A thorough knowledge of the anatomy of the heart is a prerequisite for the successful completion of the myriad procedures performed by the cardiothoracic surgeon. This chapter describes the normal anatomy of the heart, including its position and relationship to other thoracic organs. It also describes the incisions used to expose the heart for various operations and discusses in detail the cardiac chambers and valves, coronary arteries and veins, and the important but surgically invisible conduction tissues.
The overall shape of the heart is that of a three-sided pyramid located in the middle mediastinum (Fig. 2-1). When viewed from the heart’s apex, the three sides of the ventricular mass are readily apparent (Fig. 2-2). Two of the edges are named. The acute margin lies inferiorly and describes a sharp angle between the sternocostal and diaphragmatic surfaces. The obtuse margin lies superiorly and is much more diffuse. The posterior margin is unnamed but is also diffuse in its transition.
One-third of the cardiac mass lies to the right of the midline and two-thirds to the left. The long axis of the heart is oriented from the left epigastrium to the right shoulder. The short axis, which corresponds to the plane of the atrioventricular groove, is oblique and is oriented closer to the vertical than the horizontal plane (see Fig. 2-1).
Anteriorly, the heart is covered by the sternum and the costal cartilages of the third, fourth, and fifth ribs. The lungs contact the lateral surfaces of the heart, whereas the heart abuts onto the pulmonary hila posteriorly. The right lung overlies the right surface of the heart and reaches to the midline. In contrast, the left lung retracts from the midline in the area of the cardiac notch. The heart has an extensive diaphragmatic surface inferiorly. Posteriorly, the heart lies on the esophagus and the tracheal bifurcation and bronchi that extend into the lung. The sternum lies anteriorly and provides rigid protection to the heart during blunt trauma and is aided by the cushioning effects of the lungs.
The heart lies within the pericardium, which is attached to the walls of the great vessels and the diaphragm. The pericardium can be visualized best as a bag into which the heart has been placed apex first. The inner layer, in direct contact with the heart, is the visceral epicardium, which encases the heart and extends several centimeters back onto the walls of the great vessels. The outer layer forms the parietal pericardium, which lines the inner surface of the tough fibrous pericardial sack. A thin film of lubricating fluid lies within the pericardial cavity between the two serous layers. Two identifiable recesses lie within the pericardium and are lined by the serous layer. The first is the transverse sinus, which is delineated anteriorly by the posterior surface of the aorta and pulmonary trunk and posteriorly by the anterior surface of the interatrial groove. The second is the oblique sinus, a cul-de-sac located behind the left atrium, delineated by serous pericardial reflections from the pulmonary veins and the inferior vena cava.
The vagus and phrenic nerves descend through the mediastinum in close relationship to the heart (Fig. 2-3). They enter through the thoracic inlet, with the phrenic nerve located anteriorly on the surface of the anterior scalene muscle and lying just posterior to the internal thoracic artery (internal mammary artery) at the thoracic inlet. In this position, the phrenic nerve is vulnerable to injury during dissection and preparation of the internal thoracic artery for use in coronary arterial bypass grafting. On the right side, the phrenic nerve courses on the lateral surface of the superior vena cava, again in harm’s way during dissection for venous cannulation for cardiopulmonary bypass. The nerve then descends anterior to the pulmonary hilum before reflecting onto the right diaphragm, where it branches to provide its innervation. In the presence of a left-sided superior vena cava, the left phrenic nerve is applied directly to its lateral surface. The nerve passes anterior to the pulmonary hilum and eventually branches on the surface of the diaphragm. The vagus nerves enter the thorax posterior to the phrenic nerves and course along the carotid arteries. On the right side, the vagus gives off the recurrent laryngeal nerve that passes around the right subclavian artery before ascending out of the thoracic cavity. The right vagus nerve continues posterior to the pulmonary hilum, gives off branches of the right pulmonary plexus, and exits the thorax along the esophagus. On the left, the vagus nerve crosses the aortic arch, where it gives off the recurrent laryngeal branch. The recurrent nerve passes around the arterial ligament before ascending in the tracheoesophageal groove. The vagus nerve continues posterior to the pulmonary hilum, gives rise to the left pulmonary plexus, and then continues inferiorly out of the thorax along the esophagus. A delicate nerve trunk known as the subclavian loop carries fibers from the stellate ganglion to the eye and head. This branch is located adjacent to the subclavian arteries bilaterally. Excessive dissection of the subclavian artery during shunt procedures may injure these nerve roots and cause Horner syndrome.
The most common approach for operations on the heart and aortic arch is the median sternotomy. The skin incision is made from the jugular notch to just below the xiphoid process. The subcutaneous tissues and presternal fascia are incised to expose the periostium of the sternum. The sternum is divided longitudinally in the midline. After placement of a sternal spreader, the thymic fat pad is divided up to the level of the brachiocephalic vein. An avascular midline plane is identified easily but is crossed by a few thymic veins that are divided between fine silk ties or hemoclips. Either the left or right or, occasionally, both lobes of the thymus gland are removed in infants and young children to improve exposure and minimize compression on extracardiac conduits. If a portion of the thymus gland is removed, excessive traction may result in injury to the phrenic nerve. The pericardium is opened anteriorly to expose the heart. Through this incision, operations within any chamber of the heart or on the surface of the heart and operations involving the proximal aorta, pulmonary trunk, and their primary branches can be performed. Extension of the superior extent of the incision into the neck along the anterior border of the right sternocleidomastoid muscle provides further exposure of the aortic arch and its branches for procedures involving these structures. Exposure of the proximal descending thoracic aorta is facilitated by a perpendicular extension of the incision through the third intercostal space.
