Truncus arteriosus (TRUNG-kus ahr-teer-e-O-sus) is a rare heart defect that's present at birth (congenital). If your baby has truncus arteriosus, it means that one large blood vessel leads out of the heart. Normally, there are two separate vessels coming out of the heart.
In addition, the two lower chambers of the heart are missing a portion of the wall that divides them. As a result of truncus arteriosus, oxygen-poor blood that should go to the lungs and oxygen-rich blood that should go to the rest of the body are mixed together. This creates severe circulatory problems.
If left untreated, truncus arteriosus can be fatal. Surgery to repair truncus arteriosus is generally successful, especially if the repair occurs before your baby is 2 months old.