Embryology

Pulmonary atresia with ventricular septal defect and major aortopulmonary collateral arteries (PA/VSD/MAPCA[s]) is a member of the conotruncal family of defects. The embryologic development of the heart is dictated by the migration of neural crest cells, which influence both septation of the outflow tracts and development of the semilunar valves. Approximately 30% of patients with PA/VSD/MAPCAs have DiGeorge syndrome (22q11.2 deletion) and roughly three to four percent have Alagille syndrome. These associations demonstrate the close link between genetics and this form of congenital heart defect.

The embryology of the pulmonary vasculature is a complex and step-wise process. In very early gestation, the human fetus has blood vessels that originate from a systemic origin (most from the descending thoracic aorta) and supply blood flow to the lungs. At about the 20^th day of gestation, the right and left branch pulmonary arteries fuse and shortly thereafter are supposed to connect with the right ventricular outflow tract. This connection and the subsequent antegrade flow from the right ventricle are signals for involution of the systemic-based vessels. However, if the connection from the right ventricle to the pulmonary arteries does not form, and in the absence of a ductus arteriosus, the systemic-based vessels persist in what have become known as MAPCAs.