Hypoplastic Left Heart Syndrome (HLHS)

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What is hypoplastic left heart syndrome?

Hypoplastic left heart syndrome (HLHS) is one of several congenital (present at birth) heart defects in which the heart has only one fully functioning ventricle. These conditions are commonly referred to as single ventricle defects.

In an infant and child with a normal heart, oxygen-poor (blue) blood returns to the right atrium from the body, enters the right ventricle and is pumped through the pulmonary artery into the lungs to receive oxygen. The oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle and is pumped out to the body through the aorta. In HLHS, most of the structures on the left side of the heart are too small and underdeveloped (hypoplastic) to provide enough red blood flow for the body’s needs.

Wayne Tworetzky, MD, co-director of the Advanced Fetal Care Center at Boston Children's Hospital, describes the congenital heart defect HLHS

The most critical issue in HLHS is the small left ventricle, which needs to be large enough and strong enough to pump blood out to the body. If it’s too small, it simply can’t function effectively. Other left-heart structures can also be underdeveloped in varying degrees, including:

  • the mitral valve, which controls blood flow between the left atrium and left ventricle in the heart
  • the aortic valve, which regulates blood flow from the left ventricle or main pumping chamber into the aorta
  • the aorta, the largest artery in the body, which supplies oxygen-rich blood to the body and is one of the two great arteries in the heart.

Despite the severity of the heart defect, the fetus or newborn with HLHS is able to survive because of two naturally occurring “holes in the heart” — patent ductus arteriosus and patent foramen ovale — which allow for the communication of blood between the left and right sides of the heart.

Once the PDA begins to close (a natural occurrence), the baby will become extremely sick, due to lack of blood supply to the body. At this point, treatment is necessary for the baby to survive.

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