Hypoplastic left heart syndrome is a complex and rare heart defect present at birth (congenital). In hypoplastic left heart syndrome, the left side of the heart is critically underdeveloped.
If your baby is born with hypoplastic left heart syndrome, the left side of the heart can't effectively pump blood to the body, so the right side of the heart must pump blood to the lungs and to the rest of the body.
Medication to prevent closure of the connection (ductus arteriosus) between the right and left sides, followed by either surgery or a heart transplant, is necessary to treat hypoplastic left heart syndrome. With advances in care, the outlook for babies born with hypoplastic left heart syndrome is better now than in the past.
Hypoplastic left heart syndrome care at Mayo Clinic