres per second, as opposed to 0.6 metres per second in normal myocardium). The cardiac impulse therefore arrives at the apex of the heart before spreading through the ventricular walls, producing a properly coordinated ventricular ejection.

The human heart beats ceaselessly at 70 cycles every minute for many decades, maintaining perfusion of pulmonary and systemic tissues. The rate and stroke volume fluctuate in response to prevailing physiological demands. Figure 56.7 summarizes the principal events in a cardiac cycle, including: the electrical events recorded in the electrocardiogram; the mechanical sequences of diastole, atrial systole, isovolumetric contraction, ejection and isovolumetric relaxation in ventricular systole; the acoustic phenomena recorded in the phonocardiogram; the pressure profiles of right and left hearts and arterial trunks: the sequences of valvular events. Cardiac efficiency depends on precise timing of the operation in interdependent structures. Passive diastolic filling of the atria and ventricles is followed by atrial systole, stimulated by discharge from the sinu-atrial node, which completes ventricular filling. Excitation and contraction of the atria must be synchronous and finish before ventricular contraction. This is effected by a delay in the conduction of excitation from atria to ventricles. Thereafter, ventricular contraction proceeds in a precise manner. A specialized ventricular conduction system ensures that closure of atrioventricular valves is followed rapidly by a wave of excitation and contraction, which spreads from the ventricular apices towards the outflow tracts and orifices, rapidly accelerating the blood during ejection.

Cardiac contraction originates unequivocally in specialized myocytes, but neural influences are important in adapting the intrinsic cardiac rhythm to functional demands from the entire body. All cardiac myocytes are excitable, and display autonomous rhythmic depolarization and repolarization of the cell membrane, conduction of waves of excitation via gap junctions to adjacent myocytes, and excitationcontraction coupling to their actomyosin complexes. These properties are developed to different degrees in different sites and in different types of myocyte (see Ch. 6). The rate of depolarization and repolarization is slowest in the ventricular myocardium, intermediate in the atrial muscle and fastest in the myocytes of the sinoatrial node. These last over-ride those generating slower rhythms and, in the normal heart, are the locus for the rhythmic initiation of cardiac cycles. Conversely, conduction velocity is slow in nodal myocytes, intermediate in general ‘working’ cardiac myocytes and fastest in the myocytes of the ventricular conduction system.

The nodes and networks of the so-called specialized myocardial cells constitute the cardiac conduction system (Figs 56.4A, 56.16A,B and 56.17). The components of this system are the sinu-atrial and atrioventricular nodes, the atrioventricular bundle with its left and right bundle branches and the subendocardial plexus of ventricular conduction cells (Purkinje fibres). Within the system, the main pacemaker rhythm of the heart is generated (sinus), is influenced by nerves (sinus and its innervation) and is transmitted specifically from atria to ventricles (atrioventricular node and bundle) and, within the ventricles, to all their musculature. The spread of excitation is very rapid, but not instantaneous. Different parts of the ventricles are excited at slightly different times, with important functional consequences. Failure of the conduction system will not block cardiac contraction, but the system will become poorly coordinated or uncoordinated. The rhythm will be slower because it originates from a spontaneous (myogenic) activity in the working cardiac myocytes or in a subsidiary pacemaker in a more distal part of the diseased or disrupted conduction system.

Fig. 56.16 The conduction tissue of the heart: A, right aspect; B, left aspect. The elements of the conduction system are shown in purple. Note the conduction tissue accompanying fine trabeculae carneae and false chordae. In reality, the radiation of the left bundle branch is directly related to the cusps of the aortic valve.

Fig. 56.17 The basic structure of the conduction system, and its relationship with the electrocardiogram. Note foramen ovale allowing for communication between the right atrium (RA) and the left atrium (LA). SCV, superior vena cava.

(Redrawn by courtesy of Professor RH Anderson, Institute of Child Health, University College, London.)

There are no specialized internodal and interatrial conduction pathways. The excitation emanating from the sinoatrial node spreads to the atrial musculature and to the atrioventricular node through ordinary atrial working myocardium. The geometric arrangement of fibres along well-organized atrial muscle bundles, e.g. the terminal crest and the rims of the fossa ovale, ensure that conduction is marginally more rapid than elsewhere within the atrium.

