●   COMPLETE TRANSPOSITION OF THE GREAT ARTERIES

Definition, Spectrum of Disease, and Incidence

Complete transposition of the great arteries (TGA) is a common cardiac malformation with atrioventricular concordance and ventriculoarterial discordance. This implies a normal connection between the atria and ventricles; the right atrium is connected to the right ventricle through the tricuspid valve and the left atrium is connected to the left ventricle through the mitral valve, but there is a switched connection of the great vessels, the pulmonary artery arising from the left ventricle, and the aorta arising from the right ventricle. Both great arteries display a parallel course, with the aorta anterior and to the right of the pulmonary artery (Fig. 28.1), hence the term D-TGA (D = “dexter”). D-TGA is a relatively frequent cardiac anomaly occurring in 5% to 7% of all congenital cardiac malformations, with an incidence of 0.315 cases per 1,000 live births and a 2:1 male preponderance (1). D-TGA can be an isolated cardiac malformation, termed simple D-TGA, or complex, when associated with other cardiac anomalies. Figure 28.2 shows an anatomic fetal specimen of a heart with a simple D-TGA. Ventricular septal defects (VSDs) and pulmonary stenosis (left ventricular tract outflow obstruction) are common associations with D-TGA and may be present either alone or in combination in up to 30% to 40% of cases (2). Associated extracardiac malformations are rare.

Prenatal diagnosis of D-TGA is still a challenge. Prenatal screening policy for congenital heart disease, which primarily focuses on the four-chamber view alone, will undoubtedly fail in detecting TGA. Prenatal screening for congenital heart disease at a population level reports detection rates of 3% to 17% for simple TGA (3–5). The detection rate of TGA increased from 12.5% to 72.5% during a period of rapid progress and increased access to prenatal diagnosis in a population-based study (6). Given an associated decrease in neonatal morbidity and mortality in prenatally detected TGA (6–8), the assessment of the great vessels as part of the routine cardiac examination of the fetus is thus recommended, and, indeed, the inclusion of the outflow tracts was recently added to the national and international fetal cardiac screening guidelines (9, 10) (see also Chapter 2). Evidence also suggests that inclusion of the outflow tracts in screening guidelines has improved detection of TGA (8).

Ultrasound Findings

Gray Scale

The four-chamber view is typically normal in fetuses with D-TGA (Fig. 28.3A), except for an associated VSD. Visualization of the five-chamber view will show the pulmonary artery arising from the left ventricle and bifurcating, shortly after its origin, into two branches: right and left pulmonary arteries (Fig. 28.4A). The authors believe that the presence of bifurcation of the great vessel arising from the left ventricle is an important ultrasound clue to the presence of TGA. This clue may play a role in increasing prenatal detection of TGA given that the five-chamber view is now part of fetal cardiac screening (9, 10). In TGA, the aorta is noted to arise from the right ventricle in an anterior and parallel course to the pulmonary artery (Figs. 28.3 and 28.4). This parallel orientation of the great arteries in D-TGA is best obtained in an oblique plane of the heart, spatially oriented from the right shoulder to the left hip of the fetus (Figs. 28.3 and 28.5). A transverse view in the upper thorax will show in most cases, instead of the three-vessel-trachea view, a single large vessel (the transverse aortic arch) with a superior vena cava to its right (Figs. 28.4B and 28.6 to 28.8). The large vessel noted in the three-vessel-trachea view is the aorta, which is positioned anteriorly and superiorly to the pulmonary artery. It has also been described that the aorta assumes a right convex shape, also termed the I-sign, in cases of TGA at the three-vessel-trachea view (11, 12). The short-axis view at the level of the great vessels shows both the aorta and pulmonary artery as circular structures adjacent to each other instead of their normal orientation (longitudinal pulmonary artery wrapping around a circular aorta) (compare Fig. 28.6A with Fig. 8.8). On longitudinal views, the aortic arch is seen arising from the anterior right ventricle, giving rise to head and neck vessels and assuming a “hockey stick” orientation as it curves posteriorly. The pulmonary artery assumes the “candy cane” orientation in longitudinal views.

Color Doppler

Color Doppler can be helpful in diagnosing D-TGA but is not necessary (13). Color Doppler helps in demonstrating the parallel course of the great vessels (Fig. 28.9). Visualization of an associated VSD (Figs. 28.9B and 28.10), confirming patency of the foramen ovale and assessing left ventricular outflow (pulmonary artery), can be enhanced by color Doppler. In early gestation, color Doppler is essential in demonstrating the crossing of the great vessels in normal conditions or their parallel course in D-TGA.

