Truncus arteriosus is a type of heart defect in which a baby is born with one large blood vessel—normally there are two—that receives blood from both the heart's ventricles. This single blood vessel mixes oxygen-rich blood with oxygen-poor blood. Oxygen-rich blood and oxygen-poor blood normally remain in two separate blood vessels in the heart.

Babies who are born with truncus arteriosus usually die of heart failure unless they are treated very soon after birth. This condition can be corrected with surgery to divide the single large blood vessel into two separate blood vessels so that blood can circulate properly.