In heart disease associated with pulmonary atresia and ventricular septal defect, the blood flow pathway from the ventricle to the pulmonary artery is defective, resulting in hypoplasia of the pulmonary artery and the development of collateral blood vessels from the aorta or neck branches (brachiocephalic and subclavian arteries) to the pulmonary artery. The relatively large vessels in this pathway are called major aorto-pulmonary collateral arteries (MAPCAs). The MAPCAs originate from the aorta or cervical artery and then join the pulmonary artery. Pulmonary blood flow is often reduced, but pulmonary areas perfused by MAPCAs that do not have stenotic lesions have high pulmonary flow. The MAPCAs have the following characteristics (Fig. 7.1).