Left and Right Atria

Although there are exceptions, the position of the morphologic right and left atria is directly determined by the symmetry of the thoracic and abdominal contents. The right atrium lies on the side that contains the trilobed lung and the liver, whereas the left atrium connects to the thorax on the side of the bilobed lung and the spleen. These relationships are precise when there are no caval anomalies. The symmetric determination of the atrial position and morphology is ambiguous in bilateral right or left lungs and in asplenia or polysplenia.

Ventricular Looping

The next step is identification of the two ventricles as either D-loop or L-loop. These terms refer to the embryologic looping of the straight tube of the heart. A D-loop brings the cardiac apex initially to the right with the morphologic right ventricle anterior to the left ventricle; the normal heart has a D-loop. Final looping of the heart tube places the apex to the left with the left ventricle lying on the left-hand side of the interventricular septum as viewed through the tricuspid valve. A helpful but not infallible method of localizing the ventricles is the “loop rule”: the position of the aorta with respect to the pulmonary artery corresponds to a particular ventricular looping. Without regard to anteroposterior positions, when the aorta is located to the right of the pulmonary artery, a ventricular D-loop is probable. When the aorta is to the left of the pulmonary artery, a ventricular L-loop is probable.

The coronary arteries are also helpful for locating the position of the ventricles. The right coronary artery marks the atrioventricular sulcus of the right ventricle and the anterior descending artery marks the interventricular sulcus. When there is an L-loop heart, the right coronary artery is to the left of the anterior descending artery.

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.

Mesocardia

Mesocardia is a variant of dextrocardia and levocardia. In this condition, the heart lies in the midline without a distinct apex pointing to either side. These patients may have situs solitus, inversus, or ambiguus of the atria and have similar associated defects.

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.

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

* 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.

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.

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:

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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