Pulmonary atresia (PA) is a complicated congenital (present at birth) defect that occurs due to abnormal development of the fetal heart during the first 8 weeks of pregnancy.
The pulmonary valve is found between the right ventricle (RV) and the pulmonary artery. It has three leaflets that function like a one-way door, allowing oxygen depleted blood to flow from the right pumping chamber (RV) out to the lungs to pick up oxygen. This valve closes as the RV relaxes so that blood does not go backwards into the right ventricle
With pulmonary atresia, problems with valve development prevent the leaflets from opening, therefore, blood cannot flow forward from the right ventricle to the lungs. Before birth, while the fetus is developing, this actually is not a threat to life because the placenta provides oxygen for the baby and the lungs are not being used to oxygenate the blood. Blood entering the right side of the fetal heart passes through an opening called the foramen ovale that allows oxygen-rich (red) blood from the placenta to pass through to the left side of the heart and proceed to the body.
In some cases, there may be a second opening, this time in the ventricular wall, that allows blood in the right ventricle a way out. This opening is called a ventricular septal defect (VSD). If there is no VSD, the right ventricle receives little blood flow before birth and does not develop fully.
At birth, when the baby is separated from the placenta, the lungs open on the first breath and now must do the work of oxygenating the baby’s blood. However, with no pulmonary valve opening present, blood must find another route to reach the lungs and receive oxygen.
The foramen ovale normally shuts at birth, but may stay open in this situation, allowing oxygen-poor (blue) blood to pass from the right atrium to the left atrium. From there, it goes to the left ventricle, out the aorta, to the body. This situation cannot support life, since oxygen-poor (blue) blood cannot meet the body's demands. Newborns also have a connection between the aorta and the pulmonary artery, called the ductus arteriosus, that allows some of the oxygen-poor (blue) blood to pass into the lungs. Unfortunately, this ductus arteriosus normally closes within a few hours or days after birth.
Because of the low amount of oxygen provided to the body, pulmonary atresia is one of the heart problems that is labeled "blue-baby syndrome." Pulmonary atresia occurs in about one out of every 10,000 live births.