The bilateral transverse thoracosternotomy (clamshell incision) is an alternative incision for exposure of the pleural spaces and heart. This incision may be made through either the fourth or fifth intercostal space, depending on the intended procedure. After identifying the appropriate interspace, a bilateral submammary incision is made. The incision is extended down through the pectoralis major muscles to enter the hemithoraces through the appropriate intercostal space. The right and left internal thoracic arteries are dissected and ligated proximally and distally prior to transverse division of the sternum. Electrocautery dissection of the pleural reflections behind the sternum allows full exposure of both hemithoraces and the entire mediastinum. Bilateral chest spreaders are placed to maintain exposure. Morse or Haight retractors are particularly suitable with this incision. The pericardium may be opened anteriorly to allow access to the heart for intracardiac procedures. When required, standard cannulation for cardiopulmonary bypass is achieved easily. This incision is popular for bilateral sequential double-lung transplants and heart-lung transplants because of enhanced exposure of the apical pleural spaces. When made in the fourth intercostal space, the incision is useful for access to the ascending aorta, aortic arch, and descending thoracic aorta.
The right side of the heart can be exposed through a right anterolateral thoracotomy. The patient is positioned supine, with the right chest elevated to approximately 30 degrees by a roll beneath the shoulder. An anterolateral thoracotomy incision can be made that can be extended across the midline by transversely dividing the sternum if necessary. With the lung retracted posteriorly, the pericardium can be opened just anterior to the right phrenic nerve and pulmonary hilum to expose the right and left atria. The incision provides access to both the tricuspid and mitral valves and the right coronary artery. Cannulation may be performed in the ascending aorta and the superior and inferior venae cavae. Aortic cross-clamping, administration of cardioplegia, and removal of air from the heart after cardiotomy are difficult with this approach. This incision is particularly useful nonetheless for performance of the Blalock-Hanlon atrial septectomy or valve replacement after a previous procedure through a median sternotomy. A left anterolateral thoracotomy performed in a similar fashion to that on the right side may be used for isolated bypass grafting of the circumflex coronary artery or for left-sided exposure of the mitral valve.
A left posterolateral thoracotomy is used for procedures involving the distal aortic arch and descending thoracic aorta. With left thoracotomy, cannulation for cardiopulmonary bypass must be done through the femoral vessels. A number of variations of these incisions have been used for minimally invasive cardiac surgical procedures. These include partial sternotomies, parasternal incisions, and limited thoracotomies.
The surgical anatomy of the heart is best understood when the position of the cardiac chambers and great vessels is known in relation to the cardiac silhouette. The atrioventricular junction is oriented obliquely, lying much closer to the vertical than to the horizontal plane. This plane can be viewed from its atrial aspect (Fig. 2-4) if the atrial mass and great arteries are removed by a parallel cut just above the junction. The tricuspid and pulmonary valves are widely separated by the inner curvature of the heart lined by the transverse sinus. Conversely, the mitral and aortic valves lie adjacent to one another, with fibrous continuity of their leaflets. The aortic valve occupies a central position, wedged between the tricuspid and pulmonary valves. Indeed, there is fibrous continuity between the leaflets of the aortic and tricuspid valves through the central fibrous body.
With careful study of this short axis, several basic rules of cardiac anatomy become apparent. First, the atrial chambers lie to the right of their corresponding ventricles. Second, the right atrium and ventricle lie anterior to their left-sided counterparts. The septal structures between them are obliquely oriented. Third, by virtue of its wedged position, the aortic valve is directly related to all the cardiac chambers. Several other significant features of cardiac anatomy can be learned from the short-axis section. The position of the aortic valve minimizes the area of septum where the mitral and tricuspid valves attach opposite to each other. Because the tricuspid valve is attached to the septum further toward the ventricular apex than the mitral valve, it seems that a muscular atrioventricular septum interposes between the right atrium and the left ventricle. We now know that a continuation of the inferior atrioventricular groove interposes between the atrial and ventricular walls in this area, so that it is a sandwich rather than a true septum, with the fibro-adipose tissue of the atrioventricular groove forming the “meat” in the sandwich. The central fibrous body, where the leaflets of the aortic, mitral, and tricuspid valves all converge, lies cephalad and anterior to the muscular atrioventricular sandwich. The central fibrous body is the main component of the fibrous skeleton of the heart and is made up in part by the right fibrous trigone, a thickening of the right side of the area of fibrous continuity between the aortic and mitral valves, and in part by the membranous septum, the fibrous partition between the left ventricular outflow tract and the right-sided heart chambers (Fig. 2-5). The membranous septum itself is divided into two parts by the septal leaflet of the tricuspid valve, which is directly attached across it (Fig. 2-6). Thus the membranous septum has an atrioventricular component between the right atrium and left ventricle, as well as an interventricular component. Removal of the noncoronary leaflet of the aortic valve demonstrates the significance of the wedged position of the left ventricular outflow tract in relation to the other cardiac chambers. The subaortic region separates the mitral orifice from the ventricular septum; this separation influences the position of the atrioventricular conduction tissues and the position of the leaflets and tension apparatus of the mitral valve (Fig. 2-7).
