Cardiac Axis and Visceral Situs

The situs is determined by chest and abdominal radiographs. Situs solitus and situs inversus are recognized by the asymmetry of the tracheobronchial tree and by the positions of the abdominal organs. Symmetrically lobed lungs (Fig. 10-10), midline liver, gastrointestinal malrotations, asplenia, and polysplenia denote the heterotaxy syndrome (Fig. 10-11), which can also be recognized by isomerism of the atria. Isomerism means that both atria have features of the right atrium or of the left atrium. The visceral-atrial rule is that the right and left atria develop on the same side as the thoracic and abdominal viscera do. In situs solitus, the right atrium is on the right side of the mediastinum and the left atrium is on the left side. In situs inversus, the morphologic right atrium is on the left side and the left atrium lies on the right side. In situs ambiguus, right and left sides cannot be determined because the lungs and abdomen are symmetric. For example, in asplenia there are two right (trilobed) lungs and both atria are morphologically right atria. In polysplenia there are two (bilobed) left lungs and both atria are morphologic left atria.

FIGURE 10-10 Symmetric bronchi. A, Coronal spin echo acquisition from a 60-year-old woman with polysplenia, left isomerism, interruption of the inferior vena cava with azygos continuation, and coarctation of the aorta. The trachea bifurcates into two symmetric bronchi (arrows 1 and 2). The left pulmonary artery (L) passes over the left-sided bronchus; the right pulmonary artery (R) passes over the right-sided bronchus. Note that no inferior vena cava is seen passing through the liver, but a dilated azygos vein (short arrow) is found just to the right of the tracheal bifurcation. The dilated left subclavian artery (LS) originates from the distal aortic arch (Ao). AoD, descending aorta; T, trachea. B, Axial double inversion recovery image from an 8-month-old girl with complex congenital heart disease. Both the left (LP) and right (RP) main pulmonary arteries enter their respective hilum anterior to their respective right (arrow 1) and left (arrow 2) bronchi. Notice the dilated azygous vein (Az); the intrahepatic inferior vena cava is interrupted.

(Courtesy Kenneth E. Fellows, MD.)

FIGURE 10-11 Atrial septal defect with pulmonary hypertension in polysplenia and the heterotaxy syndrome. A, Frontal view shows the gastric bubble on the right and the mass of the liver on the left—abdominal situs inversus. The bulk of the cardiac silhouette is in the right chest, indicating dextrocardia. Both main stem bronchi are below their adjacent pulmonary arteries, indicating left heterotaxy. The right-sided pulmonary artery and hilar left-sided pulmonary artery are dilated, reflecting the increased pulmonary artery blood flow and pressure. The aortic arch is not well visualized, but the left-sided dilated azygos arch (arrow) is seen. B, In the lateral view, both right and left pulmonary arteries are above their respective bronchi and posterior to the trachea, again indicating bilateral left lungs.

In most people, the major portion of the heart lies slightly to the left of midline. The cardiac apex denotes the location of the heart within the thorax. Dextrocardia (Fig. 10-12), levocardia (Fig. 10-13), and mesocardia then indicate the possible positions of the heart. Using this terminology, a cardiac malposition is any heart that does not have a leftward cardiac axis in situs solitus. A malposition includes dextrocardia in situs solitus and levocardia in situs inversus (Fig. 10-14), as well as dextrocardia in situs inversus. All these positions represent deviation from normal embryologic development without necessarily implying any hemodynamic or morphologic derangement.

FIGURE 10-12 Coarctation of the aorta and ventricular septal defect in dextrocardia. The gastric air bubble lies on the left and the liver lies on the right, indicating abdominal situs solitus. The cardiac apex and bulk of the heart lie to the right, indicating dextrocardia. Rib notching and an indentation in the proximal descending aorta (arrow) denote the aortic coarctation. The enlarged central pulmonary arteries are a result of the left-to-right shunt.

FIGURE 10-13 Situs inversus with levocardia. The stomach bubble (arrow) is on the right, and the liver is on the left, indicating abdominal situs inversus. The right-sided bronchus has the appearance of a morphologic left bronchus (arrowheads); it has a long course before the right-sided upper lobe bronchus originates. This indicates thoracic situs inversus. The trachea is displaced toward the left, indicating a right-sided aortic arch. However, the cardiac apex is toward the left.

FIGURE 10-14 Complete situs inversus. The stomach bubble is on the right, and the liver is on the left, indicating abdominal situs inversus. The right-sided pulmonary artery passes over the right-sided bronchus; the left bronchus goes for a short course before giving the left-sided upper lobe branch. This indicates thoracic situs inversus. Thoracoabdominal situs concordance in situs inversus indicates complete situs inversus; the bulk of the heart lies to the right.

