Transposition of the Great Arteries

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CHAPTER 44 Transposition of the Great Arteries

Transposition of the great arteries (TGA) represents a spectrum of associated cardiac anomalies that share a common feature—ventriculoarterial discordance—meaning that the left ventricle (LV) is connected to the pulmonary trunk and the right ventricle (RV) is connected to the aorta. It is divided into two major types, complete transposition of the great arteries (CTGA) and congenitally corrected transposition of the great arteries (CCTGA). The primary lesion in CTGA is ventriculoarterial discordance alone, whereas CCTGA has both ventriculoarterial discordance and atrioventricular discordance, meaning that the connections between the two atria and two ventricles are also reversed. CTGA and CCTGA differ greatly in their clinical presentations, natural histories, management strategies, and imaging indications. They will be discussed separately in this chapter.

Historically, CTGA was first described by Matthew Baillie, a Scottish pathologist, in 1797. The first corrective surgery for CTGA was the atrial switch procedure developed by Senning ¹ in 1959 and Mustard² in 1964. Atrial switching effectively converts a CTGA to a CCTGA, in which the RV pumps the systemic circulation. Although this circulation is compatible with life, the systemic RV is prone to early failure. To address the shortcomings of atrial switching, Jatene and colleagues³ perfected the arterial switch procedure in 1975, in which the LV pumps the systemic circulation. It has become the preferred surgical treatment for CTGA. The Rastelli procedure⁴ is used when the CTGA is associated with pulmonary stenosis. CCTGA was first described by the famous Bohemian pathologist Carl von Rokitansky in 1875. Early treatments focused on correcting secondary lesions, such as ventricular septal defect (VSD). In favor of an anatomically correct circulation, Ilbawi and associates⁵ and others have advocated the double switch procedure, which combines the atrial switch and arterial switch procedures, or the atrial switch and Rastelli procedures. This procedure restores the LV as the systemic ventricle.

Definition and Classifications

CTGA is defined by ventriculoarterial discordance. It also can be described by the inlet-outlet features of the ventricles: the presence of subaortic infundibulum (Fig. 44-1), the absence of subpulmonary infundibulum, and mitral-pulmonary fibrous continuity (Fig. 44-2). Subaortic infundibulum implies a connection of the aorta to the infundibulum, which is a morphologic feature of the RV. The absence of subpulmonary infundibulum means that the pulmonary trunk is not connected to the RV. Because fibrous continuity of the inlet-outlet valves is a feature of the LV, a mitral-pulmonary fibrous continuity implies that the pulmonary trunk is connected to the LV.

FIGURE 44-1 Cardiac-gated CTA of a newborn with CTGA. This RV inlet-outlet view shows a superior vena cava (SVC) draining into a right atrium (RA). The RA is connected to the right ventricle (RV), which in turn is connected to an anteriorly located aorta (Ao). The subaortic infundibulum and the surrounding conal muscle (black arrowheads) separate the aortic valve from the tricuspid valve.

FIGURE 44-2 Cardiac-gated CTA of a newborn with CTGA. This LV inlet-outlet view shows the left atrium (LA) connected to the left ventricle (LV), which in turn is connected to the pulmonary trunk (PA). The absence of a subpulmonic infundibulum allows the pulmonary valve to be in contact with the mitral valve (arrow). They are said to be in fibrous continuity.

CTGA is known by a number of other names. Because CTGA comprises 90% of all transposition cases, it is sometimes referred simply as transposition of the great arteries. Other designations are simple transposition and dextrotransposition of the great arteries (D-TGA). The letter D refers to D-loop of the ventricles and not the positional relationship of the great arteries. D-loop refers to the rightward folding of the heart tube during early embryologic development. This crucial event leads to the normal positions of the LV and RV at birth (Fig. 44-3). Usually, D-TGA corresponds with the physiology of CTGA, but this is not always the case. In situs inversus, the CTGA physiology corresponds to L-loop ventricles and CCTGA physiology corresponds to D-loop ventricles. To avoid confusion, it is best to use the terms CTGA and CCTGA, which correctly describe the underlying physiology.

FIGURE 44-3 Cardiac-gated CTA of a patient with pulmonary hypertension. This four-chamber view shows coarse trabeculation in the RV, exaggerated by hypertrophy. The positions of the RV and LV are normal, consistent with D-loop.

There is no officially adopted classification for CTGA. Because the clinical presentation, prognosis, and treatment options are greatly influenced by the associated anomalies, it is useful to divide CTGA according to these anomalies into those with intact ventricular septum (CTGA-IVS), those with ventricular septal defect but without left ventricular outflow obstruction (CTGA-VSD), and those with ventricular septal defect and left ventricular outflow obstruction (CTGA-VSD-LVOTO).

CCTGA is defined by the presence of atrioventricular and ventriculoarterial discordance. Alternative names for CCTGA are corrected transposition, double discordance, and levotransposition of the great arteries (L-TGA). In a patient with normal situs, L-TGA implies L-loop ventricles (Fig. 44-4) and atrioventricular discordance, in addition to ventriculoarterial discordance. Like D-TGA, the term L-TGA is best avoided in favor of CCTGA.

FIGURE 44-4 MRI steady-state free precession image of a patient with CCTGA. This four-chamber view shows that the positions of the right ventricle (RV) and the left ventricle (LV) are reversed, consistent with L-loop.

