Anatomy




It is axiomatic that, to understand abnormal anatomy and to describe it adequately, it is necessary to understand normal cardiac anatomy, including the relationships of the conduction tissues and coronary arteries to the various components of the heart. I review these features in this chapter. An appreciation of normal anatomy is the key to the understanding of the terms and concepts outlined in the previous chapter. The changes in these features of normality in congenitally malformed hearts will then be emphasised in the various chapters within the book concerned with specific lesions. At this stage, I place emphasis on the diagnostic features of the chambers that permit their recognition when the heart itself is congenitally malformed.


THE HEART WITHIN THE CHEST


My account begins with a description of the normal interrelationships of the chambers within the heart, and the location of the heart itself within the chest. The heart normally occupies the middle compartment of the mediastinum, with two-thirds of its bulk to the left of the midline ( Fig. 2-1 ). The long axis shows a considerable obliquity relative to the long axis of the body, extending roughly along a line drawn through the right shoulder to the left hemidiaphragm. Despite this discrepancy between the planes of the body and those of the heart, the cardiac structures should still be described relative to the bodily coordinates, that is, in attitudinally appropriate orientation, although this basic rule of anatomy has not always been followed. 1 Usually described in terms of a triangle, the true shape of the heart as projecting to the frontal surface is more trapezoidal, with horizontal upper and lower borders, a more or less vertical right border just outside the edge of the sternum and a sloping left border extending out to the apex in the fifth intercostal space ( Fig. 2-2 ). The most instructive single plane to be found within the heart is the so-called base. In this respect, the term base is itself used in various ways. The true base of the heart is the posterior aspect of the atrial chambers relative to the mediastinum. More usually, the term is applied to the base of the ventricular mass. A section along the short axis across this ventricular base contains all four cardiac valves. When viewed in attitudinally appropriate fashion from the front, the pulmonary valve is seen to be located superiorly and to the left, with the aortic, mitral and tricuspid valves overlapping when traced in rightward and inferior direction ( Fig. 2-3 ). Interrogation of the short-axis section from the atrial aspect emphasises the central location of the aortic valve, with its leaflets, and their supporting aortic sinuses, being related to all of the cardiac chambers ( Fig. 2-4 ).




Figure 2-1


As shown by this cast of a normal heart superimposed on the frontal chest radiograph, the heart is a mediastinal structure with two-thirds of its bulk positioned to the left of the midline.



Figure 2-2


In considering the arrangement of the cardiac silhouette as seen in frontal projection as shown in Figure 2-1 , it can best be likened to a trapezium, with a longer inferior border adjacent to the diaphragm. The trapezium itself can then be broken down into atrial ( red ) and ventricular ( blue ) triangles, with the ventricular triangle having its own base and apex, the latter corresponding with the cardiac apex.



Figure 2-3


The positions of the valves are shown within the cardiac silhouette, as seen in the frontal projection.



Figure 2-4


The dissection shows the short axis of the heart viewed from its atrial aspect, and illustrates the keystone location of the aortic valve relative to the other cardiac valves.


In considering the location of the heart, note should be taken of the pericardium, and the major nerves that cross it. The fibrous pericardium can be likened to a cardiac seat belt, with its attachments to the diaphragm, along with the entrances and exits of the great veins and arterial trunks, anchoring the heart within the mediastinum. The tough fibrous pericardial sac is lined with a serous layer, the parietal pericardium, which is itself reflected onto the surface of the heart as the epicardium. Two important recesses are found within the cavity, namely the transverse and oblique sinuses ( Fig. 2-5 ).




Figure 2-5


The cartoon shows the arrangement of the pericardium relative to the heart as seen in the parasternal long-axis echocardiographic cut. The transverse sinus within the pericardial cavity lines the inner curvature, while the oblique sinus is behind the left atrium.


