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.

Ventricular Identification

Ventricles are defined by their morphologic features, not by their spatial relationships. The morphologic RV is defined by the following features:

1. Muscular connection between the free wall and the interventricular septum (moderator band) (Fig. 63-5, A)

2. The septal attachment of the AV valve of the RV (tricuspid valve) is more apically placed relative to the LV (Fig. 63-5, B)

3. Presence of a conus/infundibulum; the infundibulum is identified as a muscular cone of tissue that separates the AV valve from the semilunar valve on the same side, resulting in a lack of fibrous contiguity between the two valves (Fig. 63-5, C)

The morphologic LV is identified by the following features:

The AV valves follow the ventricle rather than the atrium. Thus the tricuspid valve is related to the morphologic RV and the mitral valve is related to the morphologic LV. The tricuspid valve typically has papillary muscle attachments to the right ventricular septal surface (septophilic valve), whereas the mitral valve is septophobic and only attaches to the free wall of the LV.

Neither the shape of the ventricle nor the degree of trabeculation or hypertrophy is considered a reliable marker for ventricular identification, because they frequently are affected by pressure or volume changes.

Great Arterial Identification

The pulmonary artery 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 a solitus relationship of the great arteries, the aortic valve annulus lies posterior and to the right of the pulmonary valve annulus.

Analysis of the Three Major Cardiac Segments

First Major Segment: Visceroatrial Situs

Situs refers to the position of the atria and viscera relative to the midline. Three types of situs exist: solitus (S), inversus (I), and ambiguous (A) (Fig. 63-8). Heterotaxy is synonymous with situs ambiguous.

Visceral situs solitus is characterized by the presence of a right-sided liver, a single or dominant left-sided spleen, a three-lobed right lung with an eparterial bronchus, and a two-lobed left lung with a hyparterial bronchus. Atrial situs solitus is characterized by the presence of the systemic venous atrium on the right and the pulmonary venous atrium on the left. Situs inversus is defined as the “mirror image” of situs solitus. Hence visceral situs inversus is characterized by a left-sided liver, a single or dominant right-sided spleen, a three-lobed left lung with an eparterial bronchus, and a two-lobed right lung with a hyparterial bronchus. Atrial situs inversus is characterized by the presence of the systemic venous atrium on the left and the pulmonary venous atrium on the right. Although the location of the cardiac apex and the stomach is usually on the left in persons with situs solitus and on the right in persons with situs inversus, they are not considered reliable markers of visceral situs because of the high incidence of variation in the setting of normal situs.

Because heterotaxy is defined as “situs other than solitus or inversus,” it is not a specific disease but a constellation of cardiac, vascular, and visceral abnormalities, including situs ambiguous of the viscera, lung symmetry, atrial appendage symmetry, anomalous systemic venous return, anomalous pulmonary venous return, and associated intracardiac defects. No single finding is pathognomonic. A tendency exists to cluster defects into two syndromes based on the predominance of right-sided or left-sided structures:

The use of the terms atrial isomerism, bilateral right-sidedness, or bilateral left-sidedness as substitutes for the syndromes of situs ambiguous is discouraged because they are semantically inaccurate representations of the anatomy found in these patients.

Second Major Segment: Ventricular Loop

Depending on the direction of ventricular looping during development, the RV may be located spatially on the right or left side of the heart (Fig. 63-9). If the bulboventricular loop occurs to the right, it is termed a D-loop, and the morphologic RV lies anterior and to the right of the morphologic LV. If the bulboventricular loop occurs to the left, it is termed an L-loop, and the morphologic RV lies anterior and to the left of the morphologic LV.

Third Major Segment: Great Arterial Relationship

Normally the aortic annulus lies posterior, inferior, and to the right of the pulmonary valve annulus. This position is called solitus (S) (Fig. 63-10, A). In situs inversus, the aorta lies posterior and to the left of the pulmonary valve, termed inversus (I) (Fig. 63-10, B). Any other position of the aorta and pulmonary artery other than solitus or inversus is termed malposition. If the aorta lies to the right of the PA, it is called D-malposition (Fig. 63-10, C). If the aorta lies to the left of the PA, it is termed L-malposition (Fig. 63-10, D).

