The results of the surgical treatment of congenital cardiac malformations have been transformed within the last half-century. Prior to the development of cardiopulmonary bypass, it was impossible to even contemplate the repair of major malformations, even though ingenious surgeons had successfully closed simple intracardiac lesions such as holes between the atrial chambers. Even when it proved possible for the surgeon to work within the heart in the setting of a bloodless operative field, the results of operative repair were far from perfect, and many patients died subsequent to the operative procedure. Nowadays, it is unusual for any patient with a relatively simple defect to die, and even those with “complex” lesions are now expected to survive. There have been many reasons for these truly remarkable advances, but without question one has been the knowledge of detailed intracardiac anatomy accrued within recent decades. In the past, it was often thought that, in the setting of so-called complex malformations, this anatomy was difficult to understand. It is now recognized that, although the combination of given lesions can truly be complex, the anatomy itself is relatively straightforward provided that it is approached in straightforward fashion and using a system of analysis that is simple and logical. To achieve success, therefore, the pediatric cardiac surgeon must hone not only the surgical skills but also the intellectual rigor needed to analyze the structure of the congenitally malformed heart. It would be foolish to suggest that problems do not still exist in obtaining the necessary morphologic understanding, but these problems can be minimized by using a simple philosophy combined with use of words in their vernacular sense. Too many of the problems of the past have reflected linguistic rather than scientific disagreements. It remains appropriate to remember the aphorism attributed to Rudolf Virchow, namely that “those who fail to learn from the mistakes of the past are condemned to repeat them.” In this chapter, therefore, we describe the basics of cardiac anatomy that permit the surgeon to diagnose and recognize the arrangement of the cardiac chambers during surgical procedures and at the same time appreciate the likely position of the vital conduction tissues. The concomitant advances made over recent years in diagnostic techniques are now such that the basic layout of the heart will have almost certainly been established prior to commencement of intracardiac procedures. Nonetheless, even in the most complex cases, preliminary diagnosis demands little more, in terms of morphology, than the distinction of a right atrium from a left atrium, a right ventricle from a left ventricle, and an aorta from a pulmonary trunk. It is such distinctions that provide the basis for simple sequential segmental analysis.¹ For the surgeon, the anatomy of holes and blockages within the heart and great vessels are of equal or even greater significance. This morphology is addressed elsewhere in the appropriate chapters of this book; therefore, we confine ourselves to showing how the anatomy of some important defects differs markedly from the normal arrangement. Our primary concern, however, is to establish the requirements for a systematic and simple approach to cardiac anatomy.

The heart lies in the mediastinum. In most instances, it is found with its apex pointing to the left, and with about two-thirds of its bulk to the left of the midline. If not thus located, the surgeon should immediately consider the possibility that complex malformations lurk within the heart. When the heart is abnormally located, it is our preference to describe the arrangement in simple terms, such as “heart mostly in the right chest with the apex pointing to the left” or as appropriate. In this way, we avoid the need to define arcane terms, such as “pivotal dextrocardia” and comparable locutions. The surgeon usually approaches the heart through the midline anteriorly or via the thoracic cavities. Nowadays, a median sternotomy or a variation on this approach is used most frequently. It is fortunate for the surgeon that the area of the mediastinum immediately behind the sternum is devoid of vital structures. Separate incisions can be made in the suprasternal notch and beneath the xiphoid process, the two being joined by blunt dissection. Splitting of the sternum exposes the pericardial sac lying between the pleural cavities. An important structure in this area is the thymus gland, particularly well developed in infants, which wraps itself over the pericardium in the area of the arterial pole. In dividing or excising the gland, the surgeon must take care of both the arterial supply, from the internal thoracic and inferior thyroid arteries, and the venous drainage through the thymic veins. The latter structures are fragile and often empty via a common trunk to the left brachiocephalic vein, which may be inadvertently damaged by undue traction.

Having exposed the pericardium, the surgeon should then remember that the vagus and phrenic nerves traverse its length, albeit well clear of the operative field (Fig. 59-1). The phrenic nerves, nonetheless, can be damaged should the pericardium be harvested for use as an intracardiac patch or baffle. Excessive traction on the pericardial cavity should be avoided, since it is easy to avulse the origin of the pericardiophrenic arteries, which accompany the phrenic nerves.

Lateral thoracotomies are made most frequently in the fourth intercostal space, using the posterior bloodless triangle between the edges of the latissimus dorsi, trapezius, and teres major muscles. An incision midway between the ribs avoids the intercostal neurovascular bundle, protected beneath the lower margin of the fourth rib. Such an incision takes the surgeon into the pleural space.

