[ASD] [VSD] [PDA] [Coarctation] [Tetralogy of Fallot] [Pulmonary Atresia] [Tricuspid Atresia] [HLHS] [Transposition] [DORV] [TAPVC] [AV Canal] [Pulmonary Stenosis] [Truncus Arteriosus] [Ebstein's Anomaly] [Coronary Anomalies] [Single Ventricle]
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You have now graduated to being able to read about a complicated condition like tricuspid atresia and the Fontan principle ! STILL, please understand that this anomaly is so complex that even cardiologists have trouble understanding its repair. So if you don't, console yourself that you are in distinguished company ! Or PREPARE yourself first: Read my introduction articles on "What is the Heart ?"
Triscupid Atresia is a condition where the TRICUSPID VALVE, which guards the junction between the right atrium and the right ventricle, is either ABSENT or is IMPERFORATE - that is, it does not have an opening to allow blood flow across it. There are many ways the valve can be imperforate - the leaflets of the valve may be formed but tightly stuck to each other, or may not be formed at all, with muscle tissue of the heart forming a wall where the valve should have been.
What is immediately apparent is that blood cannot flow across the tricuspid valve from the right atrium into the right ventricle. And if there is no alternate route, circulation of blood is not possible. But Mother Nature always has the solution to such problems ready. In addition to the absence of a normal tricuspid valve, patients with tricuspid atresia have a "hole" in the wall between the right atrium and left atrium - an ATRIAL SEPTAL DEFECT (ASD).
Blood returning to the right atrium from the veins crosses the ASD into the LEFT atrium. It then flows into the left ventricle, from where it may take a variable path. This path depends on the presence of other defects, like pulmonary stenosis and transposition of the great arteries (TGA).
We have seen, in an earlier article on Tetralogy of Fallot, about VENO-ARTERIAL MIXING. This is a situation where "impure" venous blood mixes with "pure" oxygen-rich arterial blood. It occurs in tricuspid atresia too, when blood mixes across the ASD.
As a result of veno-arterial mixing, many effects are seen in children with tricuspid atresia. First is the presence of bluish discoloration - called CYANOSIS. Cyanosis occurs because of an abnormally low amount of oxygen in blood entering the arteries. Next, the risk of paradoxical embolism is present in tricuspid atresia patients.
The further clinical course of these patients depends on other defects being present as well. If there is pulmonary stenosis - that is, narrowing of the pulmonary valve, which guards the junction of the right ventricle with the blood vessel entering the lungs (pulmonary artery) - blood flow into the lung is reduced, causing deep cyanosis and necessitating early repair. If there is transposition of great arteries, blood flow into the aorta may be reduced if the Ventricular Septal Defect (VSD), which is always present in these cases, is very small.
Although it may seem that placing an artificial valve between the right atrium and ventricle will solve the problem, it is not possible. This is because, since there has been no tricuspid valve since birth, the area where the valve SHOULD HAVE BEEN is not developed at all, and is very small. So alternate methods to repair the condition had to be devised.
After several "false starts", Dr.Francis Fontan first made the revolutionary attempt to completely "do away" with the right ventricle ! He successfully created something which today is called a "Fontan-type" circulation.
What is a Fontan-type circulation ?
Let me take some time to explain this, as it is an integral part of the entire operation for tricuspid atresia.
We - you, me, cardiologists, surgeons, and everyone else - have been accustomed thinking of the heart as having four chambers - two atria and two ventricles. These four chambers acting in unison maintain the circulation of blood.
To understand the Fontan circulation, you must make a "leap of imagination". In your mind, eliminate the right ventricle from the heart ! Tough isn't it ? And how can the heart possibly work without a right ventricle ?
Illogical as it may seem, this however was exactly what Dr.Fontan proved with his operation. In his original repair, he connected the right atrium directly to the pulmonary artery, and closed the ASD. Blood entering the right atrium from the veins passed across this surgical connection into the pulmonary artery and to the lungs. It completely bypassed the right ventricle !
Wait a minute. There must be a flaw in this somewhere ! How can the blood enter the lungs if it is not PUMPED IN by the right ventricle ? Well, that really is what makes this procedure unique - and stands testimony to the genius of Dr.Fontan ! Normally the right ventricle will do the pumping. But in tricuspid atresia - and many other conditions in which a Fontan operation is performed - there is NO right ventricle. So blood flows PASSIVELY into the lungs - without being propelled into them by a right ventricle.
Why is lung blood flow so important ? Because it is the only place in the body that blood can be purified by the addition of oxygen. So when lung blood flow is very low, oxygen supply is reduced to the entire body. This has many harmful effects, since no organ can perform its work normally without oxygen for energy.
First, you must understand that any fluid flowing in a tube will continue to move, becoming slower and slower, until the RESISTANCE offered by the tube makes it stop. In a Fontan type circulation, the left ventricle pumps blood into the aorta and arteries. This blood flows at first rapidly into the different organs. The very same force PUSHES the blood across capillaries, and through the veins, but with lesser force. Slowly, blood enters the right atrium, and then passes across the surgical connection into the lungs - all the while unaided by a right ventricle !
But by its very nature, this flow depends on many factors. For instance, if the blood vessels in the lung are thick walled and narrow before surgery, they will offer very high resistance to passive blood flow. In such a state, the Fontan operation cannot be performed, or will have a high risk of failure, since the extra energy needed to maintain lung blood flow is not available.