Van Praagh proposed a three-letter scheme that labels the sidedness of the atrial, ventricular, and arterial segments,⁶ and it has been used to categorize various anatomic manifestations of TGA. In this scheme, the first letter denotes the atrial situs, with S meaning solitus and I meaning inversus. The second letter denotes the ventricular loop, with D meaning D-loop, and L meaning L-loop. The third letter denotes the positions of the great arteries, with D meaning that the aortic root is on the right side of the pulmonary root (Fig. 44-5) and L meaning that the aortic root is on the left side of the pulmonary root (Fig. 44-6). In situs solitus and CTGA, the most common segmental anatomy is S, D, D, although S, D, L exists in a minority of cases. In situs inversus and CTGA, the most common pattern is I, L, L, which is the mirror image of S, D, D. In CCTGA, the segmental anatomy is usually S, L, L for situs solitus and I, D, D for situs inversus. Van Praagh and coworkers,⁷ in a pathology series of CCTGA, reported one case of S, L, D.

FIGURE 44-5 Cardiac-gated CTA a newborn with CTGA. This axial view shows that the aorta (Ao) is located anterior and to the right of the pulmonary artery (PA). The great arteries are in D-position.

FIGURE 44-6 MRI steady-state free precession image of a patient with CCTGA. The position of the pulmonary trunk (PA) is implied at the junction between the left and right main pulmonary arteries. This axial view shows that the aorta (Ao) is anterior and to the left of the pulmonary trunk (PA). The great arteries are in L-position.

Epidemiology and Genetics

CTGA occurs in 5% to 7% of cases of congenital heart disease, with an incidence of 20 to 30/100,000 live births. It is the second most common cyanotic heart disease, behind tetralogy of Fallot, and it is the most common cyanotic heart disease that manifests in the neonatal period. There is no known racial predilection, but there is a 2 : 1 male predominance. CTGA has been reported in trisomies 18 and 21,⁸ although the vast majority of cases have no identified genetic defect. Chromosomal 22q11 deletion, commonly found in conotruncal anomalies, is not associated with CTGA. Furthermore, the risk of recurrence in the offspring of individuals with CTGA is very low.⁹ It has a greater incidence in mothers with diabetes. These observations suggest that genetics may play a less important role in the development of CTGA than environmental factors, possibly intrauterine hormonal disturbance or exposure to teratogens. CTGA is rarely associated with other extracardiac anomalies, with the possible exception of coarctation.

CCTGA occurs in 0.5% to 1.4% of cases of congenital heart disease. It is rare among congenital cardiac anomalies and is much less common than CTGA. Its incidence is on a par with Ebstein anomaly and interrupted aortic arch. Like CTGA, its inheritance pattern is multifactorial, without clear genetic association. There is a slight male predominance.

Etiology and Pathophysiology

Embryology

The embryology of CTGA is not well understood. The loss of the normal spiral relationship between the ascending aorta and the pulmonary trunk in CTGA suggests a problem with the process of conotruncal septation. In normal development during the fifth week of gestational age, two intertwined, spiraling flow streams in the conotruncus are thought to induce the proper spiraling septation that divides the conotruncus into the aorta and pulmonary trunk.¹⁰ The spiraling flow streams may be disturbed, leading to abnormal conotruncal septation and CTGA. This may also explain why the infundibulum septum, formed during the same process, is often abnormal in CTGA.

The embryology of CCTGA, a rare condition, is also unknown. However, the origin of the reverse position of the ventricles can be traced to cardiac development during day 23 of gestational age. In normal development, the embryologic heart tube elongates and folds to the right side of midline. This leads to the normal positions of the ventricles at birth. If the heart tube folds to the left, the positions of the ventricles are reversed. In normal situs, where the atria are in their proper positions, a leftward folding of the heart tube leads to atrioventricular discordance, namely the left atrium (LA) connecting to the RV and the right atrium (RA) connecting to the LV.

Associated Cardiac Anomalies

In most cases of CTGA, the ventricular septum is intact. The most common associated anomaly is VSD, occurring in 40% to 45% of cases. Perimembranous VSD and muscular VSD are about equally common, followed by atrioventricular canal defect. The presence of a VSD is important for survival because it allows the mixing of oxygenated and deoxygenated blood in the otherwise disconnected pulmonary and systemic circulations. The second most common associated anomaly is left ventricular outflow tract (LVOT) obstruction, occurring in 25% of cases. The obstruction can be valvular at the pulmonary valve or subvalvular. In the latter, the cause of obstruction may be fibrous membrane, ring, shelf, septal muscular hypertrophy, septal aneurysm, or septal attachment of chordae tendineae of the mitral valve. In a mechanism similar to that of hypertrophic cardiomyopathy, LVOT obstruction may cause systolic anterior motion (SAM) of the mitral anterior leaflet, worsening the LVOT obstruction and causing mitral regurgitation. The physiologic effects of LVOT obstruction are decreased pulmonary flow and increased cyanosis. Uncommonly, coarctation coexists with CTGA in 5% of cases.

The relative positions of the great arteries in CTGA are variable. In normal anatomy, the aortic root is located posterior and to the right of the pulmonary root. In 90% of CTGA cases, the aortic root is located anterior and to the right of the pulmonary root. Uncommonly, in 10% of cases, the aortic root is located anterior and to the left of the pulmonary artery. This leads to the S, D, L designation in the Van Praagh segmental labeling scheme described earlier. Rarely, the aortic root can be posterior to the pulmonary root.

Knowledge of the coronary artery anatomy is crucial for the surgical management of CTGA. In CTGA, as in normal cardiac anatomy, the two main coronary arteries most often arise from the two aortic sinuses of Valsalva closest to, or facing, the pulmonary valve. In the most common coronary pattern for CTGA, which occurs in 67% of cases, the right coronary artery (RCA) originates from the right facing sinus and the left main coronary artery (LMCA) originates from the left facing sinus (Fig. 44-7). The LMCA then bifurcates into the left anterior descending (LAD) artery and left circumflex (LCx) artery, as expected. Less commonly, the LCx arises from the RCA in 22% of cases. Other variations include a single RCA (9.5%), a single LMCA (3%), reversed RCA and LMCA (3%), and others (Fig. 44-8).¹¹

FIGURE 44-7 Cardiac-gated CTA of a patient with CTGA. In this short-axis view, the aorta (Ao) and pulmonary trunk (PA) are in D-position. The left main coronary artery (LMCA) and the right coronary artery (RCA) arise from the aortic sinuses facing the pulmonary trunk. This is the most common coronary pattern for CTGA.

