Normally, blood flows through the pulmonary artery into the lungs, where it picks up oxygen. Then the blood travels back to the heart and is pumped to the aorta and the rest of the body.
Babies with an aortopulmonary window have a hole in between the aorta and pulmonary artery. Because of this hole, blood from the aorta flows into the pulmonary artery, and too much blood flows to the lungs. This causes high blood pressure in the lungs (a condition called pulmonary hypertension) and heart failure. The bigger the defect, the more blood can enter the pulmonary artery.
The condition occurs when the aorta and pulmonary artery do not divide normally as the baby develops in the womb.
Aortopulmonary window is very rare. It accounts for only 0.1% of all congenital heart defects.
This condition can occur on its own or with other heart defects such as tetralogy of Fallot or pulmonary atresia. Fifty percent of patients usually have no other heart defects.