In primary dextrocardia, the main defect is in the heart. There are two types of primary dextrocardia: (1) dextroversion, in which the heart is rotated or pivoted so that its apex lies on the right side with the atria as a fulcrum; and (2) mirror-image dextrocardia. In secondary dextrocardia, the heart is normal but the mediastinum is shifted to the right because of extracardiac abnormalities that involve the lungs, pleura, or skeleton (Box 10-1). Examples of the latter include pneumothorax, congenital herniation of the gastrointestinal tract into the thorax, and thoracolumbar scoliosis (Fig. 10-15).

FIGURE 10-15 Dextroposition. In hypoplasia of the right lung, the left lung is hyperaerated, and pushes the heart into the “empty” right chest.

Atrial Morphology

With rare exceptions, the morphology of the atria corresponds closely with the situs of the tracheobronchial tree and the abdominal viscera. Of the various criteria for distinguishing between right and left atria, the most reliable are the shape of the atrial appendage (see Figure 10-1) and the connection to the inferior vena cava (Box 10-2). The right atrial appendage is broad and pyramidal, whereas the left atrial appendage is thin with a narrow neck. The inferior vena cava almost always connects with the right atrium. This is true even in the “absence” of the inferior vena cava and azygos continuation. In this entity, there is no intrahepatic portion of the cava but the hepatic veins connect to the subdiaphragmatic portion of the inferior vena cava, which joins the right atrium.

The superior vena cava is a poor landmark of atrial morphology because bilateral cavae may be present or the right superior vena cava may be absent, and there may be a connection of either the right or left superior vena cava into either atrium or into the coronary sinus. As an example, in situs inversus totalis, the morphologic right atrium is on the left side and the morphologic left atrium lies on the right side of the body, whereas the right-sided lung has two lobes and the left-sided lung has three lobes. When thoracic isomerism exists in the heterotaxy syndrome, bilateral morphologic right atria are seen in the asplenia syndrome and bilateral left atria in polysplenia.

Ventricular Morphology

When the right and left ventricles are normal, identification of the two ventricles is relatively simple (Box 10-3). The normal right ventricle has coarse, trabeculated walls when compared with the smooth-walled left ventricle. The right ventricle has a contractile muscle called the conus or infundibulum between the tricuspid and pulmonary valves (see Figure 10-2), whereas the left ventricle has mitral-aortic continuity with no intervening muscle (Fig. 10-16). The right ventricle has trabeculations and papillary muscles on its septum, whereas in the left ventricle these structures are not present on the septum. A bicuspid (mitral) atrioventricular valve is a part of the left ventricle, whereas a tricuspid atrioventricular valve is part of the right ventricle, although either of these valves may have a cleft or be absent.

FIGURE 10-16 Axial gradient echo acquisition from a woman with pulmonary hypertension. The anterior mitral leaflet (black arrowheads) is continuous with the aortic annulus (white arrowheads). The dilated right atrium (RA) is labeled.

When some of the structures used to identify the ventricles are congenitally absent or malformed, the identification of the two ventricles becomes confusing. To clarify this situation, identify the three anatomic segments in the normal ventricle:

The inlet segment in the right ventricle is smooth and adjacent to the tricuspid valve; in the left ventricle it is between the papillary muscles and the mitral valve. The trabecular segment constitutes the body of the ventricle distal to the insertion of the papillary muscles. This trabeculated segment is a key feature in the distinction between the two ventricles. In the right ventricle, there are large, coarse trabeculations, prominent in both systole and diastole. In the left ventricle, the wall is smooth in diastole but has fine trabeculations during systole. The ventricular outlet portion of the right ventricle is a tubular muscular structure, the conus, which separates the inlet and outlet valves. In the left ventricle, the outlet is smooth and has no muscle between the inlet and outlet valves.

When one or more of these three ventricular segments is absent, the heart may be called a single ventricle. Similar terminology for hearts that lack at least one of the three ventricular segments are:

There is general agreement that an inflow tract must be present for a chamber to be considered a ventricle. The trabecular portion determines whether the chamber is of the right or left ventricular type. In these instances, the single ventricle consists of one large chamber that receives both atrioventricular valves. (Note that this definition excludes mitral or tricuspid atresia.) If only the trabecular and outflow segments are present, this structure is called an outlet chamber. Examples of such hearts are the univentricular heart of the left ventricular type, with or without a rudimentary outflow chamber.

Difficulties arise in this classification scheme when part of an inlet or outlet valve overrides the septum. In this situation, rather than make an arbitrary decision, a description of the amount of overriding is appropriate. In general, when either the inlet or outlet valve is associated with more than 50% of a ventricle, it is considered to be a part of that ventricle. Examples of this condition include straddling tricuspid valves and a double-outlet right ventricle.

Inversion of a chamber occurs when a structure that normally lies on the right side is situated on the left side, or vice versa. In the asymmetric body, situs inversus totalis is an example of inversion in which all body structures are isomers to those in situs solitus. In describing the relation of the ventricles to one another, it may be difficult to distinguish true inversion from a cardiac rotation as a result of an extrinsic abnormality. Locate the ventricular septum as seen through the mitral and tricuspid valves. In the normal, noninverted right ventricle, the septum is on the left side as viewed through the tricuspid valve. In the normal left ventricle, the septum is on the right side as viewed through the mitral valve.