On the left side, posterior retraction of the lung exposes the middle mediastinum, revealing the left lobe of the thymus overlying the pericardium and the aortic arch, with its associated nerves and vessels. The aortic isthmus and descending aorta are readily approached in this fashion, dividing the parietal pleura posterior to the vagus nerve. Care must be taken to avoid the left recurrent laryngeal nerve, which takes its origin from the vagus and curves around the inferior border of the arterial ligament or, if this structure is still patent, the arterial duct (Fig. 59-2). The thoracic duct should also be avoided; this vessel drains at the junction of the left subclavian and internal jugular veins.

A right thoracotomy provides access to the right pulmonary artery and adjacent structures. On the right side, the right recurrent laryngeal nerve passes around the subclavian artery as it runs superiorly from the vagus toward the larynx. The stellate ganglion from the sympathetic trunk is also at potential danger in this region, inappropriate damage to this structure resulting in Horner syndrome.

The nature of the cardiac chambers is readily determined by external inspection. The morphology of the appendages best distinguishes the right from the left atrium. These outpouchings usually clasp the arterial pedicle, with one appendage to each side (Fig. 59-3). The arrangement in which both appendages are on the same side of the pedicle is called juxtaposition of the atrial appendages. Should the two appendages both be left-sided, then the cardiac chambers and arterial trunks are almost always abnormally joined together. If both appendages are to the right, however, this being a much rarer finding, the heart is again likely to be malformed, but in much simpler fashion, sometimes with no more that an atrial septal defect. Juxtaposition of the appendages also distorts the internal atrial architecture, particularly the atrial septum—a situation that can produce problems unless properly recognized.

In this chapter, we use the terms “right” and “left” to indicate morphology rather than position, this being one of the basic rules of congenital cardiac morphology. If position is also abnormal, this should be described separately. The appendages are the best structures with which to distinguish the morphologically right and left atriums. The right appendage has an obviously triangular shape, in contrast to the tubular left appendage. Shape, however, can be modified by abnormal hemodynamics. In situations of uncertainty therefore, the surgeon should examine carefully the junctions between the appendages and the remainder of the atrial chambers.² The essential feature of the right atrium is that the pectinate muscles extend all around the atrioventricular junction to the back of the heart. In the left atrium, in contrast, the posterior wall is entirely smooth, the pectinate muscles being confined within the tubular appendage. When the triangular appendage is right-sided and the tubular appendage left-sided, this usual arrangement is frequently called “situs solitus.” Our preference, as already stated, is to use words which have currency in normal language, so we prefer simply to describe it as “the usual arrangement.” On rare occasion, the tubular appendage will be right-sided, with the triangular one on the left side. This is best described as “the mirror-image arrangement.” Although often called “situs inversus,” the appendages in this setting are truly mirror-imaged rather than being turned upside down. Mirror imagery is rare. In the setting of complex malformations, it is more frequent to find the arrangements in which both the appendages have comparable morphology. Both appendages can then be either broad and triangular, with pectinate muscles extending all around the atrioventricular vestibules, or else both can be tubular and narrow, with smooth posterior atrial walls on both sides (Fig. 59-4). These arrangements are accurately described as isomerism of the right or left atrial appendages, respectively. Patients with such arrangements of the appendages also typically have malposed abdominal organs, along with an isomeric arrangement of the lungs and bronchial tree.^1,2 The overall arrangement is called visceral heterotaxy and, in the past, was often categorized on the basis of splenic morphology. These syndromes are the harbingers of the most complex combinations of intracardiac lesions, but it is now unusual for such patients to be denied attempted surgical correction. By the simple expedient of inspecting the appendages, the surgeon now has the means of diagnosing these entities directly in the operating room, should the conditions not have been recognized during the diagnostic workup. Indeed, recognition of the appendages is the more important to the surgeon, since sometimes there is discordance between splenic anatomy and the arrangement of the appendages.³ It is accurate recognition of the appendages that guides the surgeon to an appreciation of the likely location of the sinus node.⁴

Thus, the surgeon should examine carefully the junctions between the appendages and the atrial venous components. If the junction is morphologically right, there will be an extensive terminal groove, marking the site of the terminal crest internally (Fig. 59-5). The sinus node is always located in the immediately subendocardial position within this groove, positioned lateral to the crest of the atrial appendage (Fig. 59-6). The morphologically left junction is never marked by any such prominent groove and similarly lacks a sinus node. Should both junctions be morphologically right, the sinus node will be duplicated and the surgeon should respect both junctions. In the setting of isomeric left appendages, in contrast, the sinus node will be hypoplastic or even absent, and the surgeon should be aware that the nodal remnant can be positioned in the smooth atrial wall close to the atrioventricular junctions.⁴

Having inspected the appendages, the surgeon should turn attention to the venoatrial connections, ensuring that all the pulmonary veins, along with the caval veins and the coronary sinus, are in their appropriate positions. Search should always be made for a persistent left superior caval vein. This will be found between the left appendage and the left pulmonary veins. If the surgeon has already established that there is isomerism of the left atrial appendages, then attention should be directed to the inferior caval vein, which is often interrupted in this setting, venous flow from the abdomen being returned through the azygos venous system, which will be correspondingly enlarged.