Sinoatrial node

The sinoatrial node is an elliptical structure, 10–20 mm long. It is located at the junction between parts of the right atrium derived from the embryonic venous sinus and the atrium proper (Figs 56.4A, 56.17). The node is often covered by a plaque of subepicardial fat, making it visible in some instances to the naked eye. It extends between 1 and 2 cm on the right from the crest of the right auricle and runs posteroinferiorly into the upper part of the terminal groove. In a small proportion of individuals, about 1 in 10, it extends in horseshoe fashion across the crest of the auricle. Nodal tissue does not occupy the full thickness of the right atrial wall from epicardium to endocardium in humans, but rather sits as a wedge of specialized tissue subepicardially within the terminal groove. Its location is marked consistently by a large central artery, which is a branch of either the left circumflex or posterior descending branch of the right coronary artery. Nodal cells are grouped circumferentially around this artery and interwoven into its dense collagenous adventitia. These cells are now considered the ‘pacemakers’, although the functional implications of this relationship are not fully understood. Many nerve fibres are present, although none appears to terminate on cells. There are no autonomic ganglion cells within the node, although many border it anteriorly and posteriorly. P cells are most abundant in the central region. They are small, empty-looking cells, 5–10 μm in greatest diameter, with a large central nucleus. Their pale appearance is attributable to the sparsity of organelles: myofibrils are few and irregularly arranged, and there is no proper sarcotubular system and little glycogen. P cells are less abundant in the periphery of the node, where they mix with slender fusiform transitional cells which are part of a heterogeneous group that is intermediate in appearance between P cells and normal working cardiac cells, and which link P cells to other cells.

Atrioventricular node

The atrioventricular node is an atrial structure that lies at the root of an extensive tree of conduction tissue reaching the apex of the ventricles, the papillary muscles and other regions of the ventricles (Fig. 56.17). The node, with its transitional zones, is located within the atrial component of the muscular atrioventricular septum. Its anatomical landmarks are the boundaries of the triangle of Koch (the attachment of the septal cusp of the tricuspid valve inferiorly, the ostium of the coronary sinus basally, and the tendon of Todaro superiorly) (Fig. 56.4A,B). The compact node is a half oval set against the central fibrous body towards the apex of this triangle. Its atrial aspect is convex, and is overlain by atrial myocardium. Its left margin is concave and abuts on the superior aspect of the central fibrous body. Its basal end projects into the atrial muscle and its anteroinferior end enters the central fibrous body to become the penetrating atrioventricular bundle. The node is pervaded by an irregular collagenous reticulum enmeshing the myocytes, but this is less dense than in the sinu-atrial node. Its arterial supply is from a characteristic vessel that originates from the dominant coronary artery at the crux of the heart. The node has a well-formed compact zone made up of interlocking nodal cells, which frequently show stratification. The transitional cell zones are found superficially and posteriorly. The larger component of atrioventricular delay is probably produced in these transitional zones of the node.

Internodal conduction pathways converge on the atrioventricular node. This is similar in general appearance to the sinoatrial node, although the collagenous component is less dense. The majority of cells are of the transitional type, but P cells resembling those of the sinuatrial node are found in a more fibrous central region. Autonomic ganglia are present between the node and the coronary sinus. In both sinoatrial and atrioventricular nodes, the intercellular contacts between P cells, and between P cells and transitional cells, are much less specialized than the intercalated discs between normal cardiac cells. A sparsity of gap junctions is consistent with the absence from these areas of connexin-43, which is a major protein component of mammalian gap junctions. This probably accounts for the observed difficulty in exciting these cells from adjacent cells. The atrioventricular delay may owe much to this relative inexcitability of the P cells, which appears to disturb the spread of potential in a manner that delays propagation. The narrow diameter of the transitional cells may contribute to the conduction delay.

