Cardiovascular Anatomy and Segmental Approach to Imaging of Congenital Heart Disease

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Chapter 63

Cardiovascular Anatomy and Segmental Approach to Imaging of Congenital Heart Disease

Because of the large variety of human hearts in nature, a standardized approach and nomenclature are needed to understand and describe cardiac anatomy and physiology in the setting of congenital heart disease (CHD). The most widely used approach is the segmental approach to heart disease, which was first proposed by Richard Van Praagh in 1972 and later modified by others. It is strongly rooted in embryologic principles and follows a logical sequence from evaluation of cardiac morphology and physiology to treatment decision making.

Embryologic Basis for the Segmental Approach to Heart Disease

Embryology of the heart is covered in Chapter 62. In this chapter, some embryologic events that are fundamental to understanding the segmental approach to heart disease are reiterated.

The heart develops from two simple epithelial tubes that fuse to form a single tube (Fig. 63-1) with the following components:

Sinus venosus consists of right and left horns. Each horn receives blood from three important veins: the umbilical vein, the common cardinal vein, and the vitelline vein.

Paired primitive atria will later fuse to form a common atrium.

The atrioventricular (AV) sulcus divides the common atrium and the primitive ventricle.

The primitive ventricle becomes the left ventricle (LV).

The interventricular sulcus divides the primitive ventricle and the bulbus cordis.

The bulbus cordis may be divided as follows: the proximal third gives rise to the body of the right ventricle (RV). The distal-most section is the truncus arteriosus, which develops into the aortic root and part of the pulmonary artery (PA). The remaining mid portion is the conus cordis, which connects the primitive RV to the truncus arteriosus. The conus cordis partitions to form the outflow tracts of the RV and LV.

Although the two ends of the heart tube remain relatively fixed, rapid growth of the middle section results in the development of a large S-shaped curve called the bulboventricular loop (see Fig. 63-1). As the heart tube grows and becomes longer, it usually bends to the right, termed D-looping by Van Praagh. D-looping is responsible for the proximal bulbus cordis (RV) lying anterior and to the right of the primitive ventricle (LV). If the heart tube loops to the left, termed L-looping, the RV will lie anterior and to the left of the LV.

In the heart tube stage, the primitive LV and the proximal bulbus cordis (primitive RV) are separated from the truncus arteriosus (which gives rise to both great arteries) by the conus or infundibulum. The conus consists of the subpulmonary and subaortic conus cushions. Normally, expansile growth of the subpulmonary conus occurs, causing it to protrude anteriorly on the left, carrying the pulmonary valve anteriorly, superiorly, and to the left of the aortic valve. Resorption of the subaortic conus occurs. Hence the aortic valve lies posterior, inferior, and right-sided, in direct fibrous contiguity with the mitral valve (Fig. 63-2). The anterior pulmonary artery rises above the anterior ventricle (RV) and leads to the posterior sixth arterial arch, which forms the branch pulmonary arteries. The posterior aorta originates above the posterior LV and leads to the anterior fourth arterial arch (which forms the aortic arch).

The segmental approach involves the analysis of the three major cardiac segments: atria, ventricles, and great arteries, along with the two connecting segments: the AV canal and the conotruncus. These segments of the heart can be distinguished in the very early embryo. Some important embryologic concepts underlie the segmental approach to heart disease:

1. The development of the suprahepatic portion of the inferior vena cava (IVC) is closely linked to the growth of the liver, and thus the anatomic right atrium (RA) and the liver almost invariably develop on the same side of the body. This concept of visceroatrial situs is fundamental to the segmental approach.

2. Ventricular looping is independent of the visceroatrial situs. This phenomenon gives rise to the concept of concordance (RA-RV and left atrium [LA]-LV) and discordance (RA-LV and LA-RV).

3. Ventricular looping and the great arterial relationship are independent entities. The direction of bulboventricular looping and the development of the conotruncus are responsible for the ultimate relationship of the great arteries to each other and to the underlying ventricles and AV valves.

