Transposition of the Great Arteries

Transposition of the great arteries (TGA) is a conotruncal malformation in which the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle, resulting in ventriculoarterial discordance. In TGA, there is atrioventricular concordance, implying normal connections between the atria and the ventricles.  In the majority of fetuses with TGA, the aorta is malpositioned anterior and to the right of the pulmonary artery, with a subaortic conus, and the pulmonary artery is posterior and to the left of the aorta, with fibrous continuity between the mitral and pulmonary valves. This great vessel arrangement, which is seen in about 88% of TGA , is referred to as D-TGA (D = “dexter”) and is the primary focus of this chapter. 

In D-TGA, the pulmonary venous blood is pumped back to the lungs through the pulmonary artery, and the systemic venous blood is pumped back to the body through the aorta. This is fairly well tolerated in the fetus because shunting across the foramen ovale and ductus arteriosus allows for blood mixing and oxygenation occurs in the placenta. 

With postnatal closure of the foramen ovale and ductus arteriosus, D-TGA results in significant neonatal cyanosis.

D-TGA accounts for 5% to 7% of all congenital cardiac anomalies with an estimated incidence of 3-4 per 10000 births. It can be simple, when it is an isolated finding or complex, when associated with other intracardiac anomalies.  Ventricular septal defects and pulmonary stenosis maybe present either alone or in combination in up to 30% or 40% of cases ( Simple TGA 605-70% and complex( 30%-40%).

Approximately 90% of TGAs are isolated findings with associated extracardiac anomalies rare. 

Transposition of the Great Arteries (TGA)
Transposition of the Great Vessels (TGV) 
 

The specific developmental aspects that result in ventriculoarterial discordance in D-TGA are not fully delineated. It is hypothesized that the morphogenesis of D-TGA is due to the abnormal growth and development of the bilateral subarterial conus.
In normal cardiac development, the subaortic conus and subpulmonary conus are present in the first month of gestation as the great arteries are positioned superior to the right ventricle. Typically, the subaortic conus is resorbed at approximately 30 to 34 days into gestation, which allows for migration of the aortic valve inferiorly and posteriorly into its normal position above the left ventricle. The pulmonary valve retains its association with the right ventricle due to the persistence of the subpulmonary conus.
In D-TGA, however, the subpulmonary conus is resorbed, which allows for posterior migration of the pulmonary valve and the development of fibrous continuity between the pulmonary and mitral valve. The unabsorbed subaortic conus forces the aortic valve anteriorly, where it abnormally engages with the morphologic right ventricle. The range in the size and orientation of the subaortic conus is thought to create much of the variability of the coronary arteries' origins and course.

The fetus tolerates the in utero circulation of D-TGA without much difficulty. Oxygen-rich blood from the umbilical vein is largely directed from the right atrium across the fossa ovalis and into the left ventricle, where it is pumped into the pulmonary artery and across the ductus arteriosus into the systemic circulation. The vascular resistance provided by the placenta is lower than the pulmonary capillary bed, which allows for right-to-left blood flow through the ductus arteriosus and into the descending aorta.

Postnatally D-TGA prior to surgical repair is physiologically uncorrected, which means that the systemic and pulmonary circulations are parallel circuits. Deoxygenated systemic venous blood drains appropriately into the right atrium and then is pumped from the right ventricle back to the systemic circulation via the aorta. Oxygenated pulmonary venous blood returns to the left atrium and ventricle and is then recirculated to the lungs via the pulmonary artery. This circulation is incompatible with life unless there is communication between the two parallel circuits. Mixing can occur either via an intracardiac route, across a patent foramen ovale or via  a ventricular or atrial septal defect (VSD or ASD), or via extracardiac connections, including a patent ductus arteriosus or the bronchopulmonary collateral circulation.

The four-chamber view is typically normal in fetuses with D-TGA except for an associated VSD. Visualization of the great vessels’ origin and course is essential to make the diagnosis. Visualization of the left ventricular outflow tract will show the pulmonary artery arising from the left ventricle, and bifurcating into the left and right branch pulmonary arteries, with the branch left pulmonary artery being most visible. The visualization by ultrasound displaying the left pulmonary artery as a branch of the left ventricular outflow tract is a very important clue to the presence of D-TGA as it is uniformly present in all D-TGA cases. 

In D-TGA, the aorta is noted to arise from the right ventricle, in an anterior and parallel course to the pulmonary artery. 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 . 

The three-vessel-trachea plane will show, in most D-TGA cases, a single large vessel (the transverse aortic arch), with a superior vena cava to its’ right. The large vessel noted in the three-vessel-trachea view is the aorta, which is positioned anteriorly and superiorly to the pulmonary artery. The aorta can assume a right convex shape, or be in a straight orientation, termed (I-Sign), in cases of D-TGA at the three-vessel-trachea view. 

