The aorticopulmonary (AP) septum is derived from neural crest cells that migrate into the primitive ventricular outflow tract. It is responsible for separating the truncus arteriosus into the aorta and pulmonary artery. As the septum descends, it spirals 180 degrees so that the aorta becomes the left ventricular outflow tract and the pulmonary trunk becomes the right ventricular outflow tract. Failure of spiraling leads to congenital malformations that involve right-to-left shunting and early cyanosis in the newborn period.
- Persistent truncus arteriosus results from abnormal migration of neural crest cells and subsequent failure of formation of the AP septum. Therefore, separation of the left ventricular and right ventricular outflow tracts never occurs. The aorta and pulmonary trunk form a common tract leaving the ventricles, which allows mixing of oxygenated and deoxygenated blood.
- Transposition of the great vessels occurs when the AP septum fails to spiral 180 degrees. The left ventricle (LV) is connected to the pulmonary trunk, and the right ventricle (RV) is connected to the aorta. This condition results in a complete right-to-left shunt and early cyanosis.
- Tetralogy of Fallot is caused by anterior displacement of the AP septum. The four abnormalities are overriding aorta, pulmonic stenosis, RV hypertrophy, and VSD. The primary defect is termed an “overriding aorta,” because the misplaced aorta partially obstructs the right ventricular outflow tract, leading to right ventricular outflow obstruction (pulmonic stenosis). Pulmonic stenosis leads to increased pressures in the RV and subsequent right ventricular hypertrophy. The membranous VSD results from a failure of fusion between the AP septum and the muscular portion of the intraventricular septum (IVS). Right-to-left shunting results in early cyanosis.