Atrioventricular Connections

The atrioventricular connections are called concordant when the right atrium connects to the right ventricle (see Figure 10-4) and the left atrium connects to the left ventricle. When the right atrium connects to the left ventricle and the left atrium connects to the right ventricle, the ventricles are discordant in relation to the atria (see Figure 10-5). In the heterotaxy syndrome, in which either two right atria or two left atria may exist, the atrioventricular connection is ambiguous. This schema is less clear when either atresia of one of the atrioventricular valves exists or when one of the atrioventricular valves straddles the interventricular septum. When there is a double-inlet or a straddling atrioventricular valve, the tensor apparatus (the attachments of the chordae tendineae) may connect to either side of the interventricular septum.

Relations of the Great Arteries

The position of the aorta is described relative to the pulmonary artery in both the anteroposterior and lateral planes (Fig. 10-17). In the normal heart, the aorta is to the right of and posterior to the pulmonary artery (see Figure 10-6). An anterior aorta to the right of the pulmonary artery is common in transposition of the great arteries (TGA; see Figures 10-7, 10-9). An aorta to the left of and anterior to the pulmonary artery is typical in, but not diagnostic of, corrected transposition (Fig. 10-18). Then there is a characteristic leftward convexity of the aorta (Fig. 10-19).

FIGURE 10-17 Congenitally corrected transposition of the great arteries. The abnormal leftward course of the ascending aorta is visible as a mediastinal line (arrow) as it becomes the aortic arch.

FIGURE 10-18 Coronal spin echo acquisition from a 23-year-old woman with corrected transposition of the great arteries. The right heart-border-forming right atrium (RA) connects with the left ventricle (LV), characterizing atrioventricular discordance. Furthermore, the LV supports the pulmonary artery (PA), indicating ventriculoarterial discordance, and lies to the right of the left heart-border-forming ascending aorta (AoA). The left-sided right ventricle is anterior and out of the imaging plane.

FIGURE 10-19 Levomalposition of the aorta. A, A left ventriculogram in congenitally corrected transposition of the great arteries shows the pulmonary artery to the right of the aorta. B, On the levophase, the right ventricle connects to the aorta, which is on the left side of the pulmonary artery. The left aortic arch forms the border of the left side of the mediastinum.

Ventriculoarterial Connections

The pulmonary valve is part of the pulmonary artery (not part of the right ventricle) and the aortic valve is part of the aorta. When the pulmonary artery or the aorta is related to, or overrides more than 50% of, a particular ventricle, it is defined as being connected to that ventricle. This association is particularly strong when there is a continuity between an atrioventricular valve and the semilunar valve. Concordant connections exist when the left ventricle is connected to the aorta and the right ventricle to the pulmonary artery (see Figures 10-2, 10-8). Discordant connections result when the left ventricle is connected to the pulmonary artery and the right ventricle to the aorta (see Figure 10-9). This latter connection is also called transposition. When both great arteries arise predominantly from one ventricle, there is a double-outlet right ventricle or double-outlet left ventricle. The final type of arterial connection is a single-outlet heart, of which there are three varieties:

The ventricular outflow tracts may be one of four distinct types:

Certain types of conus are associated with particular malformations:

FIGURE 10-20 Transposition of the great arteries with levomalposition of the aorta. A, Both the pulmonary artery and aorta fill from a right ventricular injection. The left aortic arch is the convexity in the left superior mediastinum. The right-subclavian-artery-to-rightpulmonary-artery graft (Blalock-Taussig shunt) is patent (arrow). Early (B) and late (C) frames in a lateral projection show a large ventricular septal defect. The mitral valve (MV) below the conus to the pulmonary artery is shown in diastole. The crista supraventricularis (C) is the lucency between the bilateral conus. The aortic valve is anterior to the pulmonary valve and at the same height. Note the bilateral conus in B.

(Courtesy Kenneth E. Fellows, MD.)

Associated Malformations

These defects may actually dominate the clinical presentation. Septal defects may be in the atrial septum, the interventricular septum, or peripherally between the aorta and pulmonary arteries. An example of the latter is patent ductus arteriosus (PDA) and aortic-pulmonary window. Stenoses, atresias, hypoplasia, and regurgitation may exist at the atrioventricular or arterial valves or in relation to the inflow and outflow regions of these valves. Finally, there are anomalies in the connection of the systemic veins (particularly with a left superior vena cava) and in the connections of the pulmonary veins. Abnormal pulmonary and systemic venous connections are particularly prevalent in the asplenia and polysplenia syndromes. In asplenia, there are bilateral right atria so that the pulmonary veins connect to either the superior vena cava or the portal system. In polysplenia with two left atria, the pulmonary veins may connect to either or both atria.

