Exposure of the Heart
Within the pericardium lies the heart, a hollow, muscular, four-chambered organ suspended at its base by the great vessels. In situ the heart occupies an asymmetric position, with its apex pointing anteriorly, inferiorly, and about 60 degrees toward the left. Its four chambers are arranged in two functionally similar pairs, separated from each other by the cardiac septum (see Plate 1-5). Each pair consists of a thin-walled atrium and a thicker- walled ventricle.
The anatomic nomenclature of the heart removes it from the body and places it on its apex, and thus the cardiac septum is in a sagittal plane. This practice has led to misconceptions and difficulties in orientation among cardiologists and surgeons. On a chest radiograph, for example, the left cardiac border is formed by the left ventricle, but the right border is formed by the right atrium, not the right ventricle, which lies anterior. The major and important part of the left atrium lies directly posterior and in the midline in front of the spine and esophagus, allowing the pulmonary veins to be as short as possible.
On removing the anterior chest wall and opening the pericardium, most of the presenting part of the heart is formed by the right ventricle, with its exposed surface triangular in shape. The right atrium lies to the right of the right ventricle.
The term “auricle” is often improperly used instead of atrium. The true auricle is then regrettably called “auricular appendage” instead of atrial appendage, which is morphologically correct. The term “auricular fibrillation” is clinically incorrect and should be atrial fibrillation.
The right atrium and right ventricle are separated by the right atrioventricular (coronary) sulcus, through which runs the right coronary artery, embedded in a variable amount of fat. To the left of the right ventricle, a small segment of the left ventricle is visible, separated from it by the anterior interventricular sulcus (groove). The anterior interventricular (descending) branch of the left coronary artery (see Plate 1-5) lies in this groove, again embedded in fat.
Superiorly, the pulmonary trunk is seen originating from the right ventricle and leaving the pericardium just before it bifurcates into its two main branches: the right and left pulmonary arteries. To the right of the pulmonary trunk lies the intrapericardial portion of the ascending aorta, the base of which is largely covered by the right auricle (right atrial appendage). The base of the aorta, including the first part of the right coronary artery, is surrounded by lobules of fatty tissue called Rindfleisch folds, the largest and uppermost of which is rather constant.
After removal of the heart from the pericardium, its posterior (basilar) and diaphragmatic aspects can be inspected. The superior vena cava (SVC) and inferior vena cava (IVC) enter the right atrium, with the long axis of both cavae inclined slightly forward and the IVC in a more medial position. A pronounced groove, the sulcus terminalis, separates the right aspect of the SVC from the base of the right auricle. As this groove descends along the posterior aspect of the right atrium, it becomes less distinct.
The right pulmonary veins (usually two but occasionally three) arise from the right lung and cross the right atrium posteriorly to enter the right side of the left atrium. The two left pulmonary veins enter the left side of the left atrium, sometimes by a large common stem. The posterior wall of the left atrium forms the anterior wall of the oblique pericardial sinus. Normally, the left atrium is not in contact with the diaphragm.
The bifurcation of the pulmonary trunk lies on the roof of the left atrium. The left pulmonary artery courses immediately toward the left lung, and the right pulmonary artery runs behind the proximal SVC and above the right pulmonary veins to the right lung.
The aortic arch crosses the pulmonary artery bifurcation after giving off its three main branches: the brachiocephalic (innominate), left common carotid, and left subclavian arteries. Variations in this pattern occur and usually are not significant.
The coronary sinus lies between the left atrium and the left ventricle in the posterior (diaphragmatic) portion of the left atrioventricular groove (coronary sulcus). The cardiac veins enter the coronary sinus, which has the appearance of a short, wide vein. However, its wall consists of cardiac muscle, and because of its embryonic origin, the coronary sinus should be considered a true cardiac structure. Its right extremity turns forward and upward to enter the right atrium.
The diaphragmatic surfaces of the right ventricle and the left ventricle are separated by the posterior interventricular sulcus (groove). This sulcus is continuous with the anterior interventricular groove just to the right of the cardiac apex, which in a normal heart is formed by the left ventricle. The posterior inter-ventricular (descending) artery and middle cardiac vein lie in the posterior interventricular sulcus, embedded in fat.