The cause of pulmonary atresia is not clear. During the first six weeks of pregnancy, the baby's heart begins to form and starts beating. The major blood vessels that run to and from the heart also begin to develop during this critical time. It's at this point in a baby's development that a congenital heart defect such as pulmonary atresia may begin to develop.
To understand how pulmonary atresia occurs, it may be helpful to know how the heart works.
How the heart works
The typical heart is made of four chambers. There are two upper chambers, called atria, and two lower chambers, called ventricles.
The right side of the heart moves blood to the lungs. In the lungs, blood picks up oxygen and then returns it to the heart's left side. The left side of the heart then pumps the blood through the body's main artery, called the aorta. The blood goes to the rest of the body.
In pulmonary atresia, the pulmonary valve doesn't form as usual so it can't open. Blood can't flow from the right ventricle to the lungs.
Before birth, not having a pulmonary valve opening doesn't affect the baby's oxygen. That's because the baby gets oxygen from the tissue that connects the baby to the womb, called the placenta. The oxygen-rich blood from the placenta goes to the baby's right upper heart chamber.
The blood going into the right side of the baby's heart then passes through a hole between the top chambers of the baby's heart. The hole is called the foramen ovale. It lets oxygen-rich blood move to the rest of the baby's body through the aorta.
After birth, the lungs are needed for oxygen. In pulmonary atresia, without a working pulmonary valve, blood must find another way to reach the baby's lungs.
Blood from the right side of the heart can cross over the foramen ovale to the left heart. From there it can be pumped to the aorta. Newborn babies have a temporary opening called the ductus arteriosus between the aorta and the pulmonary artery. This opening lets some blood travel to the lungs. There the blood picks up oxygen to send to the rest of the baby's body.
The ductus arteriosus most often closes soon after birth. But medicines can keep it open.
Sometimes there's a second hole in the tissue between the main pumping chambers of the baby's heart. This hole is a ventricular septal defect (VSD).
The VSD lets blood flow from the right lower heart chamber to the left lower heart chamber. People with pulmonary atresia and a VSD often have other changes with the lungs and the arteries that bring blood to the lungs.
If there's no VSD, the right lower heart chamber gets little blood flow before birth. The chamber often doesn't form fully. This is a condition called pulmonary atresia with intact ventricular septum (PA/IVS).