Cardiac Connections and Positions

Most cardiac malpositions can be unequivocally described using the basic concepts of segmental analysis of the atria, the ventricles, and the great vessels and their associated relations, connections, and associated malformations.

Dextrocardia

Dextrocardia signifies that the apex of the heart is directed toward the right. Primary dextrocardia exists because of an embryologic abnormality. This type of dextrocardia can exist with any type of situs position (Fig. 10-21). When dextrocardia exists with situs inversus, the atrial and ventricular relations are a mirror image of their positions in the usual situs solitus. When the dextrocardia exists in situs solitus, the term isolated dextrocardia is frequently applied. It is clear then that dextrocardia can occur in situs solitus, inversus, and ambiguus. Many associated cardiac anomalies exist in primary dextrocardia. Frequent conditions include ventricular septal defect, TGA, corrected TGA, double-outlet right ventricle, and juxtaposition of the atrial appendages.

FIGURE 10-21 Primary dextrocardia (dextroversion). The levophase of a pulmonary angiogram outlines a smooth-walled left ventricle (LV), which is the anterior ventricle because the heart is rotated to the right. An anomalous right inferior pulmonary vein (arrows) connects to the inferior vena cava (scimitar syndrome).

The goal of echocardiography, magnetic resonance imaging (MRI), and angiography is to define the position and location of each chamber of the heart and their connections and relations with one another and with the great arteries. In those malpositioned hearts in which the location of the interventricular septum is not known before angiography, posteroanterior and lateral projections serve as initial guidelines. Frequently, the projections can be reversed for a malposition; that is, those structures that are normally best seen in the left anterior oblique projection in the normal heart would be studied in the right anterior oblique projection in dextrocardia. As a rule, the dextrocardia itself does not cause clinical problems but rather the associated malformations mandate medical or surgical alleviation.

Levocardia

Strictly speaking, levocardia means that the cardiac apex is left sided. Isolated levocardias are those hearts that are left sided when situs inversus is present. This anomaly occurs in less than 1% of all patients with congenital cardiac malformation compared with a 2% incidence of dextrocardia in patients with CHD. With levocardia, the position of the thoracic and abdominal organs ranges from partial to complete situs inversus and also to heterotaxy (Fig. 10-22). Severe malformations are always associated with levocardia and frequently include ventricular septal defect, complete atrioventricular canal defects, and pulmonary stenosis or atresia. Isolated levocardia may be suspected on the chest film with a right-sided stomach bubble and left-sided liver shadow and a left cardiac apex. In contrast, in extrinsic levocardia the heart is intrinsically normal but the mediastinum is shifted from skeletal or pulmonary abnormalities (Fig. 10-23).

FIGURE 10-22 Levocardia with polysplenia. A, The catheter in the inferior vena cava has a high, left-sided loop indicating hemiazygos continuation before it enters the right superior vena cava. B, Posteroanterior view shows a single ventricle with a subaortic conus. The right aortic arch supplied a patent ductus arteriosus. The ventricle had pulmonary atresia and a complete atrioventricular canal.

FIGURE 10-23 Secondary levocardia. Because of a left pneumonectomy, the heart has moved with the mediastinum into the extreme left side of the thorax. The huge right lung now crosses the midline to fill much of the left hemithorax. The heart is invisible because of the oblique interface the right lung makes with the shifted mediastinum.

Heterotaxy and the Syndromes of Asplenia and Polysplenia

Heterotaxy is the failure of the developing embryo to establish normal left-right asymmetry. Asplenia and polysplenia are part of this spectrum. In these patients the thoracic and abdominal contents have a degree of symmetry, unlike those in situs solitus or inversus in which right- and left-sided organs exist together. In the thorax, both lungs may be trilobed with bilateral epiarterial bronchi, or both lungs may be bilobed with bilateral hypoarterial bronchi. In the abdomen, the asymmetry is also frequently lost. The liver may be midline. The attachment of the mesentery, which usually runs from the left upper quadrant to the right lower quadrant, may have a midline attachment. The spleen may be absent (asplenia), bilobed with multiple accessory spleens, or multiple small spleens (Fig. 10-24) may be found throughout the mesentery (polysplenia). Situs ambiguus exists either when the right and left sides of the lungs, heart, and abdomen are similar or where a right-left relationship is difficult to identify. Boxes 10-5 and 10-6 summarize the characteristics of asplenia and polysplenia.)

FIGURE 10-24 Heterotaxia. A, The main stem bronchi are below the pulmonary arteries bilaterally. B, The abdominal situs is inverted with multiple small spleens occupying the right side of the abdomen. The liver is midline and occupies nearly equal space in the right and left sides of the abdomen.

