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.

Differences Between Neonatal and Adult Heart

The neonatal heart has the following characteristics:

• 
  

  The heart lies more horizontally in the chest at approximately 45° in the frontal plane as opposed to 60° for the adult heart.

  
  
• 
  

  The right ventricular wall has the same thickness as the left ventricular wall. The right ventricular wall thins out gradually during the first month of life.³

  
  
• 
  

  The ductus arteriosus and foramen ovale, which are prominent during fetal life, are present at birth.

Upon the opening of the thorax anteriorly, the first structure encountered is the pericardial sac. Pericardium is dense fibrous tissue which encloses the heart and anchors at base to great vessels and inferiorly to the diaphragm (Figure 3-5). It is highly echogenic. The pericardium is separated from the outer layer of heart muscle or epicardium by the pericardial sac containing a small amount of fluid that lubricates and allows for free contraction of the heart. Under pathological conditions, a large amount of fluid under pressure may accumulate in the pericardial sac and may impede cardiac filling (cardiac tamponade).

The right heart chambers are to the same extent anterior as they are to the right of left heart chambers. The extent of overlap and their relative position is depicted in Figures 3-5 and 3-6. Similarly, the right atrial appendage extends more anteriorly than the left atrial appendage.

Location of the Heart Valves and the Great Vessels

The locations of heart valves are illustrated in frontal and lateral views in Figure 3-7. The pulmonary valve is the most anterior and superior, whereas the tricuspid valve is the most inferior and the mitral valve the most posterior of the cardiac valves. The aortic valve is located in the center.

Imaging Planes of the Heart

In echocardiography, cardiac chambers are imaged from specific acoustic windows through a sector scan. Sector scans do not follow the usual anatomic (coronal, sagittal, and transverse) planes. However, a few approximate these planes and are illustrated here to demonstrate cross-sectional cardiac anatomy. For example, the subcostal coronal planes and the suprasternal coronal posterior plane “crab view” approximate anatomical coronal planes. A complete list of sector scans obtained during neonatal echocardiography is reviewed later in this chapter.

The subcostal coronal anterior plane depicts the heart cut through an anterior frontal plane closest to the anterior chest wall (Figure 3-8). The triangular-shaped right ventricle, right ventricular outflow including crista supraventricularis (a muscle band that separates the upper part of the right ventricle underneath the pulmonary valve), and the initial segment of main pulmonary artery and a small anterior segment of left ventricle are seen in this view. The pulmonary valve, as well as the anterior segment of tricuspid valve, are also seen in this cut.

A subcostal coronal plane at the level of the aortic valve (Figure 3-9) reveals the right atrium, the posterior segment of tricuspid valve, the inflow segment of right ventricle, the aortic valve, the proximal segment of ascending aorta, the anterior segment of the left atrium, and the left atrial appendage, as well as a smooth-walled, ellipsoidal-shaped left ventricle. Only the anterior segment of mitral valve ring is seen in this view.

A posterior subcostal coronal plane (Figure 3-10) depicts the crux of the heart, with septal attachments of the mitral and tricuspid valves, the posterior segment of the interatrial septum, which includes a patent foramen ovale, the posterior segment of the right atrium, the large segment of the left atrium, which often includes left atrial appendage, a small posterior segment of right ventricular inflow, and variable segments of the left ventricle. At this level, the mitral valve, which is the most posterior of heart valves, is well visualized, whereas the tricuspid valve is only partially visualized and aortic and pulmonary valves (located anterior to the plane) are not seen.

The suprasternal coronal posterior “crab view” is the most posterior frontal plane obtained from suprasternal window, imaging the posterior wall of the left atrium, pulmonary veins, the left atrial appendage, a cross section of ascending aorta, and the pulmonary artery at its bifurcation (Figure 3-11). Finally, an intact heart displaying the normal posterior structures is shown in (Figure 3-12).