FIGURE 44-8 Cardiac-gated CTA of a patient with CTGA. This is an uncommon coronary pattern in which the right coronary artery (RCA) and left anterior descending artery (LAD) arise together from the left-facing sinus, whereas the left circumflex artery (LCx) arises alone from the posterior right-facing sinus. Transplantation of these coronary arteries in an arterial switch operation was thought to be impossible, and the patient underwent atrial switching.

Only a small fraction (1% to 9%) of CCTGA cases are isolated without any associated anomalies.¹² The most common associations are VSD and pulmonary stenosis. VSD occurs in 60% to 80% of cases (Fig. 44-9). It can be of any type and the VSD can be multiple. Pulmonary stenosis occurs in 30% to 50% of cases, and it can be valvular or subvalvular. There is a frequent association with tricuspid abnormalities, which include dysplastic leaflets, Ebstein-like displacement of the leaflets, and a tricuspid valve straddling the ventricular septum. Other less common associated anomalies are atrial septal defect (ASD), patent ductus arteriosus (PDA), pulmonary atresia, mitral valve abnormalities, coarctation, and interrupted aortic arch. CCTGA manifests with dextrocardia in 25% of cases and situs inversus in 5% of cases.

FIGURE 44-9 MRI steady-state free precession image of a patient with CCTGA. This LV inlet-outlet view shows a large ventricular septal defect (arrow) shunting flow into the left ventricle (LV) and then out the pulmonary artery. A pulmonary artery band is in place (arrowheads) to restrict this flow and prevent pulmonary vascular obstructive disease. Note that the right atrium (RA) is connected to the morphologic LV, consistent with atrioventricular discordance.

An important feature of CCTGA is the abnormal development and position of the atrioventricular (AV) node. In normal anatomy, the AV node is located at a relatively posterior position near the tricuspid septal leaflet in an area known as the triangle of Koch. In CCTGA, this posterior AV node may be absent and replaced by an anterior AV node. This anterior node is fragile and can be damaged by fibrosis or surgical trauma, leading to heart block. In some cases, both the posterior and anterior AV nodes are present. Their expected locations must be recognized during surgery to avoid damage.

The aortic root position is typically anterior and to the left of the pulmonary root. Together with normal situs and L-loop ventricle, S, L, L is the most common segmental description for CCTGA. Variations in coronary anatomy are common, but the branching pattern of the coronary arteries generally follows the ventricles. Because the left ventricle is positioned on the right, the LMCA commonly arises from the right-facing sinus and travels to the right before branching into a LAD and a LCx. The latter lies in the right atrioventricular groove. Similarly, the RCA commonly arises from the left-facing sinus and lies in the left atrioventricular groove. Single coronary origin is also common.¹³

MANIFESTATIONS

Clinical Presentation

The early onset of cyanosis in CTGA can be understood from its unique physiology. In utero, the fetal circulation is composed of parallel pulmonary and systemic circulations. Two right-to-left shunts are established at the fossa ovalis of the atrial septum and at the PDA. Both shunts reduce pulmonary flow through the nonfunctioning lungs. The organ in which oxygen exchange occurs is the placenta, connected to the systemic circulation through the umbilical vein and arteries. Transposing the fetal great arteries produces no physiologic effect because they are connected by the PDA. This arrangement allows the fetus to grow normally.

At birth, the placenta no longer serves as the organ of oxygen exchange. During the transition from fetal to adult circulation, the fossa ovalis and PDA close, bringing the ventilated lungs into serial connection with the systemic circulation. CTGA, by conducting deoxygenated blood from the RV to the aorta and oxygenated blood from the LV to the pulmonary artery, disconnects the pulmonary circulation from the systemic circulation. This configuration is not viable.

In practice, some mixing of the oxygenated and deoxygenated blood occurs through shunts between the pulmonary and systemic circulations, providing some oxygen to the systemic organs. The degree of cyanosis depends on (1) the size and direction of these shunts and (2) the degree of obstruction to the pulmonary flow. In CTGA-IVS, the most common anatomic arrangement, shunting is limited and the neonate presents with cyanosis within hours of birth. Cyanosis worsens as the PDA closes in the first few days of life. Prognosis in this scenario is poor, with a 30% survival rate at 1 month. In CTGA-VSD, the VSD is usually large and mixing occurs freely at the ventricular level. Patients may present with mild cyanosis, even after the PDA closes. As the pulmonary resistance drops in the first week of life, a large shunting from the systemic RV to pulmonary LV may occur. This can lead to congestive heart failure and later pulmonary vascular obstructive disease. The latter is the cause of pulmonary hypertension and Eisenmenger syndrome. The survival rate at 1 year is 50%, which is better than that for CTGA-IVS. In CTGA-VSD-LVOTO, the LVOT obstruction limits flow to the pulmonary circulation. The physiology in this situation is similar to that of tetralogy of Fallot. Severe obstruction would lead to diminished pulmonary flow and worsening cyanosis. Little obstruction would lead to large shunting and the same outcome as CTGA-VSD. If the obstruction is balanced just right, the pulmonary circulation may be protected from harmful shunt flow at a level of tolerable cyanosis. The survival rate for CTGA-VSD-LVOTO is 50% at 3 years, with some surviving into adulthood. Left untreated, the overall survival rates for CTGA are 70% at 1 week, 50% at 1 month, and 10% at 1 year.¹⁴

