There is no known cause of pulmonary atresia. It's thought that the problems that cause pulmonary atresia begin early in the pregnancy. To understand the problems pulmonary atresia causes, it's helpful to know how the heart works.
How the heart works
The heart is divided into four hollow chambers, two on the right and two on the left. In performing its basic job — pumping blood throughout the body — the heart uses its left and right sides for different tasks. The right side of the heart moves blood to the lungs through vessels called pulmonary arteries. In the lungs, blood picks up oxygen then returns to the heart's left side through the pulmonary veins. The left side of the heart then pumps the blood through the aorta and out to the rest of the body to supply your baby's body with oxygen. Blood moves through your baby's heart in one direction through valves that open and close as the heart beats. The valve that allows blood out of your baby's heart and into the lungs to pick up oxygen is called the pulmonary valve.
In pulmonary atresia, the pulmonary valve doesn't develop properly, preventing it from opening. Blood can't flow from the right ventricle to the lungs. Before birth, the improperly formed valve isn't life-threatening, because the placenta provides oxygen for your baby instead of the lungs. Blood entering the right side of your baby's heart passes through a foramen ovale — a hole between the top chambers of your baby's heart (atria) that lets oxygen-rich blood move to the left side of the heart and be pumped on to the rest of your baby's body.
After birth, your baby's lungs must provide oxygen for his or her body. In pulmonary atresia, without a working pulmonary valve, blood must find another route to reach your baby's lungs.
The foramen ovale often shuts soon after birth, but may stay open in pulmonary atresia, allowing oxygen-poor blood to pass through the upper chambers of the heart. From there, it goes to the left ventricle, out the aorta (the body's main artery), on to the rest of the body. However, this blood flow can't adequately supply your baby's body with oxygen.
Newborn babies also have a temporary connection between the body's aorta and the pulmonary artery, called the ductus arteriosus. This allows some of the oxygen-poor blood to pass into the lungs where it can pick up oxygen to supply your baby's body. The ductus arteriosus normally closes within a few hours or days after birth, but can be kept open with medications.
In some cases, there may be a second hole in the tissue that separates the main pumping chambers of your baby's heart, called a ventricular septal defect (VSD). The VSD allows a pathway out for blood to enter the left ventricle. If there's no VSD, the right ventricle receives little blood flow before birth and often doesn't develop fully. This is a condition called pulmonary atresia with intact ventricular septum (PA/IVS).
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