Transposition of the Great Arteries

Synonyms: TGA, complete transposition of the great arteries, d-TGA, simple transposition, ventriculoarterial discordance

The aorta and pulmonary artery are transposed so that the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. The aetiology is unknown and inheritance is presumed to be multifactorial. Transposition is often associated with other heart defects, e.g. ventricular septal defect, left ventricular outflow obstruction, atrial septal defect, patent ductus arteriosus. The presence or absence of associated cardiac anomalies determines the presentation and management.

• Transposition is the most common cyanotic congenital heart lesion presenting in the neonate.
• The overall annual incidence is 20-30 per 100,000 live births.
• It is more common in males than females with a ratio of about 3:1.
• Maternal factors associated with an increased risk include rubella or other viral illness during pregnancy, alcoholism, maternal age over 40 and diabetes.
• Transposition is rarely associated with syndromes or extra-cardiac malformations.

• Most affected infants become cyanosed within the first few hours or days after birth, but may be mild and not diagnosed until the end of the first week of life or beyond.
• Those with a large ventricular septal defect or patent ductus arteriosus may not be diagnosed until several weeks of age.
• Infants may develop breathlessness and heart failure over the first 3-6 weeks as pulmonary blood flow increases.
• If there is a VSD and left ventricular outflow tract obstruction, the presentation may be similar to that of an infant with Fallot's tetralogy.

Signs

• Cyanosis and may also be breathless with tachycardia and tachypnoea.
• Heart sounds are often normal with no audible murmurs.
• In patients with a ventricular septal defect there may be a systolic murmur, which increases in intensity as the pulmonary vascular resistance falls.
• An ejection systolic murmur is usually present in those with left ventricular outflow tract obstruction.

• Other causes of a severely ill neonate, e.g. infection
• Pulmonary atresia
• Fallot's tetralogy
• Total anomalous pulmonary venous connection
• Tricuspid atresia
• Truncus arteriosus

• Chest x-ray may appear normal in newborns with TGA and intact ventricular septum but may demonstrate the classic "egg on a string" appearance (heart is slightly enlarged and appears like an egg lying on its side, narrow vascular pedicle because aorta and pulmonary artery lie one in front of the other, and increased vascular lung markings).
• With an associated ventricular septal defect, the chest x-ray usually shows cardiomegaly with increased pulmonary arterial vascular markings.
• Initial investigations also include pulse oximetry, ECG and assessment of an unwell baby, e.g. infection screen, full blood count, renal function, electrolytes.
• Echocardiogram images are diagnostic of transposition and associated anatomic lesions.
• Cardiac catheterisation may be needed if echocardiogram does not adequately demonstrate the anatomical abnormality.

• Initial treatment consists of maintaining ductal patency with continuous IV prostaglandin E1 infusion.
• All patients require antibiotic prophylaxis prior to dental and indicated surgical procedures in order to reduce the risk of subacute bacterial endocarditis.

Surgical

• Cardiac catheterisation and balloon atrial septostomy is used to increase the atrial level shunt and to improve mixing.
• The definitive corrective procedure is the arterial switch operation, which has been found to have similar mortality and less morbidity than atrial (Mustard or Senning) repairs.¹
• Most full-term neonates with uncomplicated TGA can undergo an arterial switch procedure in one operation, with minimal mortality.

• Congestive heart failure
• Arrhythmias
• Right ventricular dysfunction in long-term survivors
• Eisenmenger's syndrome
• Polycythaemia and hyperviscosity syndrome
• Thrombocytopenia

• The mortality rate in untreated patients is approximately 30% in the first week, 50% in the first month, and 90% by the end of the first year.
• Death is usually due to anoxia, acidosis, heart failure and complications associated with polycythaemia, including thromboembolic events.
• The overall survival rate following arterial switch operation is 90%.