Introduction

Transposition of the great arteries (TGA) accounts for about 10 per cent of children with congenital heart disease. The male-to-female ratio is 2:1. In TGA, the aorta arises from the morphological right ventricle (mRV), and the pulmonary artery (PA) arises from the morphological left ventricle (mLV). This is referred to as ‘ventriculo-arterial (or V-A) discordance’. This means that the systemic venous return of desaturated blood passes from the right atrium (RA) through the tricuspid valve to the right ventricle (RV) and aorta. The oxygenated pulmonary venous return flows into the left atrium (LA) through the mitral valve into the left ventricle (LV) and PAs. Unless there is mixing of the blood at atrial, ventricular or ductal levels, the patient is inadequately oxygenated and dies. Most untreated patients die within the first few hours or days of life.

The external morphology of the heart is such that the aorta is positioned anterior and to the right of the PA, the so-called dextro-transposition or normal transposition (Figure 16.1). Associated cardiac malformations include ventricular septal defects (VSDs – peri-membranous or muscular), aortic arch coarctation/interruption and pulmonary stenosis. Overall, 60 to 70 per cent of all cases of TGA have an intact ventricular septum, 20 to 25 per cent have a VSD and the remaining 10 to 15 per cent are referred to as ‘complex’, implying a combination of VSD with arch anomalies or outflow tract stenosis.

Figure 16.1 Anatomical relationship for transposition of the great arteries in dextro-TGA. The aorta is anterior and to the right, arising from the morphological right ventricle.

Historically, patients survived only if there was as adequate-sized atrial or ventricular septal defect that allowed oxygenated and deoxygenated blood to mix. Surgical atrial septectomy, by Blalock and Hanlon (1950), followed by successful balloon atrial septostomy by Rashkind and Miller (1966) revolutionized the management of these patients, allowing mixing at the atrial level. Senning in 1959 and then Mustard in 1964 introduced an atrial baffling technique that re-directed the pulmonary venous return to the mRV and the systemic venous return to the mLV, thus restoring the normal physiological circulation. However, in this atrial switch, the mLV remained in the pulmonary circulation and the mRV in the systemic circulation. For the majority of patients, the mRV coped very well with the systemic circulation at first. However, the natural history for these patients has been the variable development of mRV failure and associated tricuspid regurgitation leading to congestive heart failure later in life.

True anatomic repair – i.e. restoring the mLV to the systemic circulation – was the goal, and in 1975, Jatene and colleagues described the first successful arterial switch procedure where the aorta and pulmonary artery were relocated to their respective left and right ventricles. The coronary arteries were also relocated to the new aorta in the same procedure. This successful report set in train a change to the so-called arterial switch procedure for normal transposition. This also opened the possibility of infant repair, and in 1984, Castaneda in Boston and other centres reported successful neonatal repairs of TGA by the arterial switch procedure. The great advantage of the neonatal switch was that the LV could be restored to the systemic circulation before the ventricular muscle involuted and became weaker under the low-resistance pulmonary circuit. The other challenge in the arterial switch procedure is relocation of the coronary arteries. Unusual coronary artery patterns presented a technical challenge at the outset of the arterial switch procedure, but now different types of coronary artery patterns can be relocated in the arterial switch procedure without an increased risk.

Presentation and Diagnosis

Clinical presentation is usually of a very cyanosed sick baby, which occasionally may collapse and require urgent resuscitation. The standard form of resuscitation includes administration of prostaglandin E1 intravenously to reopen the duct and allow mixing at arterial level, correction of acid-base abnormalities and a balloon atrial septostomy often under echo Doppler control. Invariably, the baby can be stabilized with satisfactory systemic oxygen saturations after the balloon atrial septostomy. The prostaglandin E1 infusion may be stopped. Rarely, mixing is not adequate, and an emergency arterial switch is necessary. TGAs with and without VSDs are corrected by arterial switch within the first two weeks of life.

If there is no VSD, then the mLV will rapidly lose its muscle mass as it is working in the pulmonary circulation. Thus, it is essential that the arterial switch is performed before the mLV has begun to ‘involute’ in this way, necessitating surgery within the first two weeks of life. In rare cases of late presentation, the switch can be done at up to six to seven weeks of age, but the older the baby, the greater is the risk that the mLV will not cope in the systemic circulation (cases may need extra-corporeal membrane oxygenation (ECMO) support postoperatively or require ‘re-training’ with the placement of a band on the PA). However, if there is a VSD, then the pressure in the mLV will be sustained, and it may be possible to delay the surgery for a few weeks – although in practice most centres would plan for repair within the first two weeks of life regardless. Other forms of TGA with arch and coarctation or interruption are also managed usually with a one-stage repair in the first two weeks of life. Where there is left ventricular outflow tract (LVOT) obstruction and VSD, the circulation may be well balanced and not need any initial intervention; however, if there is severe outflow tract obstruction, the pulmonary circulation is usually improved with a systemic shunt. Corrective surgery can usually be deferred until the first year or two of life.

Diagnosis is by echocardiography and (apart from the need in the majority for a balloon atrial septostomy to create a shunt at the atrial level) invasive investigation are not necessary. Echo Doppler can clearly define the great vessel anatomy, the presence of single or multiple VSDs, the presence of LVOT obstruction and aortic arch anomalies. In the presence of aortic arch anomalies, there may also be the potential for a RVOT obstruction beneath the aortic valve. In the great majority of cases, the coronary artery anatomy can be clearly defined as well. Occasionally, there are associated non-cardiac abnormalities that need addressing as well. The majority of cases are diagnosed post-natally, but antenatal diagnosis is increasing and can help in planning immediate management post-partum.