Atrioventricular bundle

The atrioventricular bundle is the direct continuation of the atrioventricular node, becoming oval, quadrangular or triangular in transverse sectional profile as it enters the central fibrous body (Fig. 56.4A). Traversing the fibrous body, it branches on the crest of the muscular interventricular septum, the branching tract being sandwiched between the muscular and the membranous components of the septum. The right branch of the bundle (crus dextrum) is a narrow, discrete round group of fascicles that courses at first within the myocardium and then subendocardially towards the apex of the ventricle, entering the septomarginal trabecula to reach the anterior papillary muscle. It gives few branches to the ventricular walls in its septal course. At the origin of the anterior papillary muscle, it divides profusely into fine subendocardial fascicles that diverge and, first, embrace the papillary muscle, and then recurve sub-endocardially to be distributed to the remaining ventricular walls. The left branch (crus sinistrum) arises as numerous fine intermingling fascicles that leave the left margin of the branching bundle through much of its course along the crest of the muscular ventricular septum (Fig. 56.16A,B). These fascicles form a flattened sheet down the smooth left ventricular septal surface. The sheet diverges apically and subendocardially across the left aspect of the ventricular septum, separating into anterior, septal and posterior divisions. Fine branches leave the sheets, forming subendocardial networks, which first surround the papillary muscles and then curve back subendocardially to be distributed to all parts of the ventricle.

The principal branches of the bundle are insulated from the surrounding myocardium by sheaths of connective tissue. Functional contacts between ventricular conduction and working myocytes become numerous only in the subendocardial terminal ramifications. Hence, papillary muscles contract first, followed by a wave of excitation and ensuing contraction that travels from the apex of the ventricle to the arterial outflow tract. Because the Purkinje network is subendocardial, muscular excitation proceeds from the endocardial to the epicardial aspect. In the developing heart, it can be shown that the bundle responsible for atrioventricular conduction is a much more extensive structure. Immunohistochemical analysis has revealed that the precursor of the system is a ring of cells that surrounds the inlet and outlet components of the developing ventricular loop (Ch. 59). This ring becomes modified after septation of the ventricles, so that it encircles the right atrioventricular orifice and the aortic outlet from the left ventricle. With subsequent growth, only the septal components of this ‘figure of eight’ persist as the atrioventricular conduction tract. However, remnants of the aortic ring can persist as a ‘dead-end tract’.

The Wolff–Parkinson–White syndrome is caused by abnormal small strands of otherwise unremarkable ventricular myocardium, which connect the atrial and ventricular myocardial masses at some point around the atrioventricular junctions. Histologically, they are strands of working myocardium running through the fibroareolar tissue of the atrioventricular groove.

Cardiac pacing

Temporary pacing wires are usually inserted by cannulation of either the internal jugular vein or the subclavian vein. The subclavian vein approach carries a slightly greater risk of a pneumothorax because of the proximity of the pleural cavity. Other potential risks are brachial plexus injury if the entry site is too posterior, and thoracic duct injury if the left subclavian vein is cannulated. With permanent pacemakers, the most common site at which to locate the device is in a subcutaneous pocket on the anterior chest wall. Access to the heart and the endocardium of the right ventricle and right atrium is gained via the cephalic vein as it lies in the deltopectoral groove.

Cardiac conduction studies

Intracardiac electrocardiography and electrophysiology are used to assess cardiac conduction and rhythm abnormalities. A catheter is inserted via the femoral, subclavian or internal jugular veins using a guidewire technique. Fluoroscopy, echocardiography, and more recently cardiac magnetic resonance, are used to guide accurate placement of the catheters in the appropriate position. The sites of study are high right atrium (for assessing the atrioventricular bundle and right bundle branch) and the coronary sinus (for evaluating atrioventricular junctional arrhythmias and accessory pathways). The multipolar electrodes provide detailed electroanatomical mapping of the sequence of excitation from the atria, atrioventricular junction and ventricles. The origin of supraventricular arrhythmias, ventricular tachycardias, accessory conduction pathways and re-entrant pathways can be identified and used to guide radiofrequency ablation.

Congenital conduction abnormalities

The majority of congenital conduction abnormalities used to have an anatomical basis for their origin, being the product of either accessory pathways or abnormal morphogenesis within the conduction tissue at any point from the atrioventricular node through the atrioventricular bundle. However today, conduction abnormalities are increasingly likely to relate to either long-standing haemodynamic problems and/or the effects of previous surgery for patients with congenital heart defects. This reflects the fact that although surgery for most of these defects has been available for the past four decades, surgery itself has not been curative and patients more often than not develop conduction abnormalities (and arrhythmia) secondary to haemodynamic problems (chamber dilation and/or hypertrophy), due to surgical scars. Very occasionally, conduction abnormalities are caused by tumours such as multifocal Purkinje cell tumours, or benign congenital polycystic tumours of the atrioventricular node.

