TRANSPOSITION OF THE GREAT ARTERIES

P. SYAMASUNDAR RAO, MD

Introduction

In the preceding chapters, we addressed wide-ranging topics in a more general fashion. In this and in subsequent chapters, we will discuss more commonly encountered individual cardiac defects.

Transposition of the great arteries (TGA) constitutes 5% of all cyanotic congenital heart defects (CHDs) and 10% of all neonatal cyanotic CHDs¹ and is the most common cyanotic CHD in the neonate. A variety of descriptions have been used to characterize TGA, but the most precise portrayal is “a CHD in which the morphologic right ventricle (RV) gives rise to the aorta (Ao) and the morphologic left ventricle (LV) gives rise to the pulmonary artery (PA).” In the most frequent type, namely, complete transposition, there is atrial situs solitus (atria are normal in position), atrio-ventricular concordance (right atrium (RA) connected to the RV and the left atrium (LA) to the LV) is present, d-loop of the ventricles (RV is on the right and LV on the left) exists, ventriculo-arterial discordance (Ao arises from the RV and the PA from the LV) occurs, and the aortic valve is positioned to the right of the pulmonary valve (PV) (d-TGA). Therefore, the systemic venous return from the vena cavae (VC) goes into the RA and RV and from there into the Ao, whereas the pulmonary venous blood comes into the LA and LV and from there into the PA (Figure 28.1). Consequently, the circulation is parallel instead of the normal in-series circulation. This results in not delivering the pulmonary venous blood to the body and not getting the systemic venous blood to the lungs for oxygenation. With this state of circulation, the newborn will not survive unless there is intercirculatory mixing via defects such as patent foramen ovale/atrial septal defect (PFO)/(ASD), patent ductus arteriosus (PDA), or ventricular septal defect (VSD).

The infants in Group III, that is, TGA with VSD and PS, do have a variable presentation; this is dependent upon the extent of intercirculatory mixing and the severity of pulmonary outflow tract obstruction. If poor mixing is present the babies will present early in life and mimic Group I TGA patients with an intact ventricular septum. If the pulmonary outflow tract obstruction is severe, the presentation is basically akin to that seen with tetralogy of Fallot (TOF).2 When PS is moderate the presentation is likely to be late. When PS is mild, signs of CHF may develop in a manner similar to that seen in Group II babies with TGA with VSD.

Physical Examination

The Group I TGA babies with intact ventricular septum typically have severe cyanosis. However, they are not usually in distress until severe hypoxemia and acidosis ensue. Similarly, clubbing is not seen in the neonatal period and may not be seen until 3 to 6 months of age. Increased RV impulse and single second heart sound are present. No cardiac murmurs are usually heard in neonates with intact ventricular septum. Rarely, a grade I to II/VI nonspecific ejection systolic murmur may be auscultated along the left sternal border.

In Group II, TGA babies with VSD, tachypnea, and tachycardia are usually seen. They are minimally cyanotic and have hepatomegaly. Increased RV and LV impulses, single second sound, a grade III to IV/VI holosystolic murmur at the left lower sternal border are the usual findings. In babies with a large ventricular shunt a mid-diastolic flow murmur may be heard at apex.

In Group III, TGA babies with VSD and PS, the physical examination is similar to TGA with intact septum, TGA with VSD, or TOF depending upon the degree of mixing and severity of pulmonary outflow tract obstruction. However, grade III to IV/VI long ejection systolic murmur at the left upper sternal border and/or a holosystolic murmur at the left lower sternal border are usually heard in most of these babies.

Noninvasive Evaluation

Chest x-ray

Chest roentgenogram in Group I TGA neonates with intact ventricular septum appears benign with normal to minimal cardiac enlargement and normal to slightly increased pulmonary vascular markings (Figure 28.2). The thymic shadow rapidly involutes giving the appearance of a narrow pedicle (superior mediastinum). Amalgamation of the above signs may at times have the appearance of an “egg-shaped” heart on a posteroanterior chest x-ray. In Group II TGA babies with VSD, moderate-to-severe cardiac enlargement, and increased pulmonary vascular markings are typically visualized. In Group III TGA babies with VSD and PS, mild-to-moderate cardiac enlargement may be noted. The pulmonary vascular marking may be decreased, normal, or increased; this is largely determined by the severity of PS.