Different Types of Human Hearts

The segmental possibilities at each level are as follows:

Based on the various permutations and combinations of atrial, ventricular, and great arterial relationships, as well as on the anatomy of the conus, Van Praagh provided an overview of the diversity that exists in human hearts (Fig. 63-11). It becomes immediately apparent that a pattern-based approach or an approach based on connections or the direction of flow of blood will not do justice to the complexity of the disease process. The segmental approach not only takes into account the morphologic variations at each level but also provides structural landmarks to distinguish the variations from each other and determine their impact on physiology, thereby allowing informed decision making regarding management.

Analysis of the Connecting Segments

First Connecting Segment: Atrioventricular Junction

The anatomic possibilities at the AV junction level may be classified on the basis of the number of ventricles as follows:

The types of biventricular AV connections (Fig. 63-12) include:

The types of univentricular AV connections (Fig. 63-13) include:

In the setting of univentricular AV connections, one of the AV valves is atretic, or both AV valves open partly or wholly into the same ventricle. The second ventricular chamber subsequently is very small or is just an outflow chamber and is of little functional use, apart from serving as a conduit for one of the great arteries. Single ventricles may be of an LV, RV, or indeterminate morphology. The functional single LV is characterized by the presence of a bulboventricular foramen, which connects the LV chamber to the small anteriorly located infundibular outlet chamber of the RV. The RV sinus (inflow portion) is typically absent. A functional single RV is characterized by the presence of a septal band, and in most cases, by a rudimentary posterolaterally located LV chamber.

Second Connecting Segment: Conotruncus or Ventriculoarterial Junction

The development of the conotruncus is the most important variable in the genesis of outflow tract anomalies. The differential growth of the subpulmonary and subaortic conus cushions largely determines the relationship between the semilunar valves, between the semilunar valves and the ventricles, and between the semilunar valves and the atrioventricular valves. It also determines the presence of distal infundibular stenosis and the location of the ventricular septal defect in outflow tract anomalies. Based on conal development, the following anatomic types of conus (Fig. 63-14) may be recognized:

1. Development of subpulmonary conus and resorption of the subaortic conus results in ventriculoarterial concordance and normal relationship of great arteries. The aorta lies posterior and to the right in a D-loop heart (solitus), and posterior and to the left in an L-looped heart (inversus).

2. Development of the subaortic conus and resorption of the subpulmonary conus results in ventriculoarterial discordance and transposition of the great arteries. Transposition means that the LV is connected to the main pulmonary artery and the RV is connected to the aorta. The aorta lies anterior and to the right in a D-looped heart (D-malposition) (Fig. 63-15) and anterior and to the left in an L-looped heart (L-malposition).

3. Persistence and development of both the subaortic and subpulmonary conus leads to a double-outlet RV, meaning that both great vessels arise predominantly from the RV (Fig. 63-16). Variable development of the pulmonary and aortic conus may occur, resulting in variable location of the ventricular septal defect in relation to the great arteries.

4. Resorption of both the subpulmonary and subaortic conus results in double-outlet LV, meaning that both vessels arise predominantly from the LV.

A rare form of conal maldevelopment is anatomically corrected malposition of the great arteries, in which the malposed aorta originates above the LV and the malposed pulmonary artery originates above the RV. Malalignment of the conal septum also is the cause of infundibular obstruction in tetralogy of Fallot (anterior malalignment) and subaortic stenosis with interrupted aortic arch (posterior malalignment).

Evaluation of Function

The final step in the segmental approach to heart disease is to determine how the segmental combinations and connections, with the associated anomalies, function. Broadly, abnormal function of the congenitally malformed heart may be classified into the following subgroups:

When all relevant information regarding morphology and function has been collected, decisions regarding management may be made, which may be in the form of medical therapy, surgical therapy, or both.

Illustrated Case Discussion

A case study of a segmental approach to imaging of CHD is illustrated in Figure 63-17.