Coronary arterial supply

The right and left coronary arteries arise from the ascending aorta in its anterior and left posterior sinuses (Figs 56.8, 56.9, 56.18A-E). The levels of the coronary ostia are variable. The two arteries, as indicated by their name, form an oblique inverted crown, in which an anastomotic circle in the atrioventricular groove is connected by marginal and interventricular (descending) loops intersecting at the cardiac apex (Fig. 56.18A–E). This is, of course, only an approximation. The degree of anastomosis varies and is usually insignificant. The main arteries and major branches are usually subepicardial, but those in the atrioventricular and interventricular grooves are often deeply sited, and occasionally hidden by overlapping myocardium or embedded in it.

Fig. 56.18 Anterior views of the coronary arterial system, with the principal variations. The right coronary arterial tree is shown in magenta, the left in full red. In both cases posterior distribution is shown in a paler shade. A, The most common arrangement. B, A common variation in the origin of the sinoatrial nodal artery. C, An example of left ‘dominance’ by the left coronary artery, showing also an uncommon origin of the sinu-atrial artery. Posteroinferior views of the coronary arterial system. The right coronary arterial tree is shown in magenta, the left in full red. D, An example of the more normal distribution in right ‘dominance’. E, A less common form of left ‘dominance’. In these ‘posterior’ views, the diaphragmatic (inferior) surface of the ventricular part of the heart has been artificially displaced and foreshortening ignored, to clarify the details of the so-called posterior (inferior) distribution of the coronary arteries.

The term ‘dominant’ is used to refer to the coronary artery giving off the posterior interventricular (descending) branch, which supplies the posterior part of the ventricular septum and often part of the posterolateral wall of the left ventricle. The dominant artery is usually the right (60%). Anastomoses between right and left coronary arteries are abundant during fetal life, but are much reduced by the end of the first year of life. Anastomoses providing collateral circulation may become prominent in conditions of hypoxia and in coronary artery disease. An additional collateral circulation is provided by small branches from mediastinal, pericardial and bronchial vessels.

The calibre of coronary arteries, both main stems and larger branches, based on measurements of arterial casts or angiograms, ranges between 1.5 and 5.5 mm for the coronary arteries at their origins. The left exceed the right in 60% of hearts, the right being larger in 17%, and both vessels being approximately equal in 23%. The diameters of the coronary arteries may increase up to the 30th year.

Cardiac veins

The heart is drained by the coronary sinus and its tributaries, the anterior cardiac veins and the small cardiac veins. The coronary sinus and its tributaries return blood to the right atrium from the entire heart (including its septa) except for the anterior region of the right ventricle and small, variable parts of both atria and the left ventricle. The anterior cardiac veins drain an anterior region of the right ventricle and, when the right marginal vein joins this group, a region around the right cardiac border, and end principally in the right atrium. The small cardiac veins (Thebesius’ veins) open into the right atrium and ventricle and, to a lesser extent, the left atrium and sometimes the left ventricle.

Coronary sinus

The large majority of cardiac veins drain into the wide coronary sinus, 2 or 3 cm long, lying in the posterior atrioventricular groove between the left atrium and ventricle (Fig. 56.2B,D; Fig. 56.19). The sinus opens into the right atrium between the opening of the inferior vena cava and the right atrioventricular orifice; the opening is guarded by an endocardial fold (semilunar valve of the coronary sinus; Fig. 56.4A). Its tributaries are the great, small and middle cardiac veins, the posterior vein of the left ventricle and the oblique vein of the left atrium; all except the last have valves at their orifices.

Fig. 56.19 The principal veins of the heart.

Great cardiac vein

The great cardiac vein begins at the cardiac apex, ascends in the anterior interventricular groove to the atrioventricular groove and follows this, passing to the left and posteriorly to enter the coronary sinus at its origin (Fig. 56.19). It receives tributaries from the left atrium and both ventricles, including the large left marginal vein that ascends the left aspect (obtuse border) of the heart.

Small cardiac vein

The small cardiac vein lies in the posterior atrioventricular groove between the right atrium and ventricle and opens into the coronary sinus near its atrial end (Fig. 56.19). It receives blood from the posterior part of the right atrium and ventricle. The right marginal vein passes right, along the inferior cardiac margin (acute border). It may join the small cardiac vein in the atrioventricular groove, but more often opens directly into the right atrium.

