I hope you still remember about the great arteries of the body, and the chambers of the heart from which they arise. In this article, we will discuss the condition called Transposition of the Great Arteries (TGA) in which these great vessels are inter-changed in position.
Here’s what you’ll learn about:
- What is Transposition of the Great Arteries (TGA) ?
- What happens in TGA ?
- What if TGA is left untreated ?
- When should TGA be treated ?
- What are the treatment options ?
- Initial stabilization
- Palliative treatment
- Definitive treatment
- Arterial Switch Operation (ASO)
- Atrial Switch Procedures
- Some Special Circumstances
What is Transposition of the Great Arteries?
The two “great” arteries of the body are the aorta and the pulmonary artery. The aorta comes out of the left ventricle, and carries pure blood to the rest of the body. The pulmonary artery arises from the right ventricle and carries impure blood from the veins into the lungs for oxygenation.
Transposition of the Great Arteries – TGA, for short – as the name implies, means the two arteries are “transposed”. That is, the aorta arises from the right ventricle, and the pulmonary artery from the left ventricle.
What happens in TGA?
You have read all about the circulation of blood in the body. So it must be clear to you that a situation like TGA will not permit normal circulation and oxygen supply to the different organ systems of the body.
When “impure” venous blood returns to the right ventricle (instead of going to the lungs as it normally does), it passes into the transposed aorta. This impure blood is then distributed to the rest of the body. So every organ of the body will receive blood with very little oxygen and a lot of waste materials. The patient looks “blue” – cyanosis – and so TGA is another example of a congenital cyanotic heart disease, like Tetralogy of Fallot and Pulmonary Atresia.
On the contrary, when “purified” blood with oxygen returns to the left ventricle from the lungs, instead of being pumped into the aorta, it passes into the transposed pulmonary artery. This “pure” blood makes another trip to the lungs. As it is already fully saturated with oxygen, this trip is useless.
If the two sides of the circulation were completely separate, life would not be possible beyond a few minutes. But once again Nature plays its wonderful role. By creating an additional defect, the child is kept alive for a few hours or days, so that medical care can be given, and the condition repaired.
So what is Nature’s protective method?
The basic problem of transposed arteries cannot be corrected without operation. However, if the two sides of the circulation could be made to mix with each other, there would be at least partial improvement. The child could live a little longer; long enough for surgery to be performed.
How then can this mixing be brought about?
In many cases of TGA, there is an associated hole or defect in the wall between the atrium or ventricle – called Atrial Septal Defect (ASD) or Ventricular Septal Defect (VSD). Across these defects, blood mixes. But even when these defects are absent, there is still another chance.
This other site of mixing is the Patent Ductus Arteriosus (PDA). As I mentioned in another article, PDA is a tube connecting the aorta and pulmonary artery. It normally closes within minutes or hours after birth. But it can be kept open artificially by using a drug called prostaglandin.
Prostaglandin is a “miracle” drug, that has been a boon for children with complex birth defects of the heart. It is also the mainstay in treatment of TGA patients. Prostaglandin infusion is started as soon as the diagnosis of TGA is made, and continued until surgery is possible, usually within a few hours or days.
What are the problems if TGA is left untreated?
By now it must be clear to you that TGA is a condition that is not compatible with survival. Unless treated, over 90% of patients will not live to see their first birthday.
There are a few exceptional circumstances.
First is the child with TGA and a large VSD. “Mixing” of blood from both sides of the heart takes place across the VSD. So there is enough oxygen supplied to the different organs to permit adequate, though not normal, function. These children will develop symptoms at a later age.
Although there is some oxygen supply, it is not enough for active play or work. The child with TGA and VSD will tire easily during any prolonged activity. Giddiness and fainting spells may occur. In addition there are problems related to the VSD. These include frequent chest infections, breathlessness and in long standing cases, development of high pressure and wall thickening in the blood vessels of the lung – Pulmonary Hypertension.
And then there is the extremely fortuitous combination of TGA with VSD along with narrowing of the pulmonary valve – Pulmonary Stenosis. These patients have all the benefits of a TGA with VSD, with one added advantage. The pulmonary valve which guards blood flow into the lungs is narrow. This prevents too much blood entering the lungs, and avoids chest infection, breathlessness, and most important, the later development of pulmonary hypertension. These patients sometimes survive into their 20’s and 30’s without treatment.
When should TGA be treated?
The answer in most cases is “at birth“. There are many reasons for this. Most cases of TGA will not survive without immediate prostaglandin therapy. The results of operation are best when performed early. And suitability for different types of operation vary as the child becomes older.
