What is hypoplastic left heart syndrome?

Hypoplastic left heart syndrome (HLHS) is a combination of several abnormalities of the heart and great blood vessels. It is a congenital (present at birth) syndrome, meaning that the heart defects occur due to abnormal underdevelopment of sections of the fetal heart during the first 8 weeks of pregnancy.

In the normal heart, oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped through the pulmonary artery into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, and then is pumped out to the body through the aorta.

In hypoplastic left heart syndrome, most of the structures on the left side of the heart are small and underdeveloped. The degree of underdevelopment differs from child to child. The structures affected usually include the following:

  • Mitral valve. The valve that