• Situs solitus of the atria (S): The coronary sinus and IVC enter the right-sided atrium, which therefore represents the morphologic RA. The pulmonary veins enter the morphologic LA. The RA connects to the right pulmonary artery via an atriopulmonary anastomosis. The SVC also connects to the right pulmonary artery.

• AV connection: The right AV valve (tricuspid valve) is atretic and fatty replaced. The left AV valve (mitral valve) is normal.

• D-looping of the ventricles: The large left-sided ventricle has a smooth septal surface consistent with a morphologic LV. The small blind-ending chamber to the right is the infundibular outlet chamber of the RV, which fills via a bulboventricular foramen.

• Ventriculoarterial connection: RV-PA outflow is atretic. Aortic outflow from the LV is normal.

• Solitus relationship of the great arteries (S): The aortic valve lies posterior and to the right of the atretic pulmonary valve.

The final diagnosis is {S,D,S} tricuspid atresia, pulmonary atresia, status postoperative Fontan completion.

Conclusion

In summary, the segmental approach to heart disease provides an elegantly simple method to break down the complexity of CHD on cross-sectional imaging. It creates a template for standardizing nomenclature in CHD, thereby ensuring that caregivers from different specialties understand each other. It is firmly rooted in embryologic principles and is therefore fairly intuitive to the beginner in the field. MRI and CT almost rival pathologic evaluation of specimens in the detail that they provide on cardiovascular morphology. Additionally, MRI provides unique information regarding function, flow, and tissue characterization that decreases the need for more invasive means of diagnosis. Thus MRI and CT are ideal vehicles for the segmental approach to heart disease.

Key Points

The segmental approach is a simple, logical, step-by-step approach to diagnosis and decision making in CHD and is rooted in sound embryologic concepts.

The segmental approach involves the analysis of the three major cardiac segments (atria, ventricles, and great arteries), the two connecting segments (the AV canal and the conotruncus), and associated anomalies involving the atrial and ventricular septum, great arteries, and systemic and pulmonary veins. The final step is evaluation of physiology and function, which paves the way for decision making regarding management.

Reliable identification of the cardiac chambers based on specific morphologic features is the first step in the segmental approach to heart disease. These features may be appreciated easily on cross-sectional imaging.

Characterizing the specific type of visceroatrial situs, ventricular looping, and great arterial situs is the next step. Based on the various permutations and combinations of atrial, ventricular, and great arterial relationships, Van Praagh created a chart that outlines the diversity that exists in human hearts.

Analysis of connecting segments—the AV junction and the ventriculoarterial junction—leads to an understanding of the pathogenesis of single ventricle and outflow tract anomalies, respectively.

The final step in the segmental approach to heart disease is to determine how the segmental combinations and connections, with the associated anomalies, function. This determination paves the way for appropriate decision making regarding management, which may be medical, surgical, or both.

Suggested Readings

Colvin, EV. Single ventricle. In: Garson A, Jr., Bricker JT, McNamara DG, eds. The science and practice of pediatric cardiology. Philadelphia: Lea and Febiger, 1990.

Krishnamurthy, R. Embryologic basis and segmental approach to imaging of congenital heart disease. In Ho V, Reddy GP, eds.: Cardiovascular imaging, 1st ed, Philadelphia: Saunders/Elsevier, 2010.

Shinebourne, EA, Macartney, FJ, Anderson, RH. Sequential chamber localization-logical approach to diagnosis in congenital heart disease. Br Heart J. 1976;38:327–340.

Van Praagh, R. Diagnosis of complex congenital heart disease: morphologic-anatomic method and terminology. Cardiovasc Intervent Radiol. 1984;7:115–120.

Van Praagh, R. The segmental approach to diagnosis in congenital heart disease. Birth Defects. 1972;8:4–23.

Van Praagh, S, Kreutzer, J, Alday, L, et al. Systemic and pulmonary venous connections in visceral heterotaxy, with emphasis on the diagnosis of the atrial situs: a study of 109 postmortem cases. In: Clark E, Takao A, eds. Developmental cardiology, morphogenesis and function. Mt. Kisco, NY: Futura, 1990.

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