Embryology of anomalous pulmonary venous connections. (A) Partial anomalous pulmonary venous connection occurs when one or more of the pulmonary veins fail to establish connection with the CPV before connections with the splanchnic venous system have regressed. (B) Total anomalous pulmonary venous connection occurs when all the pulmonary veins fail to establish connection with the CPV before connections with the splanchnic venous system have regressed. CPV, common pulmonary vein; LA, left atrium; LCCV, left common cardinal vein; LLB, left lung bud; RA, right atrium; RCCV, right common cardinal vein; RLB, right lung bud; UV, umbilical vein.