Cardiovascular Anatomy and Segmental Approach to Imaging of Congenital Heart Disease

Embryologic Basis for the Segmental Approach to Heart Disease

Embryology of the heart is covered in a separate chapter (see Chapter 62 ). In this section, 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 together to form a common atrium.

• Atrioventricular sulcus: Divides the common atrium and the primitive ventricle.

• Primitive ventricle: Becomes the left ventricle, and the inflow portion of the right ventricle.

• Interventricular sulcus: Divides the left and right ventricles.

• Bulbus cordis: This may be divided as follows: the proximal one-third contributes to the body of the right ventricle; the distal one-third is called the truncus arteriosus, which develops into the aortic root and part of the pulmonary artery (PA); and the middle third is called the conus cordis and connects the primitive right ventricle to the truncus arteriosus. The conus cordis partitions to form the outflow tracts of the right and left ventricles.

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 by Van Praagh as D-looping. 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, there is expansile growth of the subpulmonary conus, causing it to protrude anteriorly on the left, carrying the pulmonary valve anteriorly, superiorly, and to the left of the aortic valve. There is resorption of the subaortic conus. Hence, the aortic valve lies posterior, inferior, and right-sided, in direct fibrous contiguity with the mitral valve ( Fig. 63.2 ) . The anterior PA arises 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, and the two connecting segments: the atrioventricular canal and the conotruncus. These segments of the heart can be distinguished in the very early embryo. There are some important embryologic concepts that 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, so that the anatomic right atrium 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.

  Another important concept underlying the segmental approach is the recognition that the ventricular looping is independent of the visceroatrial situs. This gives rise to the concept of concordance (RA-RV and LA-LV) and discordance (RA-LV and LA-RV).

• 3.

  Similarly, ventricular looping and great arterial relationship are independent entities. The direction of bulboventricular looping, and the development of the conotruncus is responsible for the ultimate relationship of the great arteries to each other, and to the underlying ventricles and atrioventricular valves.

The Segmental Approach to Diagnosis of Congenital Heart Disease

Any imaginable combination of visceral, atrial, ventricular, and great vessel morphology can and does occur in congenital heart disease. 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. This approach is known as the segmental approach to heart disease, first described by Richard Van Praagh in 1972.

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. There are two staircases: the atrioventricular 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:

• 1.

  What is the anatomic type of each of the three major cardiac segments: the atria, the ventricles, and great arteries?

• 2.

  How is each segment connected to the adjacent segment?

• 3.

  What are the associated anomalies involving the valves, atrial and ventricular septum, the great vessels, and the systemic and pulmonary veins?

• 4.

  How do the segmental combinations and connections function, with or without the associated malformations?

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

Van Praagh used a segmental set to provide a shorthand description of the floorplan 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 right atrium will be on the right side of the body in situs solitus and on the left side in situs inversus.

Atrial Identification

The defining features of the morphologic right atrium (systemic venous atrium) and left atrium (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 right atrium. Hence, the morphologic right atrium receives the inferior and superior vena cavae, and orifice of coronary sinus. However, the superior vena cava (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 SVCs with the left SVC draining to an unroofed coronary sinus. In these cases, the SVC would appear to drain into the left atrium. 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 right atrium 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 right atrium, allowing accurate identification.

The morphologic left atrium 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 left atrium 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 right atrium.

Robert Anderson has described the morphologic right atrium (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 atrioventricular junction. The morphologic left atrium is characterized by a tubular narrow-based appendage and lack of pectinate muscle extension to the atrioventricular junction. Because determination of pectinate muscle morphology is beyond the resolution of MRI or 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 right and left atrium, 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 the cases.

Great Arterial Identification

The PA gives rise to branches to the lungs and no branches to the body. The aorta gives rise to branches to the body as well as the coronary arteries. A common vessel arising from the ventricles that gives rise to the coronaries and branches to the body as well the lungs is termed a common arterial trunk or truncus, and a segmental relationship is not assigned (labeled X for undetermined). On transverse cross-sectional imaging at the level of the outflow tract, the coronary artery origin is used to identify the aortic annulus. The intercoronary commissure of the aortic valve is pointed toward the right-left commissure of the pulmonary valve and is an important landmark for determining great arterial relationship ( Fig. 63.7 ). In solitus relationship of the great arteries, the aortic valve annulus lies posterior and to the right of the pulmonary valve annulus.