Transposition of Great Arteries (TGA)

Anatomy & Physiology

In "simple transposition" the aorta is connected to the RV and the pulmonary artery to the LV. Systemic venous return thus comes into the RA and passes through the RV straight back to the aorta without having gone through the lungs. Blood in the lungs drains to LA, through the LV and back into the lungs again. Clearly without some mixing of the circulation life cannot be sustained. The potential areas of mixing are atrial or ventricular septal defects and a PDA.

The degree of cyanosis thus depends upon the degree of mixing.

VSD is a common association with TGA and another variant TGA/VSD with pulmonary stenosis also occurs in which the surgical management is different.

Presentation

Simple TGA usually presents within the 1st 24-48 hours, as the ductus closes, with cyanosis and mild tachypnoea. There are usually no murmurs and unless cyanosis is severe the baby is usually surprisingly well.

TGA/VSD may present later as there is often better mixing and cyanosis is less obvious. The VSD murmur is not obvious as the defect is usually large.

TGA/VSD/PS often presents in the neonatal period due to the ejection murmur at the upper left sternal edge. The cyanosis is usually dependent upon the degree of pulmonary stenosis.

Investigations

Hyperoxia Test

A reasonable way to distinguish cardiac causes from respiratory causes of cyanosis is the hyperoxia test. The saturations are recorded prior to giving 100% oxygen for 10 minutes. The saturations should increase by at least 10% if the cyanosis is of respiratory origin. In addition if it is due to a respiratory cause the capillary pCO₂is usually elevated.

Electrocardiogram

This is usually within normal limits for a neonate. If presentation is later then right ventricular hypertrophy is seen.

Chest X-ray

This is classically described as an "egg on side" appearance as the mediastinum is narrower than normal due to the antero-posterior relationship of the aorta and pulmonary artery compared to normal. The cardiothoracic ration is usually normal or only mildly increased. The pulmonary vascular markings are increased due to the volume loading of the left heart seen in TGA.

Echocardiogram

This is diagnostic with the artery bifurcating close to the origin (PA) seen to arise from the LV and the artery giving rise to the coronary and arch vessels arising from the RV. The parallel vessels arrangement can also be seen.

The associated features (ASD, VSD, PS) are also readily assessed.

Natural History

Without surgery the most of those with simple transposition will die within the first month of life although some survive for a year depending upon the degree of mixing. Those with TGA/VSD have a greater life expectancy although they will develop pulmonary vascular disease due to the large VSD. TGA/VSD/PS may well be balanced and survival into adulthood is not unknown. If the PS is severe life expectancy will be shorter due to the intense cyanosis.

Management

The PDA is reopened or patency maintained with an infusion of prostaglandin given intravenously. The principle side effect is apnea and respiratory support may be necessary. If the foramen ovale is too small to allow adequate mixing between left and right atrial a balloon atrial septostomy is required. Once stable and preferably about a week of age definitive surgery is undertaken with an arterial switch procedure. This is suitable for both simple transposition and for TGA/VSD. Those with additional pulmonary stenosis need a different surgical strategy.

If the pulmonary stenosis is severe then a shunt will be required prior to a Rastelli procedure. If the pulmonary stenosis is significant then the shunt through the VSD is minimal, the circulation balanced and the Rastelli procedure can be done at about 1-2 years of age or later.

Prognosis

The mortality for the arterial switch procedure in simple transposition and TGA/VSD is now less than 5% in most centres. The long term outlook is also excellent. The commonest reason for reoperation is pulmonary stenosis. Late deaths do occur but in the series reported by Losay et al (Circulation 2001 Sep 18;104(12 Suppl 1):I121-6) no deaths after 5 years were seen and survival of 88% to 15 years was found. The prognosis for TGA/VSD/PS is not as good as multiple surgical procedures are required.

This page was last edited 16/2/2004

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