The ECG of the newborn in Group I (TGA and intact ventricular septum) may appear normal with the usual RV preponderance seen at this age (Figure 28.3). However, definitive RV hypertrophy becomes evident in older infants and in children. In addition, RA enlargement may be seen. In babies with Group II (TGA with VSD), biventricular hypertrophy and LA enlargement are usually observed. In newborn babies with Group III (TGA with VSD and PS), RV or biventricular hypertrophy, depending upon the degree of PS may be seen.

Echocardiogram is useful both in confirming the diagnosis and in the assessment of associated abnormalities. Because of the fact that atrial and ventricular anatomy is normal, as are the aortic and PVs, the diagnosis of TGA may be somewhat difficult by echo, particularly for the neophyte. An indirect sign of somewhat a posterior course of the great vessel coming off of the LV in a precordial long-axis view (Figure 28.4A), indicating that this vessel is a PA, is helpful. This is in contradistinction to an anteriorly coursing ascending Ao (Figure 28.4B) in normal newborns. When the great vessel arising from, the LV is traced and its bifurcation demonstrated (Figure 28.5), that vessel is identified as a PA and the diagnosis of TGA is confirmed. Other helpful signs suggestive of TGA are parallel position of both great arteries in a long-axis view of the heart (Figures 28.4A and 28.6A) and simultaneous on-end visualization of both the Ao and PA on a precordial short-axis view (Figure 28.6B). The atrial septum should be evaluated in the subcostal views both by 2-dimensional (2D) imaging (Figure 28.7A) and color flow mapping (Figure 28.7B and C); the interatrial communication is usually a PFO. Occasionally, a true ASD is present. An interatrial defect of 5 mm diameter or larger is generally considered adequate for providing sufficient interatrial mixing to avoid balloon atrial septostomy. PDA may also be present and the shunt across it by color and pulsed Doppler should be assessed (Figures 28.8 and 28.9).

Figure 28.4. Precordial long-axis echocardiographic views of 2 neonates, first, A: with TGA and second, B: with normally related great arteries. A. Note that the posterior vessel arising from the LV is coursing backward (posteriorly) after its origin from the LV and is likely to be the PA, suggesting TGA. This vessel should be traced and its bifurcation demonstrated (see Figure 11.38) to confirm that it is indeed PA. In B, the posterior vessel arising from the LV courses somewhat anteriorly, indicating that it is likely to be the Ao. Ao, aorta; LA, left atrium; RV, right ventricle.

Figure 28.5. The vessel arising from the LV of the infant shown in Figure 28.4 was traced which showed bifurcation in 2D (A) and color flow (B) images into right (RPA) and left (LPA) pulmonary arteries suggesting that this vessel is main pulmonary artery (MPA), confirming the diagnosis of TGA.

Furthermore, visualization of VSD and PS will help categorization of the babies into the respective groups. Shunting across the VSD by color Doppler (Figure 28.10) and degree of obstruction across the pulmonary outflow tract (Figure 28.11) by pulsed and continuous (or high-frequency pulsed) Doppler (not shown) should be evaluated and these are helpful in assessing physiological state of the baby.

Pulse oxymetry is valuable in identifying the cyanotic infant.3 However, blood gas values are more useful in indicating the extent of hypoxemia and ventilatory status. Serum glucose (or dextrostix), calcium, hemoglobin, and hematocrit values are also helpful in the overall assessment of the baby as reviewed elsewhere⁴ and are discussed in Chapter 13.

Cardiac Catheterization and Angiography

Because of high accuracy of echocardiography in confirming the diagnosis of TGA, invasive catheterization studies are not required for the diagnostic purposes. However, necessity for speedy relief of hypoxemia and acidosis by balloon atrial septostomy, and on occasion, the need for a better definition of coronary arterial anatomy (Figure 28.12) preceding arterial switch operation, may call for catheterization and angiography.

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