Box 10-5 Characteristics of patients with asplenia

Predominately male

Present with cyanosis

Have a high risk of infection

Frequently have severe malformations:

• Transposition of the great arteries with pulmonary stenosis or atresia

• Total anomalous pulmonary venous connection

Box 10-6 Characteristics of patients with polysplenia

Predominately female

Have more benign malformations:

• Azygos continuation of inferior vena cava

• Bilateral superior vena cava

• Anomalous pulmonary venous connections

• Atrial septal defects

Splenic anomalies with malpositions and malformations in multiple organ systems have been recognized since 1826 when Martin and later Ivemark described the absence of the spleen in cyanotic CHD. Complex cardiac malformations are typical when the type of thoracic and abdominal situs abnormality is uncertain or has features of both situs solitus and situs inversus (Table 10-1).

TABLE 10-1 Cardiovascular abnormalities in asplenia and polysplenia

Abnormality	Asplenia (%)	Polysplenia (%)
SUPERIOR VENA CAVA
Bilateral	53	33
Right	34	33
Left	10	33
Uncertain	3	—
INFERIOR VENA CAVA
Right sided	60	—
Left sided	28	—
Uncertain	12	—
Azygos continuation	—	84
Anomalous pulmonary veins	84	50
Total anomalous connection	72	—
Partial anomalous connection	12	—
CARDIAC APEX
Left	56	58
Right	41	42
Uncertain	3	—
AORTIC ARCH
Left	56	33
Right	38	67
Unknown	6	—
Great vessels
Normally related	19	84
Transposition of great arteries	72	8
Double-outlet right ventricle	9	8
PULMONARY VALVE
Normal	22	58
Stenosis	34	33
Atresia	44	9
Patent ductus arteriosus	56	50
Absent coronary sinus	85	42
Single ventricle	44	8
Ventricular septal defects	90*	67

* Of the ventricular septal defects in asplenia, 84% were of the atrioventricular canal type.

Modified from Rose V, Izukawa T, Moes CAF: Syndromes of asplenia and polysplenia; a review of cardiac and non-cardiac malformations in 60 cases with special reference to diagnosis and prognosis. Br Heart J 37:840-852, 1975.

Segmental analysis of the defects in hearts associated with asplenia begins with the atria and the atrial septum. On the chest film, the external contours of the heart frequently do not conform to the expected heart chambers (Fig. 10-25). Almost all these hearts show a common atrioventricular valve, frequently associated with separate, large atrial septal defects in the primum and secundum location. The size and location of these atrial defects are such that the malformation is called a common atrium. The ventricles also almost invariably have major malformations. About one fourth of the ventricles are inverted (as seen in corrected transposition), and half of the hearts have a univentricular chamber with a rudimentary outflow tract.

FIGURE 10-25 Heterotaxy syndrome with dextrocardia. A, The stomach bubble (St) is on the right and the liver (Li) is on the left; this is abdominal situs inversus. However, the appearance of the bronchi indicate thoracic situs solitus; this is thoracoabdominal situs discordance. The main pulmonary artery (PA) central pulmonary artery segments are dilated, indicating shunt. Notice the dilated left-sided azygos vein (arrow), reflecting interruption of the IVC with azygos connection. B, Angiography is performed from a left upper extremity venous approach with catheter passage from the left-sided superior vena cava to the right atrium. The angiogram shows that the right side of the heart is formed by the right ventricle, which connects to the pulmonary artery. The aorta fills simultaneously through a ventricular septal defect and forms the broad curve in the right anterior mediastinum (arrowheads).

Anomalies of the great vessels, including TGA and double-outlet right ventricle, have an incidence of 3% to 30%. Angiographically, the posteroanterior and lateral projections are best to allow identification of their right-left relationships. Anomalies in the ventricular septum and semilunar valve stenosis are common, so that filming is also done with the x-ray beam parallel to the interventricular septum with cranial angulation. Because two thirds of persons with asplenia have anomalous systemic venous or pulmonary venous connections, or both, these malformations frequently complicate catheterization.

The features of the heart in polysplenia are quite variable, and in fact, there is occasionally no cardiac malformation. With a femoral vein approach, you can recognize azygos continuation by the course of the catheter around the azygos arch (Figures 10-10A, 10-26). You should not make the diagnosis of tricuspid atresia if the catheter tip fails to pass leftward through the heart above the diaphragm but instead should continue advancing the catheter superiorly until it goes around the azygos arch.

FIGURE 10-26 Hemiazygos continuation in a patient with polysplenia and dextrocardia. An injection into the left femoral vein opacified the hemiazygos vein adjacent to the costovertebral sulcus before it emptied into a left superior vena cava.

Atrioventricular Discordance

In the schema of segmental cardiac analysis, after recognition of the atria and ventricles, the next step is to determine whether the connections between them are concordant or discordant. Atrioventricular discordance means that the right atrium is connected to the left ventricle and the left atrium is connected to the right ventricle (see Figures 10-4, 10-5). Implicit in this definition is the presence of two atria, two atrioventricular valves, and two ventricles. This diagnosis is not appropriate when atrial identification is indeterminate in situs ambiguus or when there is a single common atrioventricular valve. Similarly, distinct right and left ventricles are necessary for this definition, although a ventricular septal defect may exist.

