Background

Transposition of the great arteries (TGA) is a form of congenital heart defect (CHD) defined by ventricular–arterial discordance with the aorta arising from the right ventricle and the pulmonary artery (PA) arising from the left ventricle. Overall TGA accounts for 5% to 7% of all forms of CHD. The incidence ranges from 21.1 to 30.3 per 100,000 live births. Males are more commonly affected then females, accounting for 60% to 70% of cases. Untreated, 90% of infants will die within the first year of life, and 30% will die within the first week. In 1966, Rashkind and Miller performed the balloon atrial septostomy for the first time in a baby with TGA; this intervention dramatically influenced the survival of neonates with TGA.1 Unlike other forms of CHD that have a high association with extracardiac anomalies, only 10% of patients with TGA have a coexisting extracardiac defect.2

TGA can take on many different anatomic forms with an assorted range of physiologic characteristics. Two of the most common forms of transposition are often referred to as dextro-TGA (d-TGA), or classic transposition, and levo-TGA (l-TGA). The letters d (dextro or rightward) and l (levo or leftward) stem from the Van Praagh segmental nomenclature system and refer to the anatomic position of the aortic valve in relation to the pulmonary valve.

The term d-TGA represents patients who have atria and ventricles in their correct anatomic position but have an aorta arising from the morphologic right ventricle (RV) and a PA originating from the morphologic left ventricle (LV). Patients with d-TGA most commonly have an aorta that is rightward and anterior to the PA (Figure 9-1).

In l-TGA, also known as congenitally corrected TGA (ccTGA), the right atrium drains into the anatomic LV (which is right-sided), and the left atrium drains to the anatomic RV (on the left side of the heart). The aorta arises from the left-sided morphologic RV, and the PA arises from the right-sided morphologic LV (Figure 9-2). Although these patients have the anatomic diagnosis of TGA, patients with l-TGA have “normal physiology,” with an LV pumping deoxygenated blood to the lungs and an RV providing oxygenated blood to the body.

Dextro-Transposition of the Great Arteries

Definition and Epidemiology

d-TGA is the most common form of transposition. Approximately one half of patients with TGA have another cardiac defect, not including patent foramen ovale (PFO) or patent ductus arteriosis (PDA). The most common associated cardiac defect is a ventricular septal defect (VSD), occurring in 40% to 45% of patients with TGA. However, only about one third of these VSDs are hemodynamically insignificant.2 Other associated cardiac defects seen with TGA and their relative frequencies are listed in Table 9-1.

Pathophysiology

In d-TGA, the pulmonary and systemic circulations are in parallel, rather than series. Therefore, deoxygenated blood returning from the body to the right atrium (RA) crosses the tricuspid valve into the RV and exits the RV into the aorta, back out to the body and completely bypassing the lungs. Likewise, pulmonary venous return to the left atrium (LA) crosses the mitral valve into the LV and exits into the PA, headed back to the lungs. Without mixing of oxygenated and deoxygenated blood, it is easy to see how this physiology will quickly lead to profound cyanosis, acidosis, and death. In infants with a sizeable VSD, mixing at the ventricular level will provide more oxygenated blood to the body. In the absence of a VSD, newborns with TGA have 2 other potential areas for circulatory mixing, a PFO and PDA (Figure 9-1).

After fetal transition and a resultant drop in pulmonary vascular resistance, blood will shunt through the PDA mostly from aorta to PA. Although this shunt will not directly provide oxygenated blood to the body, increasing pulmonary blood flow will increase the overall amount of oxygenated blood and increase the amount of pulmonary venous return to the LA. This increase in blood volume will increase LA size and pressure, promoting mixing at the PFO.

The PFO itself can be very small to quite large in the newborn. In neonates with TGA and no VSD, mixing of oxygenated blood in the LA and deoxygenated right atrial blood via the PFO is the most important determinant of systemic saturation and overall stability. If the mixing is inadequate, as reflected by a low arterial saturation and partial pressure of oxygen in arterial blood (Pao2), a procedure to enlarge the PFO must be undertaken. This procedure is known as a balloon atrial septostomy, or Rashkind procedure. For more information, see Chapter 5.

Clinical Presentation and Diagnosis

In some cases, d-TGA is diagnosed prenatally by fetal echocardiography. However, prenatal diagnosis can be challenging because the intracardiac anatomy can look essentially normal, with 4 chambers and 2 great vessels. In addition, TGA often occurs in isolation; therefore, it is uncommon for extracardiac anomalies to prompt more specialized evaluation and fetal echocardiography.

In the absence of prenatal diagnosis, patients with d-TGA almost always present in the newborn period with cyanosis. The degree of cyanosis is completely dependent on the amount of mixing between the systemic and pulmonary circulations as discussed earlier. Fifty-six percent of newborns with TGA (without VSD) are recognized within the first hour of life and 92% within the first day.2 Table 9-2 describes physical examination, electrocardiogram, and chest x-ray findings seen in d-TGA. Echocardiography is the diagnostic modality of choice and should be sought for any infant in whom TGA is suspected.