No other congenital heart defect better demonstrates the close co-operation between cardiologist, interventionalist and surgeon in bringing about excellent outcome as does TGA.
What are the treatment options?
This is a rather complicated topic, so I will split it into sections – initial stabilization, palliative treatment and definitive treatment.
Initial stabilization of the sick patient with TGA
As I mentioned earlier, the first treatment measure is to improve mixing of venous and arterial blood. And the best method is to start a “prostaglandin drip”. This must only be done in a well equipped hospital, under the guidance of qualified physicians. Prostaglandin can suppress breathing, and sometimes the child may need to be connected to an artificial respirator. Oxygen may need to be administered by an “oxygen tent” or mask.
Most complex heart defects may need to be treated in stages. The first procedure aims at improving the condition of the child, so that he or she is better prepared to tolerate the more rigorous definitive operation. In TGA, palliation is usually achieved by trans-catheter methods.
A catheter is a special thin tube passed into the blood vessels through a small needle-stick in the groin or forearm, and guided into the heart. Through this catheter, a special device that resembles a balloon is passed into the heart. The wall between the right and left atrium is punctured and the catheter device pushed through the small hole thus created. The balloon is then inflated, and the catheter is pulled back through the small hole, tearing it and making it larger.
This procedure is called a Balloon Atrial Septostomy (BAS) and was first devised by Dr.Rashkind. The aim is to create a large enough opening between the two atria, so that blood can freely mix across it, and improve oxygen supply.
Sometimes, a balloon atrial septostomy does not provide adequate mixing, or cannot be carried out at all. In this situation, surgical creation of a “hole” in the wall between the atria is necessary. The operation was first described by Drs.Blalock and Hanlon, and is called an atrial septectomy.
It is a “closed-heart” operation, in which a part of the atrial septum is surgically removed after applying special clamps on the heart to prevent bleeding. The results of atrial septectomy are usually good.
And finally “definitive treatment”
Basically, there are two ways to deal with the abnormality of TGA. First, the venous blood returning to the right atrium can be diverted to the left ventricle, from where it will go to the lungs through the pulmonary artery. At the same time, the pure blood returning from the lungs will be diverted to the right ventricle which will pump it into the aorta. This operation is performed from within the atrium, and is called an Atrial Switch Operation.
The second approach is easier to understand. The aorta and pulmonary artery are divided, their positions “switched” and stitched back to their correct ventricles – the aorta to the left ventricle and the pulmonary artery to the right. This is called the Arterial Switch Operation. Of course, nothing in congenital heart surgery is really simple, and there are any number of special circumstances that make modifications of the treatment approach necessary.
Arterial Switch Operation
This elegant operation is simple in concept. Yet it was only first successfully performed in the late 1970s by Dr.Jatene in Brazil. This was mainly due to the technical difficulty in connecting the coronary arteries to the new aorta.
Coronary arteries as you know are the first branches of the aorta, and supply blood to the heart muscle itself.
How is an arterial switch operation (ASO) done?
Surgery is carried out through an opening in the middle of the chest. The heart will have to be stopped temporarily during the operation. So the surgeon will first hook up the patient to the heart-lung machine. The aorta and pulmonary artery are disconnected from their abnormal attachments. Their positions are then “switched“.
The aorta is stitched back to the left ventricle and the pulmonary artery to the right ventricle. A VSD is closed, if present. The coronary arteries are also freed, and connected back to the aorta using very delicate hair-thin sutures. When you consider that the size of these coronary arteries in a new-born is hardly a millimeter, you can imagine the technical skill and expertise that the surgeon must possess to carry out this connection without mishap.
What are the advantages of an arterial switch operation?
- It restores normal structure to the heart. After the operation, the anatomy of the heart is just as in normal people. The heart is almost as good as new.
- The left ventricle is what pumps blood to the rest of the body. In the atrial switch operation, it is the right ventricle that does this job. Nature designed the right ventricle for less severe work – pumping blood to the lungs alone. When it is made responsible for the blood flow to the rest of the body, it may not be able to cope with the increased demands. Over many years, it may “fail”. With the left ventricle, there is no such problem.
- Since there is very little suturing to be done inside the heart itself, the chances of later scarring causing rhythm disturbances – arrhythmias – are less with arterial switch operation.
What are the problems with an ARTERIAL switch operation (ASO)?
ASO is a technically demanding and difficult operation, and may take sometime to perfect. Different surgeons and institutions have varying results. ASO is not suited for all patients. Abnormalities in coronary arteries greatly increase the difficulty of an ASO. And an ASO cannot be performed in patients who come to medical attention at an older age, or who have severe pulmonary valve abnormalities.