Atrioventricular discordance may be present in either situs solitus or situs inversus and may be accompanied by ventriculoarterial concordance or discordance. Discordance of both the atrioventricular and the ventriculoarterial segments is called congenitally corrected TGA. Atrioventricular discordance with concordance of the ventricles and great arteries, a rare malformation, is called (isolated) ventricular inversion.

Congenitally Corrected Transposition of the Great Vessels (Levotransposition of the Great Arteries)

In 1875 Rokitansky reported a form of transposition in which blood passed in normal serial fashion through the pulmonary and systemic circuits. The right atrium was connected to the left ventricle, which was connected to the pulmonary artery. On the oxygenated side of the lungs, the left atrium was connected to the right ventricle, which was connected to the aorta. The atrioventricular valves always correspond with their ventricles, even when there is atrioventricular discordance. That is, the mitral valve is a left ventricular structure, and the tricuspid valve is a right ventricular structure. In congenitally corrected transposition of the great vessels, the aorta lies to the left of and anterior to the pulmonary artery, whereas the pulmonary valve lies to the right and posterior. The aortic valve is usually somewhat anterior to the pulmonary valve, although the two great vessels may be exactly lateral to each other. The ascending aorta frequently has an unusual course, passing in a direction toward the left shoulder so that occasionally a distinctive contour in the left side of the mediastinum is visible on the chest film.

If there are no other defects, this malformation causes no hemodynamic problems and may go undetected during a normal life span. Unfortunately, associated malformations are the rule, and their site and severity determine the clinical course. Ventricular septal defects are frequent (Figures 10-27, 10-28) and may be large enough to cause pulmonary arterial hypertension. These defects are usually in the membranous septum adjacent to the pulmonary valve; muscular defects and supracristal defects are less common. Generally, the left-sided atrioventricular valve (i.e., the valve between the left atrium and the right ventricle) is displaced slightly into the ventricle in a manner resembling Ebstein anomaly. If the displacement is more than a few millimeters (because the tricuspid valve is usually displaced to the apex by that amount), the diagnosis of Ebstein anomaly is quite likely. Pulmonary stenosis is frequently associated with ventricular septal defect and may be caused by a malformed valve, a subpulmonary membrane, aneurysms of the membranous ventricular septum, or rarely, accessory tissue in the atrioventricular valve or a muscular bar in the subpulmonary region.

FIGURE 10-27 Congenitally corrected transposition of the great arteries with ventricular septal defect. The leftward course of the ascending aorta is not apparent on the chest film. The leftward cardiac apex represents the right ventricle, which is enlarged because of the ventricular septal defect. The main pulmonary artery is not part of the mediastinal interface with the lung because it is central; the hilar and peripheral pulmonary arteries are large from the left-to-right shunt.

FIGURE 10-28 Off-coronal spin echo acquisition from a 7-month-old boy with double inlet left ventricle and corrected transposition of the great arteries. Drainage from the right atrium (RA) into the dominant left ventricle (LV) is shown. LV blood passes directly to the medially placed main pulmonary artery (MP) and through a bulboventricular foramen (arrow) into a hypoplastic right ventricular outflow chamber (R), which supports a left-sided ascending aorta (Ao).

Imaging establishes the atrioventricular connections, the morphology of the ventricles, and the position of the aorta and pulmonary artery. The position of the venous and arterial catheters frequently give the first clue to a corrected transposition (Fig. 10-29). In situs solitus and levocardia, the venous catheter passes through the heart in the midline to reach the pulmonary arteries. The catheter in the pulmonary artery is posterior to its usual location, which is where the aorta should be in normal hearts. The retrograde arterial catheter has a distinctive curve in the ascending aorta as its course becomes convex medially and to the left before entering the heart (see Figure 10-18). On the lateral view, the aortic catheter is anterior and superior to the venous catheter. The venous and arterial catheters indicate the fundamental relationship between the aorta and the pulmonary artery in corrected transposition with situs solitus and levocardia; the pulmonary artery lies to the right and posterior, whereas the aorta is anterior and to the left.

FIGURE 10-29 Catheter positions in congenitally corrected transposition of the great arteries. A, The catheter in the inferior vena cava passes through the right atrium and left ventricle to end in the right pulmonary artery. B, The retrograde aortic catheter ends in the left-sided right ventricle. The ascending aorta lies to the left of the pulmonary valve.

In corrected transposition with situs solitus and levocardia, the ventricles lie nearly side by side with the interventricular septum, seen in the anterior-posterior orientation. The left ventricle lies slightly inferior to the right ventricle and has a triangular shape, with the mitral valve lying medially and to the right. The mitral valve of the left ventricle connects to the right atrium and lies in continuity with the pulmonary valve. The left ventricular outflow region is short and vertically oriented, with the anterior leaflet of the mitral valve on the medial side and the membranous portion of the interventricular septum forming the superior and lateral wall.

