What is truncus arteriosus?

Truncus arteriosus is a congenital (present at birth) defect that occurs due to abnormal development of the fetal heart during the first eight weeks of pregnancy. The heart begins as a hollow tube, and the chambers, valves, and great arteries develop throughout the first eight weeks of pregnancy. The aorta and pulmonary artery start as a single blood vessel, which eventually divides and becomes two separate arteries. Truncus arteriosus occurs when the single great vessel fails to separate completely, leaving a connection between the aorta and pulmonary artery.

Truncus arteriosus is a complex defect where there is a single (normally there are two separate arteries) vessel arising from the heart that forms the aorta and pulmonary artery. Another congenital heart defect that occurs with truncus arteriosus is a ventricular septal defect (ventricular septum, or dividing wall between the two lower chambers of the heart known as the right and left ventricles).

Normally, there are two separate arteries (the aorta and the pulmonary artery. Oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped through the pulmonary artery into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, and then is pumped through the aorta out to the body.

In truncus arteriosus, oxygen-poor (blue) and oxygen-rich (red) blood mix back and forth through the ventricular septal defect. This mixed blood then flows through the common truncal vessel. Some of it will flow through the branch that becomes the pulmonary artery and on to the lungs, and some of the mixed blood will go into the aortic branch and continue to the body. The mixed blood that goes to the body does not have as much oxygen as normal, and will cause varying degrees of cyanosis (blue color of the skin, lips, and nail beds).

Truncus arteriosus occurs is uncommon, and affects 1 to 4 percent of all cases of congenital heart disease.