Above. Post-delivery graphic anatomy. Truncus arteriosus (TA) is defined by the presence of a single arterial blood vessel which yields flow to the pulmonary, systemic, and coronary circulations. The ascending aorta gives off the pulmonary arteries, and the truncal valve is single and often incompetent resulting in regurgitation. The single valve is usually of from two to four leaflets. The truncus overrides a large peri-menbranous VSD, which allows the pulmonary and systemic venous blood to mix. The size of the pulmonary artery determines the amount of pulmonary blood flow, which if excessive can lead to heart failure, while increased cyanosis may result if the pulmonary flow is inadequate. Other abnormalities include those of the coronary arteries, right aortic arch and DiGeorge syndrome.