The developing fetal heart begins as a simple tube that gradually specializes over time. In a normal fetus, blood is pumped from a single lower chamber of the heart (ventricle) through a trunk of arteries called the truncus arteriosus. As the fetus develops, a wall (septum) normally forms, dividing the single ventricle into two (left ventricle and right ventricle) and separating the truncus arteriosus into the pulmonary artery and aorta. The pulmonary artery travels from the right side of the heart to the lungs, and the aorta travels from the left side of the heart to the rest of the body.

There are rare cases in which a baby is born with an opening still present between the two ventricles (a ventricular septal defect) and the truncus arteriosus still undivided, or only partially divided. In addition, the valves that normally regulate the flow of blood between the lower ventricles and the pulmonary artery and aorta may be partially or completed fused. As a result, oxygen–rich blood and oxygen–poor blood freely mixes after the baby begins to take its first breaths of oxygen. Some oxygen–rich blood needlessly travels back to the lungs, and some oxygen–poor blood uselessly travels to the rest of the oxygen–demanding body.

This condition is called persistent truncus arteriosus or simply “truncus arteriosus.” Symptoms usually occur in the first week of life. Truncus arteriosus causes abnormal blood flow and increased pressure in the lungs, which may lead to symptoms of congestive heart failure within the first week of life. Babies born with truncus arteriosus may also develop a mild bluish tint the skin, lips, fingernails and other areas of the body due to a lack of oxygen–rich blood, a condition often called blue baby or cyanosis.

Although the condition involves complicated abnormalities of the heart and its structures, modern surgical advances have made it possible for babies born with this condition to have an excellent outlook.