Middle cardiac vein

The middle cardiac vein (Fig. 56.19) begins at the cardiac apex, and runs back in the posterior interventricular groove to end in the coronary sinus near its atrial end.

Posterior vein of the left ventricle

The posterior vein of the left ventricle (Fig. 56.19) is found on the diaphragmatic surface of the left ventricle a little to the left of the middle cardiac vein. It usually opens into the centre of the coronary sinus, but sometimes opens into the great cardiac vein.

Oblique vein of the left atrium

The small vessel that is the oblique vein of the left atrium (Fig. 56.19) descends obliquely on the back of the left atrium to join the coronary sinus near its end. It is continuous above with the ligament of the left vena cava. The two structures are remnants of the left common cardinal vein.

Lymphatic drainage of the heart

Cardiac lymphatic vessels form subendocardial, myocardial and subepicardial plexuses, the first two draining into the third. Efferents from the subepicardial plexuses form the left and right cardiac collecting trunks. Two or three left trunks ascend the anterior interventricular groove, receiving vessels from both ventricles. On reaching the atrioventricular groove, they are joined by a large vessel from the diaphragmatic surface of the left ventricle, which first ascends in the posterior interventricular groove and then turns left along the atrioventricular groove. The vessel formed by this union ascends between the pulmonary artery and the left atrium, and usually ends in an inferior tracheobronchial node. The right trunk receives afferents from the right atrium and the right border and diaphragmatic surface of the right ventricle. It ascends in the atrioventricular groove, near the right coronary artery, and then anterior to the ascending aorta to end in a brachiocephalic node, usually on the left.

Cardiac plexus

Nearing the heart, the autonomic nerves form a mixed cardiac plexus, usually described in terms of a superficial component inferior to the aortic arch lying between it and the pulmonary trunk, and a deep part between the aortic arch and tracheal bifurcation. The cardiac plexus is also described by regional names for its coronary, pulmonary, atrial and aortic extensions (Fig. 56.20). These plexuses contain ganglion cells. Ganglion cells, confined to the atrial tissues, and with a preponderance adjacent to the sinu-atrial node, are also found in the heart along the distribution of branches of the plexus. Their axons are considered to be largely, if not exclusively, postganglionic parasympathetic. Cholinergic and adrenergic fibres, arising in or passing through the cardiac plexus, are distributed most profusely to the sinu-atrial and atrioventricular nodes; the supply to the atrial and ventricular myocardium is much less dense. Adrenergic fibres supply the coronary arteries and cardiac veins. Rich plexuses of nerves containing cholinesterase, adrenergic transmitters and other peptides, e.g. neuropeptide Y, are found in the subendocardial regions of all chambers and in the cusps of the valves.

Pulmonary trunk

The pulmonary trunk, or pulmonary artery, conveys deoxygenated blood from the right ventricle to the lungs (Fig. 56.2A,C; see Fig. 57.8). About 5 cm in length and 3 cm in diameter, it is the most anterior of the cardiac vessels and arises from the base of the right ventricle (from the pulmonary anulus surmounting the conus arteriosus) above and to the left of the supraventricular crest. It slopes up and back, at first in front of the ascending aorta, then to its left. Below the aortic arch it divides, level with the fifth thoracic vertebra and to the left of the midline, into right and left pulmonary arteries of almost equal size. The pulmonary trunk bifurcation lies below, in front and to the left of the tracheal bifurcation (which is also associated with the inferior tracheobronchial lymph nodes and the deep cardiac nerve plexus). In the fetus, at the level of the bifurcation the pulmonary artery is connected to the aortic arch by the ductus arteriosus, which lies in the same direction as the pulmonary artery.

Thoracic aorta

Coarctation of the aorta

The aortic lumen is occasionally partly or completely obliterated, either above (preductal or infantile type), opposite, or just beyond (postductal or adult type), the entry of the ductus arteriosus. In the preductal type, the length of coarctation is variable, aortic arch hypoplasia is common and the left subclavian and even the brachiocephalic artery may be involved. Severe forms of infantile coarctation and its extreme form (aortic interruption) may be patent ductus arteriosus dependent, as there is no time for effective collateral circulation to develop. Prostaglandin infusion prior to transfer, and surgery at a tertiary centre, often provide a very good mid- to long-term outlook for such infants.

