Causes

Truncus arteriosus occurs when your baby's heart is developing in the womb, and is, therefore, present at birth (congenital). In most cases the cause is unknown.

The heart

Your heart has four pumping chambers that circulate your blood. The "doors" of the chambers (valves) control the flow of blood, opening and closing to ensure that blood flows in a single direction.

The heart's four chambers are:

• The right atrium, the upper right chamber, receives oxygen-poor blood from your body and delivers it into the right ventricle.
• The right ventricle, the lower right chamber, pumps the blood through a large vessel called the pulmonary artery and into the lungs, where the blood is resupplied with oxygen.
• The left atrium, the upper left chamber, receives the oxygen-rich blood from the lungs and delivers it into the left ventricle.
• The left ventricle, the lower left chamber, pumps the oxygen-rich blood through a large vessel called the aorta and on to the rest of the body.

Normal heart development

The formation of the fetal heart is complex. At a certain point, all babies have a single large vessel (truncus arteriosus) exiting the heart. During normal development of the heart, however, this very large single vessel divides into two parts.

One part becomes the lower portion of the aorta, which is attached to the left ventricle. The other part becomes the lower portion of the pulmonary artery, which is attached to the right ventricle.

Also during this process, the ventricles develop into two chambers separated by a wall (septum).

Truncus arteriosus in newborns

In babies born with truncus arteriosus, the single large vessel never finished dividing into two separate vessels and the wall separating the two ventricles never closed completely, resulting in a single blood vessel arising from the heart, and a large hole between the two chambers (ventricular septal defect).

In addition to the primary defects of truncus arteriosus, the valve controlling blood flow from the ventricles to the single large vessel (truncal valve) is often defective, so that it does not close completely, allowing blood to flow backward into the heart.