Early Gestation

D-TGA can be diagnosed at the 11- to 14-week ultrasound (Fig. 28.11), but its recognition is more difficult than in the second trimester, as reported in one series of targeted fetal echocardiography at 11 to 14 weeks where D-TGA was missed in most cases (14). An enlarged nuchal translucency in the setting of normal fetal chromosomes can be a marker for the presence of D-TGA (15). Recently, we reported on the strong association of abnormal cardiac axis and conotruncal anomalies, including TGA, in early gestation (16). The three-vessel-trachea view, with its finding of a single great vessel, may be helpful in early gestation as well (Fig. 28.11A). By rotating the transducer to view an oblique plane of the chest, the parallel course of the vessels can be demonstrated (Fig. 28.11B).

Three-Dimensional Ultrasound

Various reports, focusing on rotations along different axes and color display of rendered volumes, emphasize the role of three-dimensional (3D) ultrasound in the diagnosis of D-TGA (17–20). Tomographic ultrasound imaging (Fig. 28.12), glass-body mode (Fig. 28.13), inversion mode (Fig. 28.14A), B-flow (Fig. 28.14B), and other 3D rendered displays have the ability to enhance visualization of the spatial relationship of the great vessels as they arise from their respective cardiac chambers. The use of the reconstructed en face view with color Doppler imaging of the four cardiac valves in 3D volumes of fetuses with TGA can demonstrate the different types of spatial relationships of the arterial trunks (Fig. 28.15), which may predict the likelihood of abnormal coronary arterial distribution (21). Evaluation of 3D automated software on volumes of fetuses with TGA demonstrated the abnormality in ventriculoarterial connections in all fetuses (22).

Associated Cardiac and Extracardiac Findings

VSDs and pulmonary stenosis (left ventricular outflow obstruction) are the two most common associated cardiac findings in D-TGA. VSDs are common and occur in about 40% of cases and are typically perimembranous but can be located anywhere in the septum (1). Pulmonary stenosis coexists with a VSD in D-TGA patients in about 30% of cases, and the stenosis is usually more severe and complex than in D-TGA with intact ventricular septum (1). Abnormal course and bifurcation of coronary arteries are found in patients with D-TGA, and its prevalence is more than 50% when the great vessels are side by side or when the aorta is posterior and to the right of the pulmonary artery (23, 24). Other associated cardiac anomalies are rare and can involve the atrioventricular valves, the aortic arch, and great vessels.

Extracardiac anomalies may be present but rare, and numerical chromosomal aberrations are practically absent in D-TGA. Microdeletion of 22q11 could be present and should be ruled out, especially when extracardiac malformations or a complex D-TGA is present. Situs abnormalities can be present, such as abdominal situs inversus, and depending on the venoatrial connection, a balanced circulation may be found. Among the few associated extracardiac anomalies in D-TGA, the authors report one ear anomaly, one cleft lip, and one wide cleft lip and palate.

Differential Diagnosis

Double outlet right ventricle and corrected TGA are the two most common cardiac anomalies in the differential diagnosis of D-TGA as they all share the absence of “crossover” of the great vessels (25). Differentiating D-TGA from congenitally corrected TGA (cc-TGA) is explained in the next section.

False-positive diagnosis of TGA may occur at the level of the three-vessel-trachea view if the novice examiner assumes a parallel course of the vessels as they are imaged side by side at this level. The display of a parallel course of the great vessels in the diagnosis of TGA should include the ventricles, the semilunar valves, and the ventricular septum separating the great vessels.

Prognosis and Outcome

D-TGA is well tolerated in utero. Prenatal follow-up examination with color Doppler should focus on the presence or absence of a VSD and on the subsequent development of pulmonary stenosis, which is not commonly detected in the second trimester. Furthermore, flow assessment with color and spectral Doppler, at the level of the foramen ovale and ductus arteriosus, should be given close attention closer to term. Premature closure or narrowing of the foramen ovale and/or the ductus arteriosus is associated with worsening neonatal outcome and may require emergency postnatal procedures (26, 27).

Prognosis of a neonate with D-TGA is excellent provided the child is born at a tertiary institution with pediatric cardiology intensive services (3, 7). Prenatal detection of D-TGA and/or neonatal treatment before cyanosis appears to improve outcome (3, 7). In a study of 144 cases of TGA with a one-year follow-up, first-year mortality and presurgical mortality were significantly lower in cases with prenatal diagnosis (8). Closure of the duct before treatment, renal dysfunction and hypoxia occurred significantly more often in the group without prenatal diagnosis (8).

Prostaglandin infusion to maintain patency of the ductus arteriosus and a balloon atrial septostomy is usually required in order to enhance oxygenation and preparation for corrective surgery. An emergency balloon septostomy may be required within hours after birth in some cases of obstructed foramen flow, hence the importance of delivery at a center equipped to perform these procedures (7, 26, 27). Corrective surgery currently involves an arterial switch operation, where the aorta and pulmonary artery are transacted above the semilunar valves and switched with reimplantation of the coronary circulation.

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