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 congenital heart defects such as tricuspid atresia, the cause of congenital heart disease is unknown in most cases.
How the heart works
Your heart is divided into four chambers — two on the right (right atrium and right ventricle) and two on the left (left atrium and left ventricle). In performing its basic job — pumping blood throughout the body — your heart uses its left and right sides for different tasks.
The right side of the heart moves blood to the lungs through the pulmonary artery. 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.
When things go wrong
In tricuspid atresia, the right side of the heart can't properly pump blood to the lungs because the tricuspid valve — located between the upper right chamber (right atrium) and the lower right chamber (right 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 (hypoplastic).
Blood instead flows from the right atrium to the left atrium 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 is supposed to close soon after birth. If a baby with tricuspid atresia doesn't 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. In some cases, blood flows through a natural opening between the aorta and pulmonary artery (ductus arteriosus).
The ductus arteriosus is a normal part of a baby's circulatory system before birth and usually closes soon after birth. When necessary, this passageway is kept open using medication in babies with tricuspid atresia.
Many babies with tricuspid atresia have another heart defect — a hole between the ventricles (ventricular septal defect). In these cases, some blood can flow through the hole between the left ventricle and the right ventricle, and then blood is pumped to the lungs through the pulmonary artery.
However, the valve between the right ventricle and the pulmonary artery (pulmonary valve) may be narrowed in some babies with tricuspid atresia, which can reduce the blood flow to the lungs. If the pulmonary valve isn't narrowed and if the ventricular septal defect is large, too much blood may flow to the lungs, which can lead to heart failure.
Some babies with tricuspid atresia have another defect, in which the pulmonary artery and the aorta are switched, causing blood flow to be impaired and unable to flow through its normal cycle. If this occurs, the baby generally needs a ventricular septal defect between the ventricles, in order for the blood to flow from the left ventricle to the right ventricle. If the ventricular septal defect is narrowed and blood flow is reduced, a ductus arterious may be needed to allow blood to flow between the aorta and pulmonary artery.
Some babies may have other heart defects as well.