Truncus arteriosus is a rare type of congenital heart disease in which a single blood vessel (truncus arteriosus) comes out of the right and left ventricles, instead of the normal two (pulmonary artery and aorta).

There are different types of truncus arteriosus, depending on the anatomy of the single vessel.

In normal circulation, the pulmonary artery comes out of the right ventricle and the aorta comes out of the left ventricle, which are separate from each other. Coronary arteries (which supply blood to the heart muscle) come out of the aorta just above the valve at the entrance of the aorta.

In truncus arteriosus, a single artery comes out of the ventricles. There is usually also a large hole between the two ventricles (ventricular septal defect). As a result, the blue (without oxygen) and red (oxygen-rich) blood mix.

Some of this mixed blood goes to the lungs, some goes to the coronary arteries, and the rest goes to the body. Usually, too much blood is sent to the lungs.

If left untreated, two problems occur:

• Too much blood circulation in the lungs may cause extra fluid to build up in and around them, making it difficult to breathe.
• The blood vessels to the lungs become permanently damaged. Over time, it becomes very hard for the heart to force blood to them. This is called pulmonary hypertension and it can be life-threatening.