In Hypoplastic Left Heart Syndrome, blood flow to the body is severely restricted. After birth, a baby with Hypoplastic Left Heart Syndrome will be ill and have poor color. In some cases where the Ductus Arteriosus remains open (or patent) - known as the Patent Ductus Arteriosus (PDA in diagram at left), there may be surprisingly few initial symptoms.

However, when the Patent Ductus Arteriosus (PDA) closes, the infant enters profound shock. Without treatment, the infant will generally die within a few days to weeks of birth.