The right atrium has three basic parts: the appendage, the vestibule, and the venous component (Fig. 2-8). Externally, the right atrium is divided into the appendage and the venous component, which receives the systemic venous return. The junction of the appendage and the venous component is identified by a prominent groove, the terminal groove. This corresponds internally to the location of the terminal crest. The right atrial appendage has the shape of a blunt triangle, with a wide junction to the venous component across the terminal groove. The appendage also has an extensive junction with the vestibule of the right atrium; the latter structure is the smooth-walled atrial myocardium that inserts into the leaflets of the tricuspid valve. The most characteristic and constant feature of the morphology of the right atrium is that the pectinate muscles within the appendage extend around the entire parietal margin of the atrioventricular junction (Fig. 2-9). These muscles originate as parallel fibers that course at right angles from the terminal crest. The venous component of the right atrium extends between the terminal groove and the interatrial groove. It receives the superior and inferior venae cavae and the coronary sinus.
FIGURE 2-8
This view of the right atrium, seen in surgical orientation, shows the pectinate muscles lining the appendage, the smooth vestibule (circles) surrounding the orifice of the tricuspid valve, and the superior vena cava (SCV), inferior vena cava (ICV), and coronary sinus joining the smooth-walled venous component. Note the prominent rim enclosing the oval fossa, which is the true atrial septum (see Fig. 2-11).
FIGURE 2-9
This dissection of the short axis of the heart (compare with Fig. 2-4) shows how the pectinate muscles extend around the parietal margin of the tricuspid valve. In the left atrium, the pectinate muscles are confined within the tubular left atrial appendage, leaving the smooth vestibule around the mitral valve confluent with the pulmonary venous component of the left atrium.
The sinus node lies at the anterior and superior extent of the terminal groove, where the atrial appendage and the superior vena cava are juxtaposed. The node is a spindle-shaped structure that usually lies to the right or lateral to the superior cavoatrial junction (Fig. 2-10). In approximately 10% of cases, the node is draped across the cavoatrial junction in horseshoe fashion.1
FIGURE 2-10
This diagram shows the location of the sinus node at the superior cavoatrial junction. The node usually lies to the right (lateral ) side of the junction but may be draped in horseshoe fashion across the anterior aspect of the junction. ICV = inferior vena cava; SCV = superior vena cava.
The blood supply to the sinus node is from a prominent nodal artery that is a branch of the right coronary artery in approximately 55% of individuals and a branch of the circumflex artery in the remainder. Regardless of its artery of origin, the nodal artery usually courses along the anterior interatrial groove toward the superior cavoatrial junction, frequently within the atrial myocardium. At the cavoatrial junction, its course becomes variable and may circle either anteriorly or posteriorly or, rarely, both anteriorly and posteriorly around the cavoatrial junction to enter the node. Uncommonly, the artery arises more distally from the right coronary artery and courses laterally across the atrial appendage. This places it at risk of injury during a standard right atriotomy. The artery also may arise distally from the circumflex artery to cross the dome of the left atrium, where it is at risk of injury when using a superior approach to the mitral valve. Incisions in either the right or left atrial chambers always should be made with this anatomical variability in mind. In our experience, these vessels can be identified by careful gross inspection, and prompt modification of surgical incisions can be made accordingly.
The most common incision into the right atrium is made into the atrial appendage parallel and anterior to the terminal groove. Opening the atrium through this incision confirms that the terminal groove is the external marking of the prominent terminal crest. Anteriorly and superiorly, the crest curves in front of the orifice of the superior vena cava to become continuous with the so-called septum secundum, which, in reality, is the superior rim of the oval fossa. When the right atrium is inspected through this incision, there appears to be an extensive septal surface between the tricuspid valve and the orifices of the venae cavae. This septal surface includes the opening of the oval fossa and the orifice of the coronary sinus. The apparent extent of the septum is spurious because the true septum between the atrial chambers is virtually confined to the oval fossa2,3 (Fig. 2-11). The superior rim of the fossa, although often referred to as the septum secundum, is an extensive infolding between the venous component of the right atrium and the right pulmonary veins. The inferior rim is directly continuous with the so-called sinus septum that separates the orifices of the inferior caval vein and the coronary sinus (Fig. 2-12).
FIGURE 2-11
This transection across the middle of the oval fossa (asterisk) shows how the so-called septum secundum, the rim of the fossa, is made up of the infolded atrial walls (arrows). ICV = inferior vena cava; SCV = superior vena cava.
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