Truncus arteriosus is a cyanotic congenital heart disease in which one single great vessel with a single semilunar valve gives rise to the aorta, pulmonary arteries and the coronary arteries. There are four morphological types depending on the origin of pulmonary arteries from the truncus. In type I, a common pulmonary trunk arises from the truncus arteriosus and divides into left and right pulmonary arteries.
In type II, the two pulmonary arteries arise separately, but adjacent to each other, from the truncus. In type III, the two pulmonary arteries arise separately from either sides of the truncus. In type IV pulmonary arteries arise from the descending aorta. That is equivalent to the pseudotruncus with pulmonary atresia and major aortopulmonary collaterals (MAPCAS). There are other types of classifications as well. Another situation is hemitruncus in which one pulmonary artery arises from the aorta.
The origins of the pulmonary arteries from the truncus may be stenotic and thereby restricting the pulmonary blood flow. Sometimes one pulmonary artery may be absent. Right aortic arch is more common with truncus arteriosus. Absent pulmonary artery is on the same side as the arch in truncus while it is on the opposite side in tetralogy of Fallot.
If the origins of the pulmonary arteries are not stenotic, the babies with truncus arteriosus develop heart failure due to the large pulmonary blood flow and pulmonary obstructive vascular disease develops early. Truncal valve regurgitation is a common association with truncus arteriosus and so is a ventricular septal defect. DiGeorge syndrome is another association of truncus arteriosus.
Chest X-ray will show increased pulmonary vascularity and the high origin of the left pulmonary artery. Oligemia of the lung field is seen when one pulmonary artery is absent. Pulmonary vascular disease tends to develop early on the opposite side in such cases as the whole pulmonary blood flow goes through that lung. ECG often shows features of biventricular hypertrophy.
The parasternal long axis view on echocardiography will show the overriding great vessel which mimics the pattern in tetralogy of Fallot or pulmonary atresia. But a high parasternal view will demonstrate the pulmonary arteries originating from the posterior aspect of the truncus arteriosus.
Corrective surgery for truncus has to be done early in infancy. Otherwise the prognosis is poor. This is because truncus arteriosus is one of the conditions in which pulmonary vascular disease develops early. In situations in which corrective surgery is not immediately feasible, pulmonary artery banding can be done as a palliative procedure to decrease the pulmonary blood flow. Since the band has a tendency to slip and obstruct one branch, banding of individual pulmonary arteries is considered superior to banding of the main pulmonary artery.
Banding can cause kinking and obstruction of the pulmonary arteries. In corrective surgery, a valved homograft is the preferred conduit for connecting the right ventricle to the pulmonary arteries. Dacron graft is another option. Truncal valve may need to be replaced if it is severely incompetent. This adds to the problem as a suitable valve for the infant is difficult to obtain. In those with successful repair, conduit replacement may be required at a later date. The risk of conduit replacement alone is not that high.