Transposition of the Great Arteries – how to spot it, and its effects
Transposition of the Great Arteries (TGA) is a relatively uncommon (approximately 1:10,000 incidence) congenital cardiac condition.
In TGA, the great arteries join the wrong sides of the heart. As a result, the aorta (that supplies oxygenated blood to the body) joins to the right ventricle, which receives de-oxygenated blood from the veins. Meanwhile, the pulmonary artery (which supplies de-oxygenated blood to the lungs) joins the left side of the heart, which receives oxygenated blood from the lungs. In theory, that should mean that the lungs and the systemic circulation work on completely separate loops, with the lungs receiving a continuous supply of oxygenated blood, and the body receiving only de-oxygenated blood – the opposite of what should be happening.
In practice, at least one communication between these two circuits is always present. In practice, a “mix” of oxygenated and de-oxygenated blood occurs.
As with any congenital cardiac lesion, TGA may present with one or more other structural anomalies. Holes in the walls dividing the upper (collecting) and/or lower (pumping) chambers of the heart, known as septal defects, are relatively common in this context. Whilst holes in the heart are usually a problem, they may have a beneficial effect in the short term where TGA is present: that is because they also facilitate the mixing of oxygenated and de-oxygenated blood.
More unusually, TGA may be associated with near-“self-correction”. This is known as congenitally corrected transposition of the great arteries. In this condition, the vessels are plumbed into what should be the wrong ventricles, as described above (aorta into the right; pulmonary artery into the left). However, the collecting and pumping chambers have themselves been “rewired” in this variant. Thus, the morphologically right side of the heart receives oxygenated blood from the pulmonary veins, and pumps it into the aorta. Meanwhile, the morphologically left side of the heart receives de-oxygenated blood from the superior and inferior vena cavae and passes this to the pulmonary artery for re-oxygenation.
Whilst it sounds as though the heart has self-corrected, there is a major problem with this arrangement: the right ventricle is smaller and is not designed to do the work of the left, and over time, it will not be able to cope with this workload, with fatal consequences if left untreated.
The effects of Transposition of the Great Arteries
Children suffering from this condition will characteristically exhibit marked symptoms very early in life – quite possibly within hours of birth, and probably within the first week of life (n.b., that is not necessarily the case in congenitally corrected TGA), such as:
- appearing collapsed, grey and listless. They may be grunting or manifesting other signs of respiratory distress. Their oxygen saturation levels will be significantly lower than normal.
- appearing to be blue, otherwise known as cyanosis. Cyanosis may be present either from birth, or over the first month of life. It is normally noticeable around the lips and on the fingers, although blueness over the entirety of the face is not uncommon in children with this condition.
- having problems with feeding.
Diagnosis and treatment of TGA
TGA can be diagnosed prenatally on ultrasound. However, this is not always possible.
Assuming it is not made antenatally, diagnosis is usually made soon after birth by diagnostic imaging. There may be distinctive appearances on chest x-ray. Echocardiography (a form of ultrasound) will usually confirm the diagnosis.
It may be necessary to clarify the diagnosis by further investigation – possibly including a cardiac catheterisation. During this procedure, a camera is threaded up the blood vessels towards the heart and imaging of the affected areas is then performed.
Once the diagnosis has been made and clarified, surgical treatment is likely to be indicated as the next course of action.
The arterial switch
The treatment of choice for TGA is the arterial switch operation, either at first instance or more rarely following the insertion of a shunt – form of surgically-created communication which enables a mix of oxygenated and deoxygenated blood to occur. In this procedure, the great vessels are mobilised, and then “replumbed” into the correct sides of the heart.
This is a technically challenging procedure as it requires the relocation not only of the great arteries themselves, but also of the coronary arteries (which supply the muscles of the heart with oxygenated blood) that are connected to the aorta. In infants, the coronary vessels are extremely delicate and are only millimetres thick: very gentle and subtle surgical manoeuvres are required to relocate them. The operation that ultimately led to the Bristol Royal Infirmary paediatric cardiac scandal was an arterial switch procedure. During the course of this, there was an issue with the right coronary artery: in the words of the surgeon with conduct, “It just broke”.
Any holes in the walls of the heart are repaired. If the ductus arteriosus is still open, it is ligated (cut).
As with any paediatric cardiac procedure which involves time spent on cardiopulmonary bypass and either aortic cross-clamps or possibly total circulatory arrest, time is of the essence. There is a limited amount of time within which to effect the repair, as the likelihood of damage to the heart and other organs in the body mounts with the passage of time.
If an arterial switch isn’t an option
There are certain circumstances in which it would not be possible to offer an arterial switch operation (usually because the patient’s anatomy precludes this).
In this situation, it may be possible to offer what is known as an atrial switch procedure. In essence, this procedure delivers the inverse solution of an arterial switch procedure. In an atrial switch procedure, the vessels from which the right side of the heart receives de-oxygenated blood (the superior and inferior cavae) are reconnected to the left atrium (collecting chamber); similarly, the four pulmonary veins which supply oxygenated blood from the lungs back to the heart are connected to the right atrium.
This is a technically simpler procedure than the arterial switch operation because it does not involve relocating the coronary vessels. However, it does not offer an optimal long-term solution, because the right ventricle will have to take over the workload of the left. It is not designed to do this, and will struggle to cope over the longer term. Patients who have the atrial switch procedure are at higher risk of early death (usually arising from cardiac failure).
As with any open heart procedure, there are a number of risks in addition to death for patients. These include paralysis, epilepsy, cortical blindness and brain damage. These are mercifully rare, and the paediatric cardiac surgical research community has shifted its focus during the last twenty years from concerns over mortality (death) to management of non-lethal complications.