Segmental Approach to Diagnosis of Congenital Heart Disease

Any imaginable combination of visceral, atrial, ventricular, and great vessel morphology can and does occur in CHD. A simple, logical, step-by-step approach to diagnosis and decision making and a standardized nomenclature go a long way in advancing patient care by ensuring that different caregivers have similar understanding of the disease and are speaking the same language.

One can think of the heart as a three-level house (Fig. 63-3). The first level is the visceroatrial situs, the middle level is the ventricular loop, and the third level is the conotruncus. To describe it simply, the three levels are the atria, ventricles, and great arteries. The heart has two staircases: the AV ventricular junction and the ventriculoarterial junction. The levels represent the major cardiac segments. The staircases represent the connecting segments.

The segmental approach to heart disease comprises the following steps:

The first three steps in the segmental approach are concerned with morphology, whereas the last step concerns physiology.

Van Praagh used a segmental set to provide a shorthand description of the floor plan of the heart. The first letter stands for the visceroatrial situs, the second for the ventricular loop, and the third for the great arterial relationship. In a person with situs solitus of the viscera and atria, D-looping of the ventricles, and solitus relationship of the great arteries, the segmental set is {S,D,S}.

Identification of the Major Cardiac Segments

Reliable identification of the cardiac chambers based on specific morphologic features is the first step in the segmental approach to heart disease. It is important to remember that right and left do not refer to the side of the body on which the chamber lies but to specific morphologic criteria that identify each component of the heart. For instance, “right atrium” does not refer to the atrium that is on the right side of the body but to the atrium that receives the insertion of the IVC and the coronary sinus and has a triangular appendage with a broad base. Hence the morphologic RA will be on the right side of the body in persons with situs solitus and on the left side in persons with situs inversus.

Atrial Identification

The defining features of the morphologic RA (systemic venous atrium) and LA (pulmonary venous atrium) are based on their venous connections, as well as their appendage and pectinate muscle morphology. Using venoatrial connections for atrial identification is based on the fact that the sinus venosus, which carries the systemic venous return, is an integral part of the morphologic RA. Hence the morphologic RA receives the IVC and the superior vena cava (SVC) and the orifice of the coronary sinus. However, the SVC and coronary sinus have a high incidence of variation, which can be a source of diagnostic confusion. These variations include left SVC to an unroofed coronary sinus and bilateral SVC with the left SVC draining to an unroofed coronary sinus. In these cases, the SVC would appear to drain into the LA. In rare instances, even the IVC may drain into the coronary sinus, which may be unroofed, or the coronary sinus septum may be absent. In spite of this rare exception, the most reliable means of identifying the morphologic RA by cross-sectional imaging is by recognizing its connection to the IVC (Fig. 63-4). Even in the setting of an interrupted IVC, a suprahepatic segment of the IVC is present entering the RA, allowing accurate identification.

The morphologic LA is defined as the atrium that receives all or half of the pulmonary veins and none of the systemic veins (except an SVC to an unroofed coronary sinus). The LA is also the chamber that may receive no veins at all (in the setting of total anomalous pulmonary venous return). When all systemic veins and part or all of the pulmonary veins drain into one atrium, this atrium represents the morphologic RA.

Anderson has described the morphologic RA (systemic atrium) as being characterized by the presence of a triangular appendage with a broad junction and by the recognition of pectinate muscles extending to the AV junction. The morphologic LA is characterized by a tubular narrow-based appendage and lack of pectinate muscle extension. Because determination of pectinate muscle morphology is beyond the resolution of magnetic resonance imaging (MRI) or computed tomography (CT), atrial identification is performed by recognition of venoatrial connections and morphology of the appendages. If this analysis fails to yield a confident identification of the RA and LA, then a diagnosis of atrial situs ambiguous is made. Even in the setting of visceral situs ambiguous, reliable identification of atrial situs may be made in more than 80% of cases.

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