Coursing through the mediastinum, and embedded within the fibrous pericardium, are the vagus and phrenic nerves ( Fig. 2-6 ). Both sets of nerves traverse the length of the heart on each side, with the phrenic nerves anterior and the vagus nerves posterior to the hilums of the lungs. Note should also be taken of the recurrent laryngeal nerves, which pass round the brachiocephalic trunk on the right side and the arterial duct, or its ligamentous remnant, on the left. The thymus gland is also a prominent structure related to the anterior and lateral aspects of the pericardial sac in the region of exit of the great arterial trunks, particularly in neonates and infants, while the oesophagus is, perhaps, the most important mediastinal structure related directly to the heart.




Figure 2-6


The heart is shown as seen by the surgeon through a median sternotomy. The locations of the vagus and phrenic nerves are shown relative to the opened pericardial sac.


The Chambers within the Heart


The key to full understanding of cardiac anatomy is the realisation that the heart is not arranged in the upright fashion of a Valentine heart. 1 Instead, the long axis of the heart extends from right to left with considerable obliquity. When seen in frontal projection, the anterior surface of the silhouette is occupied for the most part by the right atrium and ventricle. The left atrium is almost entirely a posterior structure, with only its appendage projecting to the left upper border, while only a strip of left ventricle is seen down the sloping left border. The so-called right chambers of the heart, therefore, are basically anterior, with the ventricles situated to the left and inferiorly relative to their atrial counterparts ( Fig. 2-7 ). The aortic and mitral valves are closely related one to the other within the base of the left ventricle, while the pulmonary and tricuspid valves are separated in the roof of the right ventricle by the extensive supraventricular crest, known classically in its Latin form as the crista supraventricularis. The crest itself is intimately related on its posterior aspect to the aortic valve and root. The diaphragmatic border of the ventricular mass, made up of the right ventricle, exhibits a sharp angle between the sternocostal and inferior surfaces, known as the acute margin. In contrast, the left border of the ventricular mass, formed by the left ventricle, has a much gentler curve, and is the obtuse margin. Important grooves are found within the various surfaces, namely the atrioventricular, or coronary, grooves, which more or less mark the cardiac short axis, and the interventricular grooves, which indicate the long axis, and mark the location of the ventricular septum. A particularly important point is found on the diaphragmatic surface, positioned inferiorly rather than posteriorly when the heart is located within the body, where the interventricular groove joins the atrioventricular groove. This is the so-called cardiac crux.




Figure 2-7


The cavities of the heart have been cast in red for the so-called right chambers, and blue for the left chambers. The casts are positioned in attitudinally correct orientation, and show that, in reality, the right chambers are positioned anterior to their supposedly left counterparts. The tip of the left atrial appendage is the only part of the left atrium that projects to the frontal silhouette, and only a small strip of left ventricle is seen when the cardiac contour is viewed in frontal projection.


The Morphologically Right Atrium


The right atrium in the normal heart is recognised most readily as the chamber receiving the systemic venous return through the superior and inferior caval veins, along with the venous return from the heart itself through the coronary sinus. These channels open into the smooth-walled venous component of the atrium. In addition, the atrial chamber possesses a smooth-walled area that we described as the vestibule. This layer of muscle inserts into the leaflets of the tricuspid valve. The atrium also has a characteristic septal surface, and the extensive and trabeculated appendage ( Fig. 2-8 ). It is the appendage that is the most constant part. This feature, therefore, should be used to permit recognition of the chamber as the morphologically right structure in congenitally malformed hearts. Recognition of structures according to their morphology rather than their location, and using their most constant part in final arbitration, is called the morphological method. 2 As I discussed in the previous chapter, this principle is the basis of logical analysis of congenitally malformed hearts. 3




Figure 2-8


The morphologically right atrium has been opened by a cut through its appendage parallel to the right atrioventricular junction, and the wall of the appendage reflected upwards, revealing that the atrium, in addition to its appendage, possesses a vestibule along with the systemic venous sinus, and is separated by the septum from the left atrium.