What is the long term outcome after ASO?
When an ASO is performed well, the mortality risk is minimal, and long term survival is excellent. Essentially all patients are active without any limitations. Although the longest follow-up is barely 20 years, it is predicted that about 90% of operated patients will survive to this period.
Late complications are uncommon. Rare problems are obstruction at the site of connection of the pulmonary artery to the right ventricle, producing Pulmonary Stenosis, and sometimes leak of the new aortic valve – Aortic Regurgitation. A second operation has hardly ever been necessary for these complications.
Atrial Switch Operations
Now that you have read about the Arterial Switch Operation (ASO), let us now see what the other operations for TGA are.
How is an atrial switch operation done?
There are two types of atrial switch operations – the Mustard operation, and the Senning operation.
Both are similar in principle, but differ in technique.
The atrial switch operation is an open heart procedure and is carried out with the assistance of a heart-lung machine. The right atrium is opened, and the wall between the atria is fully removed. Using pericardium (Mustard) or flaps created from the atrial septum and wall (Senning), a “baffle” is constructed directing blood from the veins in the right atrium towards the left ventricle.
The same baffle also directs blood from pulmonary veins to the right ventricle. The circulation is therefore restored to normal in a functional sense.
When should an atrial switch operation be preferred?
The atrial switch was the first to be developed for TGA and was frequently performed in the 1970’s and early 80’s. It has many drawbacks, and so today, the ARTERIAL switch is the operation of choice for TGA.
Still, the Mustard and Senning operations are not obsolete.
Some patients are not suited for an arterial switch procedure. This may be due to severe abnormalities of their coronary arteries, which make it very difficult for the surgeon to re-implant them at operation. If the patient comes to medical attention at a later age – beyond 1 to 2 months – an arterial switch is not possible.
Some institutions may not have adequate experience with arterial switch, which is a difficult operation to learn. In any of these situations, an atrial switch procedure – Mustard or Senning – may be preferred.
What are the problems with an ATRIAL switch procedure?
While risks of operation are similar to ASO, the long term outcome is slightly inferior to it. Rhythm disturbances – Arrhythmias – are very frequent and occur in almost one-half of all survivors.
Exercise capacity is less than normal. Delayed failure of the right ventricle may occur. This is unusual with simple TGA, but is more common in patients with a coexisting VSD.
Another major drawback of the atrial switch is the chance of obstruction of the superior vena cava (SVC) – the vein which returns impure blood from the head and arms back to the heart. The SVC may become blocked by the “baffle” inside the atrium and produce problems needing re-operation.
Some Special Circumstances
It is after much deliberation that I decided to add this section. It is not essential, and if you don’t understand it, skip it!
TGA after 2 months of age – the Rapid Two-Stage ASO
When a child with TGA comes for treatment only after 4 to 8 weeks, the left ventricle is too weak to support the circulation after an ASO. To prepare the left ventricle, an initial operation called “banding” is done.
A tape or thread is placed around the artery arising from the left ventricle (i.e. pulmonary artery in TGA) to constrict it and make it narrow. The left ventricle is thus “trained” to work against the higher resistance produced by this narrow outflow and becomes stronger and thick walled.
Within two weeks, an ASO is then performed with a better outcome, since the left ventricle is now capable of handling its workload of pumping blood to the entire body. This is called the rapid two-stage ASO and was devised by Dr.Aldo Castaneda.
TGA with a narrow pulmonary valve – Alternate Operations.
After an ASO, the pulmonary valve is now shifted to the left ventricle, a region of higher pressures than the right. If the pulmonary valve is narrow or abnormal, it cannot function normally in the high pressure area. So other procedures have been devised for these patients.
- RASTELLI procedureIn this operation, the pulmonary valve is surgically closed. A tube or conduit in which a valve is placed, is used to connect the right ventricle to the pulmonary artery. This avoids including the abnormal pulmonary valve in the circulation. One disadvantage with this operation is that in very small children, a second operation will be needed to replace the conduit as the child outgrows it.
- LECOMPTE (or REV) procedureThis is similar to the Rastelli operation, but aims to avoid the complications related to using a conduit. The pulmonary artery is moved close to the right ventricle, and sewn to it directly, using a patch of pericardium to widen the junction.
- NIKAIDOH procedureIn this operation, the aorta is “switched” along with the aortic valve and placed in the pulmonary position. This avoids leak of a faulty pulmonary valve on the left side.