Even normally a small amount of resistance will exist across the lung blood vessels. After a Fontan operation, the pressure in the veins will therfore be higher than normal, in order to overcome this resistance and maintain lung blood flow.
The elevated pressure in the veins has a few ill effects. First, there may be swelling of the entire body due to fluid from the blood leaking out of the vein walls. There may be facial puffiness, fluid accumulation in the abdomen (ascites) or chest (pleural effusion). Sometimes even absorption of nutrients from the intestines is affected.
Ten characteristics were identified which would permit a good outcome after the Fontan operation, called the TEN COMMANDMENTS !
The ten commandments included data which could be obtained before surgery by examining the patient, and carrying out tests like echocardiography and cardiac catheterization. When more of the ten commandments are "obeyed", the better are the chances of a happy - "heavenly" - result from surgery.
In effect, all these criteria were to ensure that the resistance of blood vessels in the lung was not too high. A high resistance would interfere with passive lung blood flow. This could be produced by very small pulmonary arteries, blood vessel wall thickening and hardening, mitral valve leak or reduced function of the left ventricle. In all of these conditions, a Fontan operation would not be performed, or modified to reduce the risks.
Ever since its first description, the Fontan operation has been modified many times. Each modification aimed to avoid one of the drawbacks of the previous types. While some are definitely better, others are not very different. We are still striving to devise the "best" type of Fontan repair for each group or individual.
As I mentioned earlier, in the original Fontan operation, the venous blood was diverted to the lungs directly from the right atrium, and the ASD was closed. While Dr.Fontan used an artificial valve between the inferior vena cava (IVC) and the right atrium, future modifications eliminated this.
The aims of the "ideal" Fontan operation are
Total Cavo-pulmonary Connection (TCPC) operation
A group of surgeons headed by Dr.Marc de Laval in the United Kingdom has come up with a detailed computer simulation program. This software allows creation of a computer image of the heart after different modifications of the Fontan operation to allow study of blood flow patterns with each. The type that allows smooth flow will be preferred because less energy is wasted by "turbulence", and allows more energy to be used for achieving lung blood flow.
Based on their experimental studies performed earlier, this group suggested the idea of a LATERAL TUNNEL FONTAN repair. In this, a "tunnel" is created within the right atrium using an artificial fabric patch. The tunnel links the superior and inferior vena cava (SVC and IVC) to each other. The tunnel is then connected to the pulmonary artery. Because of this, the operation is also called TOTAL CAVO-PULMONARY CONNECTION - or TCPC. The advantage with this technique is that blood flow is laminar, and energy loss due to turbulent flow is least, and thus improving lung blood flow.
Later improvements of the TCPC operation have allowed
One other major modification was suggested by Drs.Hillel Laks and Bridges. This is the FENESTRATED FONTAN operation. When the Fontan operation has to be done in children who have an elevated lung blood vessel resistance, the outcome is unlikely to be good. This is because lung blood flow after operation will be low. Also, the pressure of blood in the veins will increase to maintain lung blood flow. And this elevated vein pressure has its own ill effects.
To avoid such problems, it was suggested that a "hole" be left in the fabric patch used to create the tunnel. Through this hole, the tunnel could "decompress" into the left atrium whenever the pressure in the tunnel becomes dangerously high. While this causes a reduction in oxygen content in arterial blood, it still avoids sudden decrease in the amount of blood pumped by the heart. This modification has proved life-saving in many cases, and has allowed a much wider usage of the Fontan operation in borderline cases. After some time, when the patient's condition has stabilised, this hole can be closed using trans-catheter techniques or by tightening a purse-string stitch that had been placed at the time of surgery.
Although not exactly a Fontan operation, a bidirectional Glenn shunt is similar to it - and is called a type of "Partial Fontan" operation. The bidirectional shunt is performed by connecting the superior vena cava (SVC) to the right branch of the pulmonary artery using fine sutures, and dividing or tying up the pulmonary artery. Now, venous blood from the head and upper limbs will pass directly to the lungs, bypassing the right ventricle. The venous blood from the lower body however will continue to enter the heart.
At a second operation, the lower body venous blood will also be diverted to the lungs, thus creating the "complete" Fontan circulation.
The bidirectional Glenn shunt is preferred in very small babies - below 2 years of age - in whom the lung vessel resistance is still quite high, and in borderline cases with abnormal pulmonary arteries. While avoiding the risk of failure of a complete Fontan operation, it also partly relieves symptoms. If all is well after the Glenn shunt, and the lung vessel resistance is low, a change to complete Fontan is possible. Otherwise, nothing further is done.
When the right ventricle is not too small, it may be included in the circulation with some benefit. In Bjork's modification of the Fontan procedure, a passage is created using pericardium, connecting the right atrium to the small right ventricle. As blood flows into it, the right ventricle grows in size. It will then help pump blood into the lungs, and avoid some of the disadvantages of a "classical" Fontan.
This new operation attempts to combine the advantages of both the above strategies. A connection is made between the superior vena cava and right pulmonary artery, just as in a bidirectional Glenn shunt. But the pulmonary artery is not interrupted, thus allowing blood from the inferior vena cava also to enter the pulmonary artery. This blood is pumped partly by a small (about "one-half normal" sized) right ventricle, hence the name "one and a half ventricle repair" !
I hope you enjoyed reading this piece as much as I did writing it !
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