The characteristic external feature of the right appendage is its broad triangular shape ( Fig. 2-9 ), along with its extensive junction with the smooth-walled venous component, this being marked by the terminal groove. Internally, the groove matches with the strap-like terminal crest ( Fig. 2-10 ). Taking origin in parallel fashion from the crest and extending laterally into the appendage are the pectinate muscles. In the morphologically right atrium, these muscles extend all round the atrioventricular junction, reaching into the diverticulum located inferior to the orifice of the coronary sinus ( Fig. 2-11 ). Although often considered to be sub-Eustachian, this diverticulum, also described as a sinus, is sub-Thebesian when the heart is seen in attitudinally appropriate position (see Fig. 2-11 ). The extent of the pectinate muscles relative to the vestibule of the right atrioventricular valve is the single most characteristic feature of the right atrium in congenitally malformed hearts. 4 In many hearts, flap-like muscular or fibrous valves take origin from the extent of the terminal crest and guard the orifices of the inferior caval vein and the coronary sinus. These are the Eustachian and Thebesian valves, respectively (see Fig. 2-11 ). The valves, however, are not uniformly present. An important structure in continuation with the Eustachian valve, nonetheless, can almost always be found. This is the tendon of Todaro, 5 which runs through the wall that separates the coronary sinus from the oval fossa, the so-called sinus septum, to insert into the fibrous root of the aorta. This tendon forms one of the borders of the triangle of Koch (see below).




Figure 2-9


The characteristic external feature of the morphologically right atrium is the triangular shape of its appendage, with the terminal groove ( red dotted line ) separating the appendage from the termination of the systemic venous tributaries in the systemic venous sinus.



Figure 2-10


This view of the interior of the morphologically right atrium, shown in attitudinally appropriate orientation, is taken in the operating room from a patient with a septal defect in the floor of the oval fossa. It shows the extensive terminal crest giving rise to the pectinate muscles.

(Courtesy of Dr. Benson R. Wilcox, University of North Carolina, Chapel Hill.)



Figure 2-11


This view of the interior of the morphologically right atrium, made possible as in Figure 2-8 by making an extensive cut parallel to the atrioventricular junction, and reflecting the wall of the appendage superiorly, shows the location of the so-called venous valves, and the position of the sinus septum.


At first sight, the septal surface of the right atrium is extensive, surrounding the oval fossa and incorporating the orifices of the superior caval vein and coronary sinus (see Figs. 2-8 and 2-11 ). This appearance is deceptive. Only the floor of the oval fossa, along with its antero-inferior rim, is made up of tissues that separate the cavities of the two atriums. The apparently extensive rims of the oval fossa, also described as the septum secundum, or the secondary septum, are largely the infolded walls of the atrial chambers. 6 This infolding is particularly prominent superiorly, where it forms the extensive fold between the superior caval and right pulmonary veins ( Fig. 2-12 ). This superior interatrial fold is also known as Waterston’s, or Sondergaard’s, groove. Part of the extensive antero-inferior margin of the oval fossa is unequivocally a septal structure. This is the part formed by muscularisation of the atrial or vestibular spine, also known as the dorsal mesenchymal protrusion. The development of this part of the atrium is discussed extensively, and illustrated, in our chapter devoted to embryology ( Chapter 3 ). Another part is an atrioventricular muscular sandwich, existing because of the more apical attachment of the leaflet of the tricuspid relative to the mitral valve (see below). In this area, an extension of the inferior atrioventricular groove separates the overlapping segments of atrial and ventricular muscle. This area is confluent with the so-called sinus septum, separating the orifices of the coronary sinus and the inferior caval vein (see Fig. 2-11 ). The sinus septum is no more than the adjacent walls of the two venous structures.




Figure 2-12


This section through the atrial chambers in four-chamber plane shows how the superior rim of the oval fossa, the so-called septum secundum, is simply the infolded walls between the origins of the superior caval vein from the right, and the right pulmonary veins from the left atriums, respectively. Note that the floor of the oval fossa, along with its antero-inferior rim, is a true septal structure interposing between the atrial cavities.


