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Causes
By Mayo Clinic staff
Tricuspid atresia occurs during fetal growth when your baby's heart is developing. While some factors, such as heredity or Down syndrome, may increase your baby's risk of tricuspid atresia, in most cases the cause is unknown.
The normal-functioning heart
Your heart is divided into four chambers, two on the right and two on the left. In performing its basic job — pumping blood throughout the body — your heart uses its left and right sides for different tasks. The right side moves blood to the lungs. In your lungs, oxygen enriches the blood, which then circulates to your heart's left side. The left side of the heart pumps blood into a large vessel called the aorta, which circulates the oxygen-rich blood to the rest of your body. Valves control the flow of blood into and out of the chambers of your heart. These valves open to allow blood to move to the next chamber or to one of the arteries, and they close to keep blood from flowing backward.
Holes in the walls
In tricuspid atresia, the right side of the heart cannot properly pump blood to the lungs because the tricuspid valve, located between the upper right chamber (atrium) and the lower right chamber (ventricle), is missing. Instead, a solid sheet of tissue blocks the flow of blood from the right atrium to the right ventricle. As a result, the right ventricle is usually very small and underdeveloped.
Blood instead flows from the upper right chamber to the upper left chamber through a hole in the wall between them (septum). This hole is either a heart defect (atrial septal defect) or an enlarged natural opening (foramen ovale) that usually closes soon after birth. If a baby with tricuspid atresia does not have an atrial septal defect, the baby may need a procedure to create this opening.
Once the blood flows from the right atrium to the left atrium, the heart's left side must pump blood both to the rest of the body and to the lungs. The blood reaches the lungs from the left side of the heart through another natural opening between the right and left sides (ductus arteriosus) that usually closes soon after birth. When necessary, this passageway is kept open using medication in babies with tricuspid atresia. (When the foramen ovale and the ductus arteriosus are open, they are referred to as being "patent.")
Some babies with tricuspid atresia have another heart defect — a hole between the lower two chambers (ventricular septal defect). In these cases, blood can flow through the hole and into the right ventricle, which pumps it to the lungs.