The physiology and natural history for CCTGA are very different from those of CTGA. The addition of atrioventricular discordance reestablishes the physiologic circulations in which the oxygenated pulmonary venous blood flows into the aorta and the deoxygenated systemic venous blood flows to the pulmonary artery. Although the circulatory physiology is correct, the anatomy is abnormal because the RV is the systemic ventricle and the LV is the pulmonary ventricle; as a result, the pressure loads to the ventricles are reversed. Complications from CCTGA can be divided into those manifesting early and those manifesting late. Early complications are usually caused by the associated anomalies, such as VSD, pulmonary stenosis, and dysfunctional tricuspid valve. In patients with a large VSD, shunting from the systemic RV to the pulmonary LV can be large. The increased volume load can lead to congestive heart failure. Coexisting tricuspid valve abnormalities that cause significant tricuspid regurgitation will increase RV volume load and exacerbate the heart failure. Pulmonary stenosis can be beneficial by decreasing the pulmonary flow and reducing the RV to LV shunting. It also increases the left ventricular pressure and promotes a rightward shift of the ventricular septum. This configuration has the effect of reducing the tricuspid valve size and tricuspid regurgitation. Given these observations, pulmonary artery banding, a form of artificially induced pulmonary stenosis, is a valid palliative treatment for CCTGA. In the rare patients who do not have any associated anomaly, they may be asymptomatic throughout young adulthood.

Late complications of CCTGA are caused by RV failure and conduction abnormalities. The RV is not structurally homologous to the LV, and the RV has not been evolved to carry lifelong systemic pressure load. In men, RV failure commonly occurs during the fifth and sixth decades of life. In women, it may manifest during the first pregnancy. The pathophysiology of the RV failure is not well understood. Exercise studies have shown that the cardiac reserve of the system RV is less than the normal systemic LV. The right coronary system may not provide adequate perfusion to support the hypertrophic RV, leaving the systemic RV vulnerable to ischemia. RV with exuberant hypertrophy is particularly at risk because the perfusion per unit weight of tissue is decreased. The closed tricuspid valve, which normally operates against a 20 mm Hg systolic pulmonary pressure, must now hold against a 100 mm Hg systolic systemic pressure. This, together with the dysplastic and dysmorphic tricuspid leaflets that often accompany CCTGA, produces substantial regurgitation and volume load to the RV, causing further stress to the RV (Fig. 44-10). Patients without associated cardiac anomalies generally fare better, but both groups show substantial decline in RV function between ages 40 and 50 years. The probabilities of developing RV failure in patients with and without associated anomalies are 55% and 25%, respectively, at age 40, and 90% and 65% at age 50 years.¹⁵

FIGURE 44-10 MRI steady-state free precession image of a patient with CCTGA. This four-chamber view shows an enlarged systemic right ventricle (RV). The septum bulges into the left ventricle (LV), and there are multiple tricuspid regurgitant jets (arrow).

The AV node is often abnormally developed in CCTGA. It is prone to fibrosis, even without surgical injury. There is a continuing risk of developing complete AV block at a rate of 2%/year. Overall, complete AV block is present in 12% to 33% of patients with CCTGA. Wolff-Parkinson-White syndrome from an accessory atrioventricular conduction pathway known as the bundle of Kent is present in 2% to 4% of patients. Other rhythm disorders include supraventricular tachycardia, atrial flutter, atrial fibrillation, and ventricular arrhythmia. Fortunately, sudden death from arrhythmia in CCTGA is rare.

DIFFERENTIAL DIAGNOSIS

From Clinical Presentation

Broadly speaking, CTGA may present with significant cyanosis, especially in CTGA-IVS, or congestive heart failure, as in CTGA-VSD. Chest radiography is helpful for separating those entities with diminished pulmonary vascularity, such as tetralogy of Fallot and Ebstein anomalies, from those with increased pulmonary vascularity, typically admixture lesions such as CTGA-VSD, truncus arteriosus, total anomalous pulmonary venous return without obstruction, tricuspid atresia, and single ventricle. Additional clinical features, physical findings, and hyperoxia and prostaglandin challenge testing can usually narrow down the diagnosis. An interesting sign of CTGA is the reverse differential cyanosis, manifested by higher oxygen saturation in the lower extremities than in the upper extremities. This is caused by oxygenated blood from the pulmonary artery shunting through the PDA to the descending artery and the lower extremities, while the deoxygenated blood from the aortic root supplies the cervical arteries. In patients with congestive heart failure, differential diagnoses include large left-to-right shunt such as truncus arteriosus, intrinsic systemic ventricular failure such as anomalous left coronary artery from the pulmonary artery (ALCAPA), and obstructive lesions such as critical aortic stenosis, interrupted aortic arch, and severe coarctation. Neonates born with profound cyanosis and metabolic acidosis at delivery may have CTGA-IVS with intact or restrictive atrial septum. The other consideration is hypoplastic left heart syndrome with intact atrial septum. Either case would require emergent balloon atrial septotomy to sustain life. Neonatal presentation of CCTGA is usually less dramatic. Ultimately, the precise diagnosis is established by imaging, usually with echocardiography.

From Imaging Findings

Both CTGA and CCTGA share the configuration of parallel ascending aorta and pulmonary trunk (Fig. 44-11). Distinguishing between the two types of transpositions requires additional imaging evidence. In CTGA, the ventricles are usually in their normal positions (see Fig. 44-3) and the aortic root tends to be on the right of the pulmonary root see (Fig. 44-5). In CCTGA, the ventricular positions are reversed (see Fig. 44-4) and the aortic root tends to be on the left of the pulmonary root (see Fig. 44-6). However, as discussed earlier, these signs are not completely reliable. It is better to trace the connections between the venous returns and great arteries. In CTGA, the pulmonary veins are ultimately connected to the pulmonary artery through the left ventricle and the venae cavae are connected to the aorta through the right ventricle. In CCTGA, blood flow from the pulmonary veins reaches the aorta through the right ventricle and blood flow from the venae cavae reaches the pulmonary trunk through the left ventricle.

