A thorough knowledge of the anatomy of the heart is a prerequisite for the successful completion of the myriad of procedures performed by the cardiothoracic surgeon. In this chapter, we describe the normal anatomy of the heart, including its position and relationship to other thoracic organs. We describe the incisions used to expose the heart for various operations, and discuss in detail the cardiac chambers and valves, coronary arteries and veins, and the important but surgically invisible conduction tissues.
Location of the Heart Relative to Surrounding Structures
The overall shape of the heart is that of a three-sided pyramid located in the middle mediastinum (Figures 1, 2). When viewed from its apex, the three sides of the ventricular mass are readily seen (Figure 2 – right-hand panel). 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 to the horizontal plane (see Figure 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 hilums of the lungs 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 (Figure 2 – right hand panel). Posteriorly, the heart lies on the esophagus and the tracheal bifurcation, and the bronchi that extend into the lung. The sternum lies anteriorly. It provides rigid protection to the heart during blunt trauma, and is aided by the cushioning effects of the lungs.
The Pericardium and Its Reflections
The heart lies within the pericardium, which is attached to the walls of the great vessels and to 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 caval vein.
Mediastinal Nerves and Their Relationships to the Heart
The vagus and phrenic nerves descend through the mediastinum in close relationship to the heart (Figure 3a). They enter through the thoracic inlet, with the phrenic nerve located anteriorly on the surface of the anterior scalene muscle. They lie just posterior to the internal thoracic, or 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 caval vein, again in harm’s way during dissection for venous cannulation for cardiopulmonary bypass (CPB). 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 caval vein, the left phrenic nerve is directly applied 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 (Figure 3b). 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’s syndrome.
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