The Morphologically Left Atrium


The left atrium, like its right-sided counterpart, possesses a venous component, an appendage, and a vestibule ( Fig. 2-13 ). Again, in keeping with its morphologically right partner, the morphologically left appendage is the most characteristic and constant component. It is a long tubular structure, usually with several constrictions along its length. Its opening with the venous component is restricted, but its most characteristic feature in malformed hearts is that its pectinate muscles are contained within the appendage, or else they spill only marginally onto the septal surface and the anterior part of the wall we described as the vestibule. The vestibule surrounding the posterior part of the atrioventricular groove, therefore, is smooth. The coronary sinus is located within the atrioventricular groove, and hence is an integral component of the morphologically left atrioventricular junction, even though it opens into the cavity of the morphologically right atrium ( Fig. 2-14 ). Its walls are separate from those of the left atrium itself. 7 The pulmonary veins open into the corners of the extensive smooth-walled venous component. The septal surface is formed by the flap valve of the oval fossa, which has a characteristically roughened appearance where it overlaps the infolded superior rim ( Fig. 2-15 ). In addition to these parts, the left atrium also possesses a significant body. The evidence for the existence of the body is seen in the setting of totally anomalous pulmonary venous connection. Even when the pulmonary venous component is lacking, there is part of the left atrial chamber that forms a site of union for the appendage, vestibule and septum. This is the body.




Figure 2-13


As with the right atrium, the morphologically left atrium is made up of an appendage, a venous component, a vestibule and a septal surface. In addition, the left atrium possesses an obvious body, joining together the other parts.



Figure 2-14


This cast of the right and left sides of the heart is photographed to show the diaphragmatic surface. The coronary sinus, formed by the union of the great cardiac and oblique veins, is an integral part of the morphologically left atrioventricular junction, but opens into the cavity of the right atrium.



Figure 2-15


In this heart, photographed to show the septal surface of the left atrium, the oval foramen was probe-patent, as shown by the probe placed between the flap valve of the septum and the infolded superior rim.


The Morphologically Right Ventricle


The muscular walls of the right ventricle extend from the discrete atrioventricular junction to their union with the fibroelastic walls of the pulmonary trunk at the anatomical ventriculo-arterial junction. Within the cavity thus demarcated, there are three components, the inlet, the apical trabecular and the outlet parts ( Fig. 2-16 ). The inlet component contains and supports the leaflets of the tricuspid valve, extending to the attachments of the valvar tension apparatus. The three leaflets of the valve take origin from the septal, inferior or mural, and anterosuperior margins of the atrioventricular junction ( Fig. 2-17 ). The septal leaflet has multiple cordal attachments to the septum. The inferior leaflet runs along the diaphragmatic surface of the ventricle, and its margin with the anterosuperior leaflet is often indistinct. When examined in terms of its pattern of closure, however, there is no doubt about its existence as a third valvar leaflet. 8 The anterosuperior leaflet is the most extensive of the three, and extends from its zone of apposition with the septal leaflet, an area supported by the medial papillary muscle, to the acute margin of the ventricle. A characteristic anterior papillary muscle arises from the prominent apical trabeculation (see below) to support this leaflet, but not always at its site of apposition with the inferior leaflet.




Figure 2-16


The parietal wall of the morphologically right ventricle has been cut away, showing how the ventricular myocardium extends from the atrioventricular ( blue dotted line ) to the ventriculo-arterial junction ( green dotted line ). The ventricle itself has inlet, apical trabecular, and outlet components.



Figure 2-17


As seen from their ventricular aspect, the leaflets of the tricuspid valve occupy anterosuperior, inferior and septal positions.


The apical trabecular part of the ventricle has particularly coarse trabeculations, this being the most constant feature of the ventricle in malformed hearts. One of these trabeculations on the septal surface is particularly prominent, diverging into two limbs at the base to clasp the supraventricular crest. This is the septomarginal trabeculation, or septal band ( Fig. 2-18 ). The medial papillary muscle arises from the posterior limb of this trabeculation, while the anterior papillary muscle springs from the body towards the ventricular apex. The moderator band continues on from the papillary muscle as a discrete muscular bundle, extending to the parietal ventricular wall. A further series of trabeculations extend from the anterior surface of the septomarginal trabeculation and run into the parietal margin of the trabecular zone. These are the septoparietal trabeculations ( Fig. 2-19 ).




Figure 2-18


Opening the right ventricle reveals its muscular roof, the supraventricular crest, which separates the leaflets of the tricuspid and pulmonary valves. The crest inserts to the septum between the limbs of a prominent muscular landmark, the septomarginal trabeculation, or septal band.