In the sagittal view, the left ventricle appears to “stand on its apex” with a conical shape whose apex is in the diaphragmatic-sternal angle. The anterior wall of the left ventricle extends superiorly into a distinctive pouch that is characteristic of inverted ventricles, namely the anteriorly placed left ventricle (Fig. 10-30). This recess is separate from both the mitral and pulmonary valves and is the most anterior and superior structure of either ventricle. The outflow portion of the left ventricle in the lateral projection is posterior and connects to a pulmonary artery, which is beside or posterior to the aorta. The posterior wall of the left ventricle beneath the pulmonary valve is the membranous septum, and the anterior wall forms a neck above the blind recess and below the pulmonary valve.

FIGURE 10-30 Angiography of the left ventricle in congenitally corrected transposition of the great arteries. A, The septum runs obliquely, inferiorly on the right, and then superiorly on the left side. The distinctive recess (arrow) is typical of an inverted ventricle. B, The lateral view shows the pulmonary valve (arrow) posterior to the aortic valve.

In the frontal view, the right ventricle is to the left of and slightly superior to the left ventricle (Fig. 10-31). In this position, the right ventricle has an oval to triangular shape and the usual coarse trabeculations. The tricuspid annulus is in the posteroanterior plane separated from the aortic valve by the muscular infundibulum. This morphologic right ventricle connects with the left atrium. The crista supraventricularis in this projection is the medial wall of the infundibulum above the tricuspid valve. In the lateral projection, the crista is the posterior wall of the infundibulum and separates the tricuspid from the aortic valve.

FIGURE 10-31 Angiography of the right ventricle in congenitally corrected transposition of the great arteries. A, The subaortic conus of the right ventricle connects to the leftward ascending aorta, the border-forming structure of the left side of the mediastinum. B, The aortic valve is anterior to the heart and lies in a horizontal plane.

The aortic valve appears higher than the pulmonary valve and is the border-forming structure in the left side of the upper half of the mediastinum. Unlike in the normal heart, the main pulmonary artery does not form any interface with the lung on the frontal chest film.

Different ventricular patterns and shapes occur in situs inversus and in other rare malformations. Because the aortic arch may lie on either the left or right side, the distinctive mediastinal contour of the aorta (which is frequently not present in situs solitus with classic corrected transposition) may not be visible on the standard chest films.

About one third of patients with corrected transposition have tricuspid regurgitation (i.e., from the right ventricle connected to the left atrium). The apically displaced tricuspid leaflets of Ebstein anomaly are usually the cause of the regurgitation, but there are occasionally other leaflet abnormalities. When there is severe regurgitation in infants, the details of the leaflets and the origin of their insertion are frequently difficult to identify. If technical factors such as arrhythmia and catheter position can be excluded, it should be presumed that severe regurgitation into the left atrium is associated with a “left-sided” Ebstein anomaly (Fig. 10-32). The tricuspid annulus is adjacent to the right coronary artery, which may be opacified during the ventriculogram.

FIGURE 10-32 Ebstein anomaly in congenitally corrected transposition of the great arteries. Injection into the right ventricle has resulted in severe regurgitation into the left atrium. In contrast to isolated Ebstein anomaly, the tricuspid leaflets are poorly seen and have little apical displacement. A ventricular septal defect has allowed opacification of the pulmonary arteries.

Coronary Artery Patterns

The coronary anatomy in congenitally corrected transposition of the great vessels is unique to inverted ventricles. The right coronary artery supplies the morphologic right ventricle and the left coronary artery provides an anterior descending branch in the interventricular sulcus and a variable circumflex branch over the morphologic left ventricle (Fig. 10-33). In congenitally corrected TGA, the right coronary artery passes to the left and inferior in the atrioventricular groove between the left atrium and right ventricle. Distally, this artery branches into the atrioventricular nodal branch, the posterior descending artery, and a variable set of branches to the inferior portion of the left ventricle (Fig. 10-34). The marginal branches over the right ventricular epicardial surface tend to be large with numerous branches. In contrast, the left coronary artery lies anterior and to the right of the right coronary artery. The left main coronary artery continues mainly as the anterior descending branch, which has numerous septal and diagonal branches. The circumflex artery in the atrioventricular groove between the right atrium and left ventricle tends to be vestigial. The position of the coronary arteries within the thorax may be different because of dextrocardia or other relative rotations, but the coronary distribution corresponds uniquely to the respective ventricle. When confusing ventricular morphology does not allow identification of the right or left ventricle, visualization of the coronary arteries permits accurate identification of the ventricles.

FIGURE 10-33 Coronary arteries in corrected transposition of the great arteries. The aortic valve is anterior and to the left of the right ventricular infundibulum (RV). The posterior descending artery (PD) originates posteriorly and follows the atrioventricular groove between the left atrium and right ventricle. The circumflex artery (Cf) lies in the atrioventricular groove between the right atrium and left ventricle, whereas the anterior descending artery (AD) follows the interventricular sulcus. With situs solitus, the coronary arteries appear to be the mirror image of those in the normal heart.