FIGURE 44-11 Cardiac-gated CTA of a newborn with CTGA. In both CTGA and CCTGA, the right ventricle (RV) is connected to the aorta (Ao) and the left ventricle (LV) is connected to the pulmonary artery (PA). The two great arteries lie in parallel. In this newborn patient, a patent ductus arteriosus (PDA) connects the two great arteries.

Differentiation between CTGA and double-outlet right ventricle (DORV) can be challenging, especially when the DORV has a doubly committed large VSD. In DORV, more than 50% of both the aorta and pulmonary trunk should be aligned with the right ventricle. In contrast, CTGA has the aorta aligned mostly with the right ventricle and the pulmonary trunk aligned mostly with the left ventricle. When both outlet valves are separated from the inlet valves by intervening conal muscle, or bilateral conus, this is considered a sign of DORV (Fig. 44-12). In CTGA, the mitral valve remains in fibrous continuity with the pulmonary valve, without intervening conal muscle.

FIGURE 44-12 Cardiac-gated CTA of a patient diagnosed with double outlet right ventricle (DORV) with transposed great arteries. A, This RV inlet-outlet view shows discordant connection between the right ventricle (RV) and the aorta (Ao). Like CTGA, there is subaortic conal muscle (arrowheads). B, This LV inlet-outlet view shows discordant connection between the left ventricle (LV) and pulmonary artery (PA). Unlike CTGA, however, there is subpulmonic conal muscle (arrowheads). The presence of conal muscle under both great arteries classifies this as DORV.

SYNOPSIS OF TREATMENT OPTIONS

Medical

Although palliative in nature, early medical therapy is crucial for stabilizing the cyanotic crisis in neonates with CTGA before definite surgery. The most important is the infusion of prostaglandin E₁ to keep the PDA from closing. The PDA allows oxygenated blood to enter the aorta from the pulmonary artery. Metabolic acidosis and oxygen desaturation may require intubation and mechanical ventilation. In patients with CCTGA, fixed lesions such as a large VSD, pulmonary stenosis, and tricuspid valve dysfunction will ultimately require surgical correction. Medical therapy is useful for the management of systemic RV failure. An angiotensin-converting enzyme (ACE) inhibitor may be given to reduce afterload, diuretics may be given to reduce preload, and digoxin may be given for its inotropic effect.

Surgical and Interventional

Surgery for Complete Transposition of the Great Arteries

In CTGA patients presenting with severe cyanosis, keeping the ductus arteriosus open may not be sufficient. Creating or enlarging the ASD to promote shunting at the atrial level is a valid palliative treatment. This is now performed at the bedside with an endovascular balloon atrial septostomy under echocardiography guidance. In this procedure, first described by Rashkind and Miller in 1966,¹⁶ a balloon catheter with deflated balloon is passed from the right atrium, through the foramen ovale, to the left atrium. The balloon is inflated and forcibly retracted back to the right atrium, tearing the atrial septum to allow increased interatrial mixing. The goal is to increase systemic oxygen saturation to the 50% to 80% range. Although generally safe and widely used, balloon septostomy has been associated with an increased risk of stroke.¹⁷

The preferred surgical correction of CTGA today is the arterial switch procedure, or the Jatene procedure, named after its creator, Adib Jatene.³ Although conceptually straightforward, the arterial switch operation is technically difficult because of the need to transpose the tiny coronary arteries with the aorta. For this reason, the atrial switch procedure was developed first. Arterial switching became possible in the 1970s when a surgical technique was developed to reconnect the coronary arteries to the transposed aorta via coronary buttons. In this procedure, the ascending aorta and pulmonary trunk are transected. The coronary ostia, along with a large button of surrounding aortic wall, are excised from the aortic sinus of Valsalva. The proximal segments of the coronary arteries are freed from the surface of the heart to prevent tension or distortion after reimplantation. In a surgical manipulation called the Lecompte maneuver, the transected upper aorta is moved behind the pulmonary trunk so that the left and right main pulmonary arteries are now straddling the aorta (Fig. 44-13). The aorta is anastomosed to the proximal pulmonary trunk, creating a neoaorta. Openings are created at the appropriate positions on the neoaorta and the coronary buttons are sutured to these openings (Fig. 44-14). Finally, the transected pulmonary trunk, now located anteriorly to the aorta, is anastomosed to the proximal aorta. Any defects created by the coronary buttons are closed. If ASD and VSD are present, they are closed during the surgery.

FIGURE 44-13 Cardiac-gated CTA of the great arteries after the Lecompte maneuver. In this volume rendered image, the neopulmonary trunk (nPA) is moved in front of the neoaorta (nAo). The left main pulmonary artery (LPA) and the right main pulmonary artery (RPA) straddle the neoaorta.

FIGURE 44-14 Cardiac-gated CTA of the transplanted coronary arteries. In this volume rendered image, the two main coronary arteries (arrows) have been transplanted from the neopulmonary artery (nPA) to the neoaorta (nAo).

The arterial switch procedure is usually performed during the first 2 to 4 weeks of life, during which the pulmonary LV remains hypertrophic enough to handle systemic pressure load after the switch. After the first month of life, pulmonary artery banding may be necessary to maintain the LV muscle mass. Contraindications for the arterial switch procedure include unfavorable coronary anatomy for reimplantation, aortic stenosis, pulmonary stenosis, and unfavorable VSD and ventricular sizes for two-ventricle repair. With increasing experience, contraindications to the arterial switch procedure have been relaxed. Some patients with coronary anatomy once thought not to be suitable for the arterial switch procedure who have undergone the atrial switch procedure are now referred for conversion to arterial switching.