External inspection also provides significant information concerning the structure of the ventricular mass. It is the positions of the interventricular branches of the coronary arteries that are the guide.⁵ In the usual situation, the anterior interventricular coronary artery is a branch of the main stem of the left coronary artery, descending close to the obtuse margin of the ventricular mass. Should this artery take its origin from a coronary artery arising in right-sided position from the aortic root, the aorta itself being anterior to the pulmonary trunk, then almost always the ventricles themselves will be arranged in mirror-imaged fashion (so-called left-hand ventricular topology) with the morphologically left ventricle then positioned on the right. Identification of such a coronary arterial pattern should alert the surgeon to the likely presence of congenitally corrected transposition, the combination of discordant atrioventricular and ventriculoarterial connections (see below). Equally obvious should be the arrangement in which two arteries of comparable size take origin from an anteriorly positioned aorta, delimiting the position of a small ventricle on the anterior surface of the ventricular mass. Such a finding is indicative of disproportion between the sizes of the ventricles. The most frequent disproportion is seen when the left ventricle is dominant and the right ventricle is small, as in double-inlet left ventricle or tricuspid atresia. Should prominent interventricular arteries not be evident on the anterior ventricular surface, suspicion should be raised that there is a solitary ventricle of indeterminate morphology. This pattern, however, is exceedingly rare. Much more frequently in this setting, the right ventricle will be dominant, with the small left ventricle hidden on the diaphragmatic surface of the ventricular mass. This is the typical pattern seen in hypoplasia of the left heart.

In examining the ventricular mass, note should also be taken of the relationship of the arterial trunks, confirming first that there are separate aortic and pulmonary trunks as opposed to a common or solitary trunk. There will usually be separate trunks, and typically the aortic trunk is then posterior- and right-sided, with the pulmonary trunk spiraling around the aorta as it divides into right and left pulmonary arteries. This is the “normal relationship.”

Abnormal relationships of the great arterial trunks are indicative of intracardiac malformations, albeit that the positions and connections of the cardiac chambers themselves cannot be inferred from such knowledge. Abnormal arterial positions, nonetheless, give important clues to the presence of particular lesions, which nowadays will almost certainly have been accurately diagnosed before the patient reaches the operating room. An anterior and right-sided aorta is the typical feature of discordant ventriculoarterial connections, usually known as “transposition,” but is also seen when both arterial trunks arise from the right ventricle. When the aorta is anterior and left-sided, most frequently the transposition will be congenitally corrected as discussed above, but this pattern can also be seen with double outlet from the right ventricle. The aorta can also be anterior and left sided when the connections are concordant across both the atrioventricular and ventriculoarterial junctions, a rare situation sometimes called “anatomically corrected malposition.”⁶ A left-sided and anterior aorta can also be found with regular transposition. All this variation serves to emphasize that relationships of the arterial trunks are at best a guide to the specific connections of the cardiac segments.

The perceptive reader will have ascertained from our descriptions thus far that one of the major features of the congenitally malformed heart is that the cardiac chambers are not always in their usual position, nor are they joined as expected to their neighbors. The most important rule of congenital cardiac anatomy, nonetheless, is that each chamber has a relatively constant anatomy irrespective of its position or its connections, albeit that a subtle change in morphology is found when the chambers are connected in abnormal fashion or when the junctions between the cardiac segments are themselves malformed. In this section, therefore, we concentrate our attention on the expected normal morphology, emphasizing when the surgeon should expect to find abnormal arrangements.

The Right Atrium

As already discussed, the most distinctive feature of the right atrium is its extensive triangular appendage, which is separated from the venous component of the atrium by the terminal groove (Fig. 59-5). When viewed by the surgeon, the superior caval vein enters the left-hand side of the venous component, with the inferior caval vein to the right-hand side. The venous component itself is then seen by the surgeon as a sleeve, being separated inferiorly from the right pulmonary veins by the extensive interatrial groove. This groove, also known as Waterston’s or Sondergaard’s groove, is a deep infolding between the right and left atrial walls.

