A hypoplastic left heart syndrome (HLHS) baby tends to be born well developed, despite the serious defects on the left side of the heart. The reason for this healthy in–utero development is that the normal fetal circulatory system has an open ductus arteriosus, or connection between the pulmonary artery and the aorta that allows blood to bypass the lungs.

In an HLHS fetus, this in-utero system allows blood to circulate normally before the baby is born, despite the fact that the left ventricle is not helping to pump blood to the rest of the body. However, the ductus arteriosus normally closes soon after the baby takes its first breath, closing off the connection between the right ventricle and the aorta. As a result, the malformed left ventricle becomes solely responsible for pumping blood to the body, and the signs of HLHS quickly begin to appear.

Most infants with HLHS will manifest signs of their condition within 48 hours of birth, while still in the nursery. In some cases, however, an infant will be discharged and develop serious complications because the ductus closes spontaneously. In this case, the child may go into serious shock and experience multiorgan failure, seizures, kidney failure and other complications. The severity of these complications, and any long-term damage, depends on how long it takes to reach a hospital and how severe the resulting injuries are. When this happens, the emergency medical team will first seek to establish blood flow to the infant, then stabilize and regain function in his or her noncardiac internal organs.

Fortunately, with the advent of modern fetal ultrasound, HLHS is frequently diagnosed before birth, thus allowing physicians and parents time to develop a treatment plan immediately after birth.