The surgical outcome of the arterial switch procedure is favorable. Depending on the series, the surgical mortality is reported to be 2% to 5%.¹⁸ Sudden death is uncommon. Late mortality related to coronary occlusion, myocardial infarction, and left ventricular failure occurs in fewer than 1%. Survival rate is 88% at 10 years and unchanged at 15 years.¹⁹ Because the coronary arteries are surgically manipulated, the long-term patency of the coronary arteries is of particular concern. In a series of 165 patients who underwent arterial switching, 7% had coronary occlusion, 5% developed more than 50% coronary stenosis, and 4% developed less than 50% coronary stenosis or coronary stretching.²⁰ The rate for surgical intervention for coronary obstruction was 2.4% in this series. Other complications include pulmonary stenosis in 5% to 25% of patients. The obstruction most often occurs at the left main pulmonary artery as it crosses the aorta (Fig. 44-15), followed by the right main pulmonary artery and the pulmonary valve compressed between the aorta and the sternum. Aortic regurgitation is detected in 16% of patients at 5-year follow-up, but only 4% had grade 2 or above, which is considered clinically significant.

FIGURE 44-15 Contrast-enhanced MRA of a patient who had an arterial switch procedure. The right pulmonary artery (RPA) is widely patent, whereas the left pulmonary artery (LPA) is stretched and compressed by the ascending neoaorta (arrow).

The atrial switch procedure, having a less favorable outcome than the arterial switch procedure, is now reserved for special cases in whom the latter is contraindicated. It may also be seen in patients whose surgery predated the arterial switch procedure. The atrial switch operations developed by Senning and Mustard differ only on the methods of constructing the atrial baffle. Senning used native atrial septal tissue for the baffle construction ¹ whereas Mustard used pericardium or Dacron material.² The postoperative anatomy and physiology of the two operations are the same. In this procedure, the normal atrial septum is resected. An atrial baffle is sutured to the atrial wall to create two compartments of very complex geometry. The posterior compartment collects flow from the pulmonary veins and channels it to the right ventricle (Fig. 44-16). The anterior compartment collects flow from the two cavae and channels it the left ventricle (Fig. 44-17). In effect, the atrial baffle creates an AV discordance, thereby converting the CTGA to a CCTGA.

FIGURE 44-16 MRI steady-state free precession image of a patient who had an atrial switch procedure. In this four-chamber view, a baffle (arrowheads) collects the pulmonary venous flow (arrows) in a posterior compartment and channels it into the right ventricle (RV). PV, pulmonary vein.

FIGURE 44-17 MRI steady-state free precession image of a patient who had an atrial switch procedure. In this oblique coronal view, the baffle (arrowheads) collects the caval flow (arrows) in an anterior compartment and channels it into the left ventricle (LV).

Because the atrial baffle does not grow with the patient, the atrial switch procedure is performed later than the arterial switch procedure, typically at 1 month to 1 year of age. The surgical mortality rate has been reported to be 1% to 10%, higher in patients with VSD.²¹ Late mortality of 7% to 15% sudden death is significantly higher than that of the arterial switch procedure. Survival rate is 90% at 10 years and 76% to 80% at 20 years.²² RV failure, as in CCTGA, is a late complication. Abnormal stretching of the atrial baffle may result in flow obstruction from channel narrowing or baffle leak from suture dehiscence. Although baffle leak is more common than flow obstruction, the former rarely requires reoperation. Flow obstruction typically occurs at the superior vena cava (SVC) channel (5% to 10%) and less commonly at the inferior vena cava (IVC) channel (1% to 2%). Pulmonary hypertension is associated with 7% of patients who had late repair of CTGA-VSD. Arrhythmia in postatrial switch patients is particularly troublesome. At 10 years, 50% of the patients develop bradyarrhythmia from sinus node dysfunction, requiring a pacemaker. Tachyarrhythmia may result from incisional atrial reentry tachycardia. Nearly 50% of patients experience one or more episodes of supraventricular tachycardia and 73% experience atrial flutter.

In CTGA patients with pulmonary stenosis, the Rastelli procedure is the indicated operation. In this procedure, the native pulmonary trunk is transected and the proximal trunk is surgically closed. An intracardiac baffle is placed in the RV to connect the VSD to the RVOT, effectively channeling the LV outflow to the transposed aorta (Fig. 44-18). An extracardiac valved pulmonary conduit is anastomosed to the RV after a ventriculostomy, and the conduit is connected to the distal pulmonary trunk, establishing the pulmonary flow (Fig. 44-19). After surgery, the LV becomes the systemic ventricle. A prerequisite for the Rastelli procedure is that the VSD must be large enough to conduct the left ventricular flow without obstruction. Surgical enlargement of the VSD risks damaging the conduction system. Surgical mortality has been reported at less than 5%.²³ Survival rates are 82% at 5 years, 80% at 10 years, 68% at 15 years, and 52% at 20 years. Causes of late mortality include sudden death secondary to arrhythmia and LV failure. Reoperation rate for the Rastelli procedure is high mainly because the pulmonary conduit develops pulmonary stenosis or regurgitation, requiring replacement every 10 to 20 years.

FIGURE 44-18 Contrast-enhanced MRA of a patient who had the Rastelli procedure. In this oblique coronal view, an intracardiac baffle (arrowheads) directs blood from the left ventricle (LV) to the aorta (Ao). The pulmonary trunk has been divided (arrow) from the branch pulmonary artery (LPA).

FIGURE 44-19 Contrast-enhanced MRA of a patient who had the Rastelli procedure. In this oblique sagittal view, an extracardiac pulmonary conduit (C) connects the right ventricle (RV) to the native pulmonary artery (PA).