The postductal type of coarctation has been attributed to abnormal extension of the ductal tissue into the aortic wall, stenosing both vessels as the duct contracts after birth. This form can permit years of normal life, allowing the development of an extensive collateral circulation to the aorta distal to the stenosis (Fig. 56.21). High vascularity of the thoracic wall is important and clinically characteristic; many arteries arising indirectly from the aorta, proximal to the coarctation segment, anastomose with vessels connected with it distal to the block, and all of these vessels become greatly enlarged. Thus, in the anterior thoracic wall, the thoraco-acromial, lateral thoracic and subscapular arteries (from the axillary artery), the suprascapular artery (from the subclavian artery), and the first and second posterior intercostal arteries (from the costocervical trunk), anastomose with other posterior intercostal arteries, and the internal thoracic artery and its terminal branches anastomose with the lower posterior intercostal and inferior epigastric arteries. Posterior intercostal arteries are always involved, and enlargement of their dorsal branches may eventually groove (‘notch’) the inferior margins of the ribs. The radiographic shadow of the enlarged left subclavian artery is also increased. Enlargement of the scapular vessels and anastomoses may lead to widespread interscapular pulsation (easily appreciated with the palm of the hand, and sometimes heard on auscultation).

Fig. 56.21 Cardiac magnetic resonance angiography showing 3D reconstruction of native aortic coarctation in an adult patient with extensive collateral flow. Note the marked dilatation of the left subclavian artery, supplying most of the collateral vessels and mild aortic arch hypoplasia.

(By courtesy of Dr Raad Mohiaddin, Royal Brompton Hospital, London.)

Brachiocephalic artery

The brachiocephalic (innominate) artery, the largest branch of the aortic arch, is 4–5 cm in length (Fig. 56.2A,C; see Fig. 28.14). It arises from the convexity of the arch posterior to the centre of the manubrium of sternum, and ascends posterolaterally to the right, at first anterior to the trachea, then on its right. Level with the upper border of the right sternoclavicular joint, it divides into the right common carotid and right subclavian arteries.

Descending thoracic aorta

The thoracic aorta is the segment of descending aorta confined to the posterior mediastinum (see Fig. 55.4B). It begins level with the lower border of the fourth thoracic vertebra, continuous with the aortic arch, and ends anterior to the lower border of the 12th thoracic vertebra in the diaphragmatic aortic aperture. At its origin it is left of the vertebral column, as it descends it approaches the midline, and at its termination is directly anterior to it.

Subclavian arteries

Common carotid arteries

The right and left carotid arteries differ in length and origin. The right carotid is exclusively cervical, and arises from the brachiocephalic trunk behind the right sternoclavicular joint. The left carotid originates directly from the aortic arch immediately posterolateral to the brachiocephalic trunk and therefore has both thoracic and cervical parts.

Superior vena cava

The superior vena cava returns blood to the heart from the tissues above the diaphragm. It is approximately 7 cm in length, and is formed by the junction of the brachiocephalic veins behind the lower border of the first right costal cartilage near the sternum. It descends vertically behind the first and second intercostal spaces, and ends in the upper right atrium behind the third right costal cartilage. Its inferior half is within the fibrous pericardium, which it pierces level with the second costal cartilage. Covered anterolaterally by serous pericardium (from which a retrocaval recess projects), it is slightly convex to the right (Figs 56.1, 56.3; see Fig. 28.14). The superior vena cava has no valves.

Inferior vena cava

The inferior vena cava returns blood to the heart from the tissues below the diaphragm. It passes through the diaphragm between the right leaf and central area of the central tendon of the diaphragm at the level of the eight and ninth thoracic vertebrae, and drains into the inferoposterior part of the right atrium (see Fig. 58.1). The thoracic part is very short, and is partly inside and partly outside the pericardial sac. The extrapericardial part is separated from the right pleura and lung by the right phrenic nerve, and the intrapericardial part is covered, except posteriorly, by inflected serous pericardium. The abdominal course of the inferior vena cava is described in Chapter 62.

Brachiocephalic veins

The right and left brachiocephalic veins join to form the superior vena cava.