Figure 2-19


A series of further muscular structures, the septoparietal trabeculations, arise from the anterior margin of the septomarginal trabeculation, and extend to the parietal wall of the right ventricle. One of these, also extending from the anterior papillary muscle, is the moderator band.


The outlet component of the ventricle is relatively smooth walled. It forms the free-standing sleeve of musculature ( Fig. 2-20 ) that supports the leaflets of the pulmonary valve. The leaflets of the valve themselves are attached in semilunar fashion within the sleeve, crossing the circular junction between ventricular muscle and the fibroelastic wall of the pulmonary trunk ( Fig. 2-21 ). Because of this arrangement, three crescents of ventricular musculature are incorporated within the bases of the sinuses of the pulmonary trunk, while three triangular areas of pulmonary trunk are incorporated within the ventricular outflow tract beneath the tips of the zones of apposition between the valvar leaflets. 8 As a result, the valvar leaflets do not possess an annulus in the sense of a fibrous ring supporting their attachments in circular fashion. Indeed, the most obvious circles within the outflow tract are either the anatomical ventriculo-arterial junction, or else the junction between the valvar sinuses and the tubular pulmonary trunk, the latter best described as the sinutubular junction, and an integral part of the valvar mechanism. There is then another ring that can be constructed by joining together the most proximal parts of the three semilunar leaflets, but this is a virtual structure, with no counterpart ( Fig. 2-22 ). Part of the free-standing subpulmonary infundibular sleeve interposes between the leaflets of the pulmonary and tricuspid valves. This is the supraventricular crest (see Fig. 2-18 ). It is often illustrated as representing a septal structure. In reality, as can be shown by removing the wall ( Fig. 2-23 ), it is largely made up from the inner curvature of the right ventricular musculature. We describe this area as the ventriculo-infundibular fold. It is also the case that a small part of the musculature between the limbs of the septomarginal trabeculation can be removed to provide access to the left ventricle, this small part truly representing a muscular outlet septum ( Fig. 2-24 ). The outlet part, however, cannot be distinguished in normal hearts from the remainder of the muscular ventricular septum. The key feature of the infundibular area, therefore, is the sleeve of free-standing musculature that supports the leaflets of the pulmonary valve, the presence of this sleeve making it possible surgically to remove the valve as an autograft in the Ross procedure. 8–10




Figure 2-20


The pulmonary trunk has been reflected forward relative to the aorta, showing the extensive sleeve of infundibular musculature that lifts the trunk away from the base of the ventricular mass.



Figure 2-21


The pulmonary trunk has been opened, and the valvar leaflets removed from their attachments, revealing the semilunar nature of these attachments. The attachment of each semilunar leaflet crosses the anatomical ventriculo-arterial junction, so that crescents of infundibular musculature are incorporated into the base of each valvar sinus, and triangles of arterial wall are incorporated into the ventricular outflow tract, extending to the level of the sinutubular junction.



Figure 2-22


The cartoon represents the anatomy as depicted in Figure 2-21 . The true rings, or annuluses, are the line over which the walls of the pulmonary trunk join the muscular infundibulum, or the anatomical ventriculo-arterial junction, depicted in yellow , and the sinutubular junction, shown in blue . A third ring can be constructed by joining together the basal attachments of the valvar leaflets, as shown in yellow . The red lines show the semilunar attachments of the valvar leaflets.



Figure 2-23


The dissection shows that the greater part of the supraventricular crest is made up of the parietal ventricular wall. As can be seen, removing this wall reveals the sinuses of the aorta.



Figure 2-24


The dissection shown in Figure 2-23 has been continued, showing that it is possible to remove a very small part of the septum, immediately between the limbs of the septomarginal trabeculation, so as to gain access to the left ventricle. This small part, representing a true muscular outlet septum, cannot be distinguished from the remainder of the septum without the aid of dissection.

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Related posts:

  1. Myocardium and Development
  2. Division of Atrial Chambers
  3. Pulmonary Venous Abnormalities
  4. Other Forms of Functionally Univentricular Hearts

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Apr 6, 2019 | Posted by in CARDIOLOGY | Comments Off on Anatomy

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