The Damus-Kaye-Stansel procedure is infrequently used in patients with TGA or Taussig-Bing anomalies. Indications are difficult coronary anatomy, subaortic stenosis, and absence of pulmonary stenosis.²⁴ In patients who had atrial switching, pulmonary artery banding may improve the systemic RV function by increasing LV pressure and shifting the ventricular septum to the right. In patients whose anatomy is amendable, a reverse switch procedure may be performed to convert the atrial switching to arterial switching. However, the pulmonary LV must first be conditioned, or trained, to handle the systemic pressure load. This is accomplished by raising the pulmonary resistance with a pulmonary artery band (Fig. 44-20) and forcing the LV to pump at a higher pressure. The band is gradually tightened until the LV pumps at near systemic pressure.

FIGURE 44-20 Cardiac-gated CTA of a patient who had pulmonary artery banding in preparation for a reverse switch procedure. In this oblique sagittal view, an atrial baffle (arrowheads) was placed previously in the atrium (LA) from an atrial switch procedure. A pulmonary artery band (arrows) creates a resistance to flow that the left ventricle (LV) must overcome with greater pressure.

Surgery for Congenitally Corrected Transposition of the Great Arteries

In infants with CCTGA, palliative procedures are geared toward finding the right balance for the pulmonary flow. In patients with a large VSD, pulmonary artery banding is indicated to restrict the pulmonary flow and reduce the risk of developing pulmonary obstructive vascular disease. In patients with severe pulmonary stenosis and VSD, a Blalock-Taussig shunt is indicated to improve pulmonary flow and reduce cyanosis. Beyond palliation, operations are commonly performed to correct the anomalies other than CCTGA. These include VSD closure to remove shunting, resection to relieve pulmonary stenosis, and repair or annuloplasty of the tricuspid valve to reduce tricuspid regurgitation. The goals of these procedures are to reduce ventricular workloads and preserve ventricular functions. Compared with repairs of the same lesions in normal ventricles, the postoperative prognosis is worse for CCTGA. This may be related to conduction abnormality, worsening tricuspid regurgitation, and coronary anomalies inherent in CCTGA. Thirty-eight percent of postoperative patients develop complete AV block. Most patients undergo reoperation for atrioventricular valve regurgitations and pulmonary stenosis. The overall surgical mortality is 6%; the late survival rate is 55% to 85% at 10 years and 48% at 20 years.²⁵ Causes of death include reoperation, sudden death from arrhythmia, and progressive systemic RV failure.

In light of the long-term risk of RV failure, Ilbawi and colleagues ⁵ and others²⁶ have advocated the double switch procedure, which combines atrial and the arterial switching, as the definitive treatment for CCTGA. The purpose of this operation is to return the LV as the systemic ventricle. However, just as in the reverse switch procedure, the LV must be trained first to handle the systemic pressure load by pulmonary artery banding. The time required for LV training depends on the age of the patient, from a few weeks for infants to 1 year for older children. There is evidence that the LV responds poorly to training in patients older than 16 years.²⁷ The reported surgical mortality is 0% to 11%. Patients are also at risk for the complications associated with the atrial switch procedure. Although the immediate postoperative survival rate of the double switch procedure is good, it has yet to demonstrate long-term benefits over conventional operations that leave the RV as the systemic ventricle.²⁸

IMAGING INDICATIONS AND REPORTING

Imaging for Complete Transposition of the Great Arteries

The manifestations of CTGA on chest radiographs are variable, from normal to shunt vascularity and increased heart size, depending on the size of the VSD. The classic cardiomediastinal silhouette of “egg on a string” is seen only in one third of cases (Fig. 44-21). Echocardiography is the definitive imaging modality. In the parasternal long-axis view, the LV is connected to the pulmonary trunk. In the short-axis view, the aorta is anterior and to the right of the pulmonary trunk. In the suprasternal view, the ascending aorta lies parallel to the pulmonary trunk. Echocardiography should assess for secondary lesions, such as ASD, VSD, and PDA, and the coronary anatomy, if possible. Doppler technique is used to detect any hemodynamically significant subvalvular and valvular stenoses of the outlet valves. Catheter angiography, CT, and MRI are generally not necessary for the initial diagnosis and management, except for the evaluation of the coronary anatomy when echocardiography is unclear. In rare cases, when the ventriculoarterial alignments and their relationship to the VSD are complex, three-dimensional imaging with cardiac-gated CT or MRI can be helpful for surgical planning.

FIGURE 44-21 Chest radiograph of a patient with CTGA corrected by atrial switching. The “egg on a string” appearance of CTGA refers to the enlarged cardiac silhouette, corresponding to the egg, and the narrowed mediastinal silhouette, corresponding to the string. The aortic arch and pulmonary trunk are absent from their normal positions (arrowheads), thereby leaving a thin mediastinum.

In the early days, after the development of the arterial switch procedure, catheter coronary angiography was routinely performed in postoperative patients to assess coronary patency. With experience and improved surgical outcome, coronary angiography is now reserved for patients presenting with signs or symptoms of cardiac ischemia. Because the obstructions likely occur in the proximal coronary arteries where they have been surgically manipulated, the stenoses can be evaluated noninvasively with MRI or CT coronary angiography (Fig. 44-22). For patients with detected coronary stenoses, a stress test such as a stress myocardial perfusion study or stress echocardiography should be performed to determine the clinical significance of the lesions. On occasion, aortic regurgitation or pulmonary stenosis develops after the arterial switch operation. These complications, along with their ventricular responses, are monitored with echocardiography, supplemented by MRI quantifications of ventricular volumes and valvular regurgitant fractions if necessary.

FIGURE 44-22 Cardiac-gated CTA of the coronary arteries after an arterial switch procedure. The left main coronary artery (LMCA) is widely patent but the right coronary artery (RCA) is atretic and occluded at the origin (arrow).