Treatment

Medical

Once the diagnosis is made, infants are maintained on prostaglandin E1 to ensure the patency of the PDA. However, they need to be closely monitored for side effects such as apnea, vasodilation, fever, rash, and edema. In addition, as mentioned earlier, balloon atrial septostomy (Rashkind procedure) may be necessary to enlarge the PFO, promoting mixing of the pulmonary venous and systemic venous blood, if persistently low arterial Pao2 levels are present in the newborn period.

Surgical

Corrective surgery for d-TGA is usually undertaken in the first 1 to 2 weeks of life, after the pulmonary vascular resistance has fallen. In the current era, the most commonly performed operation for d-TGA is called the arterial switch operation (ASO) and involves translocation of the great vessels back to their appropriate positions. This surgery was first described in 1975 by Adib Jatene, a Brazilian surgeon, and became widely used in the 1980s. The ASO is now the procedure of choice for d-TGA.

The ASO is performed by first transecting the aorta and PA, just above the sinuses of each valve. The coronary arteries are then excised from the aortic sinus, taking a rim of tissue surrounding each coronary os (these are known as the coronary buttons). The distal main pulmonary (with attached branch PAs) is passed anterior to the aorta (LeCompte maneuver) and connected to the proximal aortic root, becoming the neopulmonary root. The coronary buttons are sewn to the proximal pulmonary root or the neoaortic root, and the distal ascending aorta (which is now behind the PA) is connected to the neoaortic root (Figure 9-3).

Prior to the introduction of the ASO, patients with d-TGA underwent an atrial-level switch operation that redirected systemic venous return to the LA and pulmonary venous return to the RA. This was accomplished by 1 of 2 methods, the Mustard or Senning operation (Figure 9-4). The Senning operation constructed an atrial baffle made from native atrial tissue, using the atrial septum. The Mustard procedure used baffle made from the pericardium and cut away most of the atrial septum. Both surgeries redirected blood to the appropriate outlet, while leaving the RV as the systemic pumping ventricle. Because of atrial size and the complexity of the surgery, patients often did not have surgery until a few weeks to several months after birth.

Most early and midterm follow-up studies comparing the ASO to the Mustard or Senning operation show the ASO to be superior when comparing ventricular performance, development of arrhythmias, and incidence of early sudden death.3 Therefore, the atrial switch operation has been largely abandoned for the ASO in the current era.

In patients who have significant LV outflow tract obstruction (or pulmonary stenosis), an ASO may not be feasible because patients would be left with significant obstruction to systemic outflow (neoaorta). These patients often have large VSDs and can undergo a Rastelli procedure. In this operation, the VSD is closed to the native aorta with oversewing of the pulmonary valve. The RV is then connected to the PA via a homograft conduit.

Outpatient Management

Postoperative management of patients after surgical repair of TGA should be focused on identification of potential short-, mid-, and long-term sequelae known to occur after ASO. A list of the more common morbidities associated with the ASO is provided in Table 9-3.2-5

Postoperative Complications after ASO

Early mortality for patients after ASO is less than 10% and often associated with anatomic abnormalities, such as unusual coronary artery course, multiple VSDs, abnormal aortic arch, or atrioventricular valve malformations.2

Supravalvar pulmonary stenosis (PS) is the most common short-term complication following the ASO, with 5% to 30% of patients requiring reintervention by balloon angioplasty/stent placement via cardiac catheterization or rarely surgical repair with PA plasty. The etiology of supravalvar PS can be from compression of the PAs by the posterior aorta (with the LeCompte maneuver), circumferential narrowing at the suture line, or branch PA stenosis (stretching around the neoaorta).

Neoaortic root dilation is a known phenomenon after ASO and can be progressive. Progressive neoaortic root dilation can lead to aortic insufficiency. However, the number of patients needing surgical reintervention in childhood for root dilation or aortic insufficiency is small (<5%).4

Coronary artery patency is a major concern after ASO, given that the coronary arteries are moved from the aorta to the PA (neoaorta) at the time of the operation. Mechanical occlusion of the coronary arteries can occur in several forms, with a range of clinical manifestations (Table 9-3). In addition, the long-term effects that the ASO may have on development of coronary artery disease, long-term ventricular function, and development of arrhythmia are not yet known. The oldest survivors are currently in their third decade, and it is unknown whether coronary artery disease will occur prematurely in this patient population.3 However, it is important to keep in mind that the presentation of myocardial ischemia in this patient population may not be classic chest pain, because their hearts have been effectively deinnervated as a result of surgical suture lines. Therefore, a high suspicion of coronary artery pathology is always warranted when caring for these patients at any stage in their life.

Postoperative Complications of the Atrial Switch Operation

The atrial switch procedures, the Mustard and Senning operations, were routinely performed through the early 1990s. Therefore, there are a large number of patients in their late teens and adulthood who are living with this type of anatomic correction.

Long-term sequelae after the atrial switch operation are listed in Table 9-4. One of the most well-described effects of the atrial switch operation is RV dysfunction. The RV was not designed to perform as the systemic ventricle. Tricuspid regurgitation often accompanies RV dysfunction and tends to progressively worsen as RV function deteriorates. Treatment of RV dysfunction is challenging, and clinicians are often tempted to use β-blockers and angiotensin-converting enzyme inhibition given their known benefits in acquired heart disease. However, to date, there have been no convincing data that these, or other medications, provide any survival benefit for patients with RV dysfunction after atrial switch.5