FIGURE 10-34 Situs solitus and congenitally corrected transposition of the great arteries. The right coronary artery is inverted from its location in the normal heart in the left (A) and right (B) anterior oblique projections.

Isolated Ventricular Inversion

Atrioventricular discordance with ventriculoarterial concordance is termed isolated ventricular inversion. The segmental connections for the venous side of the heart are right atrium to left ventricle to aorta, and for the systemic side left atrium to right ventricle to pulmonary artery. As originally reported by the Van Praaghs (1966), the pulmonary artery arises anteriorly and to the left of the aorta. A large subaortic ventricular septal defect is adjacent to the septal leaf of the tricuspid valve. The aorta may also originate anterior to the pulmonary artery.

Straddling Atrioventricular Valves

Malposition of the atrioventricular valves completely or partially across a ventricular septal defect represents a mixed form of atrioventricular connection. The tensor apparatus of the mitral and tricuspid valves is complex and includes the annulus, leaflets, chordae, and papillary muscles. With a straddling atrioventricular valve, the leaflets connect across the ventricular septum through a septal defect. Peripherally, the chordae or the papillary attachments may cross the septal defect to attach in the contralateral ventricle. Most of these valves represent a type of complete atrioventricular canal defect. An overriding atrioventricular valve has its annulus on both sides of a septal defect. An atrioventricular valve then may be straddling, overriding, or both.

Ventriculoarterial Discordance

The next stage in the segmental analysis concerns the connections and relations of the great arteries with respect to the ventricles. There are a number of ventriculoarterial malformations, most of which are a TGA or one of its variants. A complete description of a ventriculoarterial defect involves three aspects:

If a conus is present, there is contractile tissue between the atrioventricular and semilunar valves.

The most common variety of ventriculoarterial discordance is TGA. In this sense, transposition means that the two great arteries are abnormally placed with respect to the interventricular septum: The aorta connects to the right ventricle and the pulmonary artery to the left ventricle. As an illustration, complete dextrotransposition of the great arteries exists when the aorta is anterior and to the right and connected to the right ventricle, whereas the pulmonary artery is posterior and to the left and connected to the left ventricle. The term partial transposition applies to variations that do not meet the strict criteria of complete transposition and includes double-outlet right ventricle and double-outlet left ventricle.

Complete Dextrotransposition of the Great Arteries

In 1797 Baillie described the heart of an infant in which the aorta connected to the right ventricle and the pulmonary artery to the left ventricle. The term transposition of the aorta and pulmonary artery is ascribed to Farre in 1814. Since that time, there has been controversy about whether it should be defined by the abnormal anteroposterior position of the great arteries or by the abnormal connections to the ventricles. TGA is a ventriculoarterial abnormality in which the aorta originates above the right ventricle and the pulmonary artery originates over the left ventricle.

After tetralogy of Fallot, complete TGA is the second most common cause of cyanosis from heart disease in infancy. In this malformation, the systemic and pulmonary circulations connect in parallel, in contrast to the serial connection in the normal infant. The blood flow through the lungs returns to the left atrium and to the left ventricle only to pass again through the lungs; in a similar fashion, the systemic venous and arterial circulations form a closed loop. For life to be sustained, mixing must occur between these two circuits. Therefore, one of the objectives of imaging is to determine the location and amount of these intracardiac or extracardiac shunts. The foramen ovale is almost always patent but is too small for adequate mixing. Occasionally, a secundum atrial septal defect will allow a large shunt to provide adequate mixing of oxygenated blood at this level.

Ventricular septal defects occur in about one third of babies with transposition (see Figure 10-7A), and when present, may result in congestive heart failure from the large blood flow. Extracardiac shunts may occur, as in patent ductus arteriosus or with bronchopulmonary connections to the pulmonary vascular bed. The ductus arteriosus remains patent in one fourth to one half of infants who do not receive prostaglandin E1 and allows blood to flow from the pulmonary artery to the aorta if the pulmonary vascular resistance is high and from the aorta to the pulmonary artery when the high fetal pulmonary artery pressures fall below the systemic blood pressure.

Besides the ventricular septal defect, the other major associated malformation is obstruction to blood entering the pulmonary arteries. About one fourth of those with TGA have some form of pulmonary stenosis (Fig. 10-35). The site of obstruction is usually in the subpulmonary region and it has a variety of causes:

• Anomalous attachment of the mitral valve

• Accessory endocardial tissue

• Subpulmonary membrane

• Subpulmonary fibromuscular tunnel

• Aneurysm of the membranous outflow tract

FIGURE 10-35 Gradient echo acquisition from a 24-year-old woman with shortness of breath. A, Cranially angulated coronal early systolic acquisition. The pulmonary valve leaflets (arrows)

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