After the atrial switch procedure, routine echocardiography is needed to assess the integrity of the atrial baffle and ventricular functions. Atrial baffle leak typically occurs at the suture site of the atrial baffle to the atrial wall and is readily detected by color Doppler duplex imaging. The baffle leak is usually small and does not require intervention. Of greater concern is any obstruction of the flow channels created by the atrial baffle. Doppler imaging may detect accelerated flow at the obstruction, which would require further evaluation by catheter measurement of pressure gradient. If hemodynamically significant, the obstruction can be relieved by stenting.

Assessment of ventricular functions is important because the systemic RV is prone to early failure. In echocardiography, special attention should be directed to the septal motion. Exaggerated leftward shift of the septum during systole reduces stroke volume of the RV and increases tricuspid annular size. The latter promotes tricuspid regurgitation and increases volume load to the RV. Furthermore, septal shift can narrow the LVOT, causing subpulmonic obstruction, SAM of the anterior leaflet of the mitral valve, and mitral regurgitation. These findings and their severity should be documented. In cases in which echocardiography is inadequate or precise quantifications of ventricular volumes and ejection fractions are desired, cardiac MRI is indicated.

After pulmonary artery banding for a planned reverse switch procedure, Doppler imaging is used to estimate the pressure gradient at the pulmonary band and the left ventricular systolic pressure from a mitral regurgitant jet. The latter helps monitor whether the trained left ventricle is ready to take on systemic pressure load. This pressure load should be confirmed by catheter measurement before surgery. In some centers, cardiac MRI is used to monitor the increase in the left ventricular mass during the LV training (Fig. 44-23). This is calculated as the epicardial volume of the LV minus the endocardial volume of the LV, multiplied by myocardial tissue density, 1.05 g/mL.²⁹ The goal of training is to attain an LV mass comparable to that of a normal systemic LV, estimated at 60 g/m² body surface area or higher.³⁰

FIGURE 44-23 Cardiac-gated CTA of a patient who underwent pulmonary artery banding for left ventricle (LV) training. A, Before pulmonary artery banding, the wall of the pulmonary LV is thin in this short-axis view, especially at the septum. B, Two years after pulmonary artery banding, the wall of the pulmonary LV is significantly thickened.

The imaging management of patients who have undergone the Rastelli procedure is similar to that of those who had complete surgical repair for tetralogy of Fallot. Complications center on the pulmonary artery conduit, stenosis causing excessive pressure load to the RV (Fig. 44-24) and regurgitation causing excessive volume load to the RV. Echocardiography and MRI are indicated to monitor RV volume, RV ejection fraction, and pulmonary regurgitation severity. MRI has the advantage of providing accurate quantitative measurements of these parameters for serial comparison. As in the case of repaired tetralogy of Fallot, the goal is to postpone conduit replacement until these parameters suggest potentially irreversible deterioration of the RV function. The critical values of these parameters are under ongoing investigation. In addition, the intracardiac baffle created by the Rastelli procedure may leak or it may cause obstruction between the VSD and transposed aortic valve. Rarely, clots can form in the blind-ended pulmonary trunk. Dislodgment of these clots into the baffled channel causes systemic thromboembolism. These complications are typically evaluated with echocardiography.

FIGURE 44-24 Gated CTA of a patient after the Rastelli operation. This volume rendering shows a pulmonary conduit (C) stenosis (arrow) at its anastomosis with the native pulmonary trunk (PA).

Imaging for Congenitally Corrected Transposition of the Great Arteries

As in the case of CTGA, chest radiographic findings for CCTGA are variable and rarely diagnostic. Shunting through a VSD may produce shunt vascularity on the radiograph, and severe tricuspid regurgitation may cause significant RA enlargement. Dextrocardia is seen in 25% of cases. During early childhood, echocardiography is the definitive diagnostic test. Later in life, MRI may yield better visualization of the abnormal cardiac connections. Regardless of the imaging modality, the diagnosis of CCTGA depends on careful analysis of each cardiac segment and its connections.

First, the atrial situs is determined. The right atrium should have a broad-based, pyramidal atrial appendage and the left atrium should have a tubular, crenellated atrial appendage. The right main pulmonary artery should be anterior to the right mainstem bronchus and the left main pulmonary artery should course above the left mainstem bronchus. The right mainstem bronchus should be more vertically oriented than the left mainstem bronchus. The right lung should have three lobes and the left lung should have two lobes. If these features are left-right reversed, then the atrial situs is inversed and the positions of the morphologic left and right atria are reversed.

Second, the ventricular morphology is evaluated. A right ventricle typically has course trabeculations (see Fig. 44-3), a prominent moderator band, and separation of the inlet-outlet valves (see Fig. 44-1). In contrast, a left ventricle has fine trabeculations except at the apex, absence of a moderator band, and fibrous continuity of the inlet-outlet valves without muscle in between (see Fig. 44-2). The diagnosis of CCTGA requires that the right atrium is aligned with the left ventricle and the left atrium aligned with the right ventricle, or atrioventricular discordance. In addition, it requires that the right ventricle be aligned with the aorta and the left ventricle is aligned with the pulmonary trunk, or ventriculoarterial discordance. Differentiation of the aorta and pulmonary trunk is usually straightforward by echocardiography and MRI.

Echocardiography should assess the presence and severity of lesions associated with CCTGA—VSD, pulmonary stenosis, and malformed tricuspid valve. Catheter measurements of the abnormal hemodynamics may be necessary. The hemodynamic significances of these lesions dictate the proper surgical interventions. If a double switch procedure is planned, imaging assessment of LV training is similar to the reverse switch procedure (see earlier). Later in life, when the LV is no longer responsive to training and double switch is not an option, echocardiography is used to monitor for signs and severity of RV failure. When medical management is maximized, cardiac transplantation is the final therapeutic option.