Major aortopulmonary collateral arteries (MAPCAs) develop early in embryonic life but regress as the normal pulmonary arteries (vessels that will supply deoxygenated blood to the lungs) develop.In certain heart conditions the pulmonary arteries do not develop and the collaterals continue to grow. Teh collaterals then become the main supply of blood to the lungs.
The pulmonary arteries, located at the base of the right ventricle, tansport deoxygenated blood from the body. The collateral arteries however carry blood which has already be oxygenated by the lungs,so are of little use in helping the body to get oxygen.
The aim of treatment of the MAPCAs is to group them together and convert their supply to deoxygenated blood from the right side of the heart.