What Is Transposition of the Great Arteries (TGA)?

TGA is a congenital cardiac malformation accounting for 5–7% of congenital cardiac anomalies. The basic abnormality in TGA is that the aorta arises from a morphologic right ventricle, and the pulmonary artery arises from a morphologic left ventricle (Figure 65.1). The common clinical entity of “simple” TGA is associated with situs solitus atria, concordant atrioventricular, and discordant ventriculo-arterial alignment. The anatomic derangement results in systemic venous blood flow predominantly to the aorta and pulmonary venous blood to the pulmonary artery creating a circulation in parallel. The deoxygenated and oxygenated blood need to mix at atrial, ventricular, or ductal level to be compatible with life. This physiologically uncorrected entity of TGA is fatal without treatment with mortality of 30% in the first week of life and 90% mortality within the first year.

Figure 65.1 Illustration of transposition of the great vessels. RA and LA, right and left atrium; RV and LV, right and left ventricles; SVC and IVC, superior and inferior vena cava respectively; TV, MV, PV, AoV, tricuspid, mitral, pulmonary, and aortic valves respectively; MPA, main pulmonary artery; PDA, patent ductus arteriosus; ASD; atrial septal defect.

Image courtesy of the Centers for Disease Control and Prevention, USA

What Are Oxygen Saturations in TGA?

Oxygen saturations greater than 85% on room air are reassuring; however, if the baby is severely cyanotic it is concerning as the baby may have pulmonary hypertension or poor mixing of the oxygenated and deoxygenated blood secondary to a restricted atrial septum.

What Is the Difference between D- and L-TGA?

In D-TGA the aorta is anterior and rightward, or dextroposed, and the circulations are parallel. In L-TGA, the circulation is in series and is often referred to as “corrected transposition” or “ventricular inversion.” Anatomically, there is atrio-ventricular and ventriculo-arterial discordance which causes the deoxygenated venous return coming to the right atrium to get to the pulmonary artery but via a morphological left ventricle. The oxygenated pulmonary venous blood returns to the aorta via the morphological right ventricle.

What Is the Status of the Atrial Septum?

The atrial septum is very important in patients with D-TGA. This is the site of mixing of blood and allows the patient to have a systemic circulation with improved oxygenation. Secondly, the nonrestrictive atrial septum allows the decompression of the left atrium and a decrease in left atrial pressures.

What Is the Role of the PDA in This Congenital Anomaly?

The size and patency of the PDA is important (in the setting of a restricted atrial septum) in determining the degree of mixing of systemic and pulmonary circulations, and thereby survival. The patency of the PDA becomes less important if the atrial septum is non-restrictive. Hence it is not unusual to discontinue the prostaglandin after an atrial septostomy.

What Other Anomalies Occur with TGA?

No other coexisting defects may be present in nearly half of the patients with TGA. A persistent PDA or patent foramen ovale (PFO) may be the only anomaly. However, in about 40–45% of patients with TGA, a VSD is the most common anomaly. A combination VSD with significant left ventricular outflow tract obstruction (LVOTO) is seen in about 10% of patients. The presence of these coexisting defects along with valvular anomalies may alter surgical planning.

The VSD may be small or large and located anywhere in the septum. Conoventricular or perimembranous VSDs account for 33% followed by malalignment for 30% and muscular defects accounting for 27%.

Is Pulmonary Stenosis Seen in Association with TGA?

Usually TGA with intact septum does not have pulmonary stenosis (PS).

Patients with TGA/VSD may have PS and can be extremely cyanotic.

How Is the Diagnosis of TGA Confirmed?

Echocardiography is the mainstay in confirming the diagnosis of d-TGA. The presence of a PDA, interatrial septal communication or VSD and other anomalies should be confirmed. These findings are important in surgical planning.

What Is the Difference in Circulation Physiology between a Normal Neonate and a Neonate Born with TGA?

The anatomic derangement results in deoxygenated systemic venous blood flow predominantly to the aorta and oxygenated pulmonary venous blood to the pulmonary artery creating a circulation in parallel. This means that the deoxygenated blood returns via the aorta back to the body and the oxygenated blood returns to the pulmonary circulation for re-oxygenation, causing a complete physiological right-to-left (to systemic circulation) and left-to-right shunt (to pulmonary circulation). The deoxygenated and oxygenated blood need to mix at the atrial, ventricular, or ductal level to be compatible with life. This mixing allows some deoxygenated blood to get oxygenated and some oxygenated blood to return to the systemic circulation.

How Is Survival Possible in a Parallel Circulation as Noted in TGA?

A parallel circulation is incompatible with life. There has to be mixing at the atrial, ventricular, or ductal level for the patient to survive. This physiologically uncorrected entity of TGA is fatal without treatment with mortality of 30% in the first week of life and 90% mortality within the first year.