Opening of the right atrium reveals the extensive terminal crest (Fig. 59-6). This muscular bundle underlies the terminal groove, encasing the orifices of both the superior and inferior caval veins, and extending anteriorly toward the atrial septum. In the region of the septum, it becomes a muscular ridge, the eustachian ridge, which separates the orifice of the inferior caval vein from the mouth of the coronary sinus. The mouths of these venous structures are often guarded by sickle-shaped fibrous folds of varying dimensions, the eustachian and thebesian valves. The fibrous commissure of these two valves buries itself in the musculature between the coronary sinus and the oval fossa; as viewed by the surgeon, it runs toward the left-hand margin of the tricuspid vestibule. This important structure, the tendon of Todaro, forms one boundary of the crucial triangle of Koch (Fig. 59-6). When seen in the operating room, the tendon forms the most distant border of the triangle, while the site of annular attachment of the septal leaflet of the tricuspid valve is closer to the surgeon. The atrioventricular node is found at the apex of this triangle, with the atrioventricular bundle penetrating from the apex to pass into the left ventricular outflow tract.

When the right atrium is viewed as shown in Figure 59-6, the impression is gained of an extensive septal surface between the right and left atriums. Sectioning shows that this is not the case (Fig. 59-7). The atrial septum, defined as the tissue that can be removed without encroaching on the pericardial cavity,⁷ is confined to the floor of the oval fossa and its anteroinferior margins.^7,8 The left-hand side of the fossa, as viewed by the surgeon, is the atrial wall overlying the aortic root. The septum secundum, between the fossa and the orifice of the superior caval vein, positioned superiorly with the heart in anatomic location but seen as an inferior structure by the surgeon, is no more than the deeply infolded walls of the interatrial groove.

The Left Atrium

Overall, the left atrium (Fig. 59-8) has a much simpler structure than the right atrium but also possesses an extensive body, which is lacking in the right atrium. The appendage accounts for less of the chamber than on the right side. The venous component, located posteriorly and superiorly, receives the four pulmonary veins, one at each corner. When seen by the surgeon entering through the atrial roof, the narrow opening of the tubular appendage is to the left hand, while the septal surface is to the right. The smooth inferior and posterior margin of the mitral vestibule overlies the coronary sinus as it runs round from the obtuse margin of the ventricular mass. The septal surface of the left atrium is much simpler than that of the right, being formed by the flap valve of the oval fossa (Fig. 59-8).

The Atrioventricular and Ventriculoarterial Junctions

Knowledge of the structure of the junctions between the cardiac components is fundamental to the proper understanding of several congenital anomalies, particularly atrioventricular septal defects in the setting of common atrioventricular junctions.⁹ The salient anatomy of both these junctions, and the relationship between the valves guarding them, is well seen when the musculature of the atrial chambers and the arterial trunks is dissected away from the ventricular base, which can then be viewed from its superior aspect (Fig. 59-9). Although each of the four cardiac valves is usually described as possessing an annulus, in reality none has a true and complete fibrous ring supporting its leaflets. It is the mitral annulus that approximates most closely to the concept of a ring, albeit that it is more akin to an oval saddle, and parietally there is often very little collagenous tissue supporting the mural leaflet of the valve. In the tricuspid orifice, it is very rare to find a collagenous annulus. Instead, it is the fibro fatty tissues of the atrioventricular groove that separate the atrial muscle from the ventricular mass. In the case of the arterial valves, the concept of a “ring” is totally deficient. For both the aortic and pulmonary valves, each of the three leaflets is attached to the underlying ventricular structures in semilunar fashion. Although encased in a circular tube, the attachments of the arterial valvar leaflets, when considered as a whole, take the form of a coronet, with the hinge lines of the leaflets tenting up to reach the sinotubular junction and sweeping down to the nadir of the attachments at the ventricular bases (Fig. 59-10).¹⁶ In the right ventricle, these basal attachments are exclusively supported by ventricular muscle, specifically by the free-standing subpulmonary infundibulum. For the aortic valve, in contrast, a good half of the valvar circumference is supported by fibrous and collagenous tissues. This is because the leaflets of the aortic valve usually have extensive fibrous continuity with those of the mitral valve and, via the membranous septum, also with the leaflets of the tricuspid valve (Fig. 59-11). The two ends of the region of aortio-mitral valvar continuity are thickened to form the right and left fibrous trigones, with the right trigone itself forming an integral part of the fibrous mass where the aortic root is continuous with the leaflets of the tricuspid valve (Fig. 59-12). This whole area is called the central fibrous body, incorporating the so-called membranous septum, which forms the medial wall of the subaortic outflow tract beneath the zone of apposition between the right coronary and noncoronary leaflets of the aortic valve. The attachment of the septal leaflet of the tricuspid valve divides this membranous septum into its atrioventricular and interventricular components.