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).

Hypertrophy of heart muscle

In hypertrophic cardiomyopathy, there is an increase in the thickening of the myocardial walls, particularly the interventricular septum, which is disproportionately thickened in comparison with the posterior wall. Echocardiography allows accurate assessment of the thickening and of systolic function. Other features in hypertrophic cardiomyopathy are dynamic left ventricular outflow obstruction, systolic anterior motion of the anterior mitral valve cusp, and midsystolic closure of the aortic valve. A degree of diastolic dysfunction is also present in some cases of hypertrophic cardiomyopathy. Serial short-axis magnetic resonance imaging (MRI) allows accurate measurement of wall thickness and is particularly useful in assessing hypertrophy confined to the apex. Gradient-echo MRI also allows some functional assessment of the hypertrophy. A number of histological changes are observed, including disarray of the cardiac myocytes with replacement fibrosis, and expansion of the collagen component. Treatment is usually medical, except for refractory cases and those in whom the left ventricular outflow tract obstruction has a gradient of greater than 50 mmHg. Ventricular septal myotomy and myectomy are performed in such cases. More recently, catheter alcohol septal ablation has been introduced as a non-surgical alternative. A number of patients may also require implantation of cardiac defibrillators to prevent sudden cardiac death.

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