CROSS-SECTIONAL NEONATAL CARDIAC ANATOMY
Cardiac anatomy is often taught on a heart excised from a cadaver, or on heart models that do not reflect the true spatial orientation of the heart as it lies in the chest cavity. However, with the advent of newer cardiac imaging techniques, diagnostic studies, and procedures on the heart, it is now essential that the structures of the heart be displayed and visualized as they lie in situ, and as they relate to the other contents of the thoracic cavity. In this chapter, we provide a comprehensive description of cardiac anatomy, particularly as depicted by echocardiography. Through the detailed use of drawings and representative echocardiographic scans, we will focus on describing all cardiac structures, specifically as they are situated in the thoracic cavity.
Cardiac Orientation in the Fetus
Up to the fourth week of gestation, the heart is completely symmetrical, with the right atrium and ventricle located to the right and the left atrium and ventricle to the left of mid chest, as shown in Figure 3-1.1 However, this echocardiographic image is obtained from the apex of the heart of a newborn and partially satisfies our preconception of how an ideal heart should appear, although this is not a realistic expectation. In this case, the transducer is placed at the apex of the heart and the cut is made mainly transversely, indicating the relationship of anterior and posterior structure of the heart (Figure 3-2). The rearrangement of cardiac position in the thoracic cavity occurs after fifth weeks of gestation and involves displacement and counterclockwise rotation (as viewed anteriorly) of the apex to the left and counterclockwise and displacement of posterior attachment of the atrial septum to the right. In this way, the posterior attachment of the atrial septum is located to the right of the spine, whereas the apex of the heart is situated at the 4th intercostal space at the anterior axillary line. Figure 3-3 shows the relative position of cardiac chambers from a postmortem cross section of a neonatal thorax at the 4th intercostal space.2 The anterior wall of the right ventricle, the right ventricular outflow, the pulmonary valve, and the initial segment of the main pulmonary are the most anterior, whereas the posterior wall of the left atrium and pulmonary veins are the most posterior structures of the heart, located directly anterior to the descending aorta and esophagus. The ventricles are inferior and to the left of their corresponding atria. The newborn heart, as is situated in the thoracic cavity at the time of birth, is depicted in Figure 3-4. Newborn infants often have a large thymus which is usually multilobar and covers the upper part of the heart and great vessels.
Apical four-chamber echocardiogram obtained from a 1-day-old neonate. Abbreviations: RA, right atrium; RV, right ventricle, LA, left atrium; LV, left ventricle.