The normal pumping chambers of the heart (ventricles) must have an inflow valve to let blood in, a good-sized pumping chamber, and an outflow to allow blood to exit the chamber during contraction. In tricuspid atresia, the tricuspid valve, which lies between the heart’s upper right chamber (atrium) and lower right chamber (ventricle), does not develop. As a result, it prevents oxygen-depleted blood that is returning to the right atrium from the body from directly flowing into the right ventricle – the chamber that normally pumps blood to the lungs where it picks up oxygen. Instead, the venous blue blood must pass through a hole between the upper filling chambers (atria), where it then mixes with oxygen-rich blood returning from the lungs via the pulmonary veins. This combined blood then passes to the left lower pumping chamber.
Commonly in tricuspid atresia, the right ventricle may be small because blood flow to the right ventricle is blocked by the non-existent tricuspid valve, and the lower right chamber does not receive a normal amount of blood. In many situations, there is a hole between the two lower pumping chambers (ventricular septal defect, VSD). Some of the blood ejected by the left ventricle passes out to the aorta on its way to the body. Some of the blood may cross the VSD to go to the lungs. Additionally there may be muscular narrowing in the right ventricle under the pulmonary valve that keeps blood from entering the pulmonary arteries. In some situations, the main vessels exiting the heart, the aorta and pulmonary arteries, may be connected to the wrong pumping chamber (transposed).
Progression and Possible Complications
How a child born with tricuspid atresia is affected after birth is dependent on how much blood flow is going to the lungs and if the flows to the lungs and body are balanced. It can be too little, too much, or just right. If there is not enough blood going to the lungs from the heart to pick up oxygen, then the ductus arteriosus must be kept open, much like opening the back door if the front door is blocked, before surgery is performed. This is done using a medication called prostaglandin E1. If there is too much blood going to the lungs, an operation such as a pulmonary artery banding procedure may have to be performed to limit the amount of blood that is allowed to pass into the lungs. This will allow a more balanced circulation and will protect the lungs from the high pressures of the left ventricle. If a baby is born with some natural narrowing, this may provide enough balance to prevent the need for an operation within the first few months of life.
Rashkind Atrial Septostomy
Occasionally, the hole between the atria – the PFO – is too small or may become too small over time to allow sufficient blood to cross it and so requires enlargement. It can be made larger through a procedure called Rashkind balloon atrial septostomy.
In this procedure, a doctor inserts a balloon-tipped catheter (a small, thin tube) into a vein in the leg, guides it into the heart’s right atrium and through the PFO into the left atrium. In the left atrium, the balloon is inflated. The catheter is then pulled back sharply into the right atrium, causing a tear in the atrial wall. This tear enlarges the opening between the two chambers to allow more blood to flow through.
To keep the ductus arteriosus open so that blood can continue to flow to the lungs from the aorta, a medication called prostaglandin (PGE1) is given through an IV (intravenous) line. The medication cannot be used for prolonged periods, so surgery is performed soon after birth to replace the ductus arteriosus with a man-made blood vessel.
The goal of surgery in treating tricuspid atresia is to separate the circulation of oxygen-poor blood (blue) and oxygen-rich blood (red) – not to duplicate the anatomy of a normal heart. A new tricuspid valve cannot simply be placed between the right atrium and right ventricle. Surgery also aims to improve oxygen delivery to the body and relieve the extra burden placed on the heart as it works harder to supply blood and oxygen to the body.
A single ventricle pathway is typically chosen for children with tricuspid atresia. Separation of the blue and red circulation is achieved through three surgical procedures:
- Soon after birth, a vessel is inserted between the aorta and the pulmonary artery to enable adequate blood flow to the lungs to mimic the function of the ductus arteriosus, which naturally will shrink with time. This new vessel is called an aorto-pulmonary shunt or modified Blalock-Taussig (B-T) shunt.
- In the second procedure, usually performed at 6 to 9 months of age, the main vein that returns blood from the head, neck and arms (the superior vena cava) is connected directly into the blood vessels going to the lungs (the pulmonary arteries). This procedure is called the bidirectional Glenn or hemi-Fontan procedure. The previously placed B-T shunt is removed at this time. It is no longer needed since oxygen depleted venous blood coming back from the upper body provides the blood supply to the lungs. Blood coming back from the veins of the lower body continues to pass directly into the heart and eventually mixes with the blood from the lungs before going out to the body. Thus the bluish discoloration of the child’s skin persists.
- When the child reaches about 18 months of age to 3 years, the third surgery is performed. In this procedure, called the Fontan procedure, the vein (inferior vena cava) carrying oxygen-depleted venous blood from the lower body is connected directly to the pulmonary arteries, either using a tube which bypasses the heart or by creating a baffle within the heart to direct the blood upward to the lungs. The pulmonary arteries then send the blood to the lungs to pick up oxygen. After completion of the Fontan procedure, the blue and red blood circulations are fully separated. However, there is no pumping chamber to propel blood through the lungs. Blood passively moves through the lungs and returns to the heart via the pulmonary veins.
Infants who undergo the Fontan procedure have only one functioning ventricle to pump blood to the body. In order for the procedure to succeed, that ventricle needs to be reasonably healthy and strong enough to “suck” blood from the pulmonary (lung) circulation and propel it forward into the body. If it is not strong enough, the procedure will fail. The valves related to the main ventricle also need to function correctly. Leakage or narrowing of these valves will place additional strain on the ventricle.
Also, after the Fontan procedure, there is a risk that the heart’s right atrium will become stretched, although this occurs less frequently with today’s more refined surgical techniques. If the atrium is stretched, abnormal heart rhythm may result. Abnormal heart rhythm will interfere with the function of the heart in pumping enough blood to the body, resulting in fatigue, fainting and heart failure.