A “normal-appearing” three-vessel-trachea view can be rarely present, however, in D-TGA, when the spatial relationship of the great arteries is in a side-by-side orientation. The short-axis view at the level of the great vessels shows both the aorta and the pulmonary artery as circular structures adjacent to each other, instead of their normal orientation (longitudinal pulmonary artery wrapping around a circular aorta

Colour Doppler
Colour Doppler can be helpful in diagnosing D-TGA, but is not necessary. Colour Doppler helps in demonstrating the parallel course of the great vessels and the branch left pulmonary artery of the left ventricular outflow tract. 

In the three-vessel-trachea view, the aorta as a solitary large vessel can be well visualized on colour Doppler. In D-TGA with side-by-side orientation of the great vessels, an almost normal three-vessel-trachea view can be visualized. In this case, the diagnosis of D-TGA is made by visualizing the left ventricular outflow tract, demonstrating the bifurcation of the pulmonary artery. Visualization of an associated VSD ( and confirming patency of the foramen ovale in D-TGA can be enhanced by colour Doppler.

Early Gestation
D-TGA can be diagnosed at the 11 to 13 weeks’ ultrasound but is often missed in series of first-trimester fetal anomalies. An enlarged nuchal translucency in the setting of normal fetal chromosomes can be a marker for the presence of D-TGA. It has been reported to be  strongly associated with  abnormal cardiac axis and conotruncal anomalies, including D-TGA, in early gestation. The three-vessel-trachea view, with its finding of a single great vessel, may be the most significant view in early gestation. At the three-vessel-trachea view, the aorta, arising from the right ventricle, shows a characteristic reverse curvature, referred recently to as the “reverse boomerang” sign. In early gestation, colour Doppler is essential in demonstrating the crossing of the great vessels in normal conditions or their parallel course in D-TGA.

3D Ultrasound
Tomographic ultrasound imaging, surface mode, glass-body mode, inversion mode, B flow, and Biplane in grayscale, and colour Doppler have the ability to enhance visualization of the spatial relationship of the great vessels as they arise from their respective cardiac chambers. 
Evaluation of 3D automated software on volumes of fetuses with TGA demonstrated the abnormality in ventricular–arterial connections in all fetuses.

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 up to 40% of cases and are typically perimembranous but can be located anywhere along the septum. Pulmonary stenosis coexists with a VSD in D-TGA patients in up to 20% of cases, and the stenosis is usually more severe and complex than in D-TGA with intact ventricular septum.

Premature closure of the foramen ovale and narrowing of the ductus arteriosus may complicate D-TGA in late gestation; thus, careful ultrasound evaluation is recommended, especially in the third trimester.  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. Other associated cardiac anomalies are rare and can involve the atrioventricular valves, the aortic arch, and great vessels.

Extracardiac anomalies may be present, but are rare, and numerical chromosomal aberrations are practically absent in D-TGA. Deletion 22q11.2 could be present and should be ruled out, especially when extracardiac malformations or a complex D-TGA are present. 

Double-outlet right ventricle and corrected TGA are the two most common cardiac anomalies in the differential diagnosis of D-TGA because they all share the absence of “crossover” of the great vessels.

Unlike many other forms of congenital heart disease (CHD), D-TGA is not associated with any particular genetic abnormality or familial inheritance pattern. The prevalence of CHD in siblings of affected children is no higher than that of the general population.
In addition, patients with D-TGA rarely have noncardiac anomalies, which commonly accompany other forms of CHD.
In particular, DiGeorge Syndrome (22q11 deletion), which is associated with other conotruncal lesions, is not commonly associated with D-TGA. Testing for the 22q11 deletion in patients with D-TGA will likely have a higher yield if there is also laterality or branching abnormalities of the aortic arch, which are more common in patients with DiGeorge syndrome.

A detailed anatomic scan of the fetus at a tertiary referral center is highly recommended to exclude other cardiac or extracardiac structural abnormalities. Fetal karyotype / microarray should be offered to the patient to exclude chromosomal abnormalities. 
Ultrasound follow up should be offered in order to assess possible signs of progression, specifically pulmonary stenosis, which may not be detected in the 2nd trimester. Colour doppler flow assessment at the level of the foramen ovale and ductus arteriosus should be evaluated close to term as premature closure or narrowing of the foramen or ductus is associated with worsening neonatal outcome and may require emergency postnatal procedures. 

A formal fetal echocardiogram with a pediatric cardiologist is needed to further evaluate fetal cardiac structures and consultation for postnatal management and delivery at a tertiary care center. 
Transfer to delivery at a tertiary care center with Pediatric Cardiology intensive care services  is recommended as this is associated with better outcomes.

1.    Abuhamad A, Chaoui R. A Practical Guide to Fetal Echocardiography: Normal and Abnormal Hearts. Edition, 4th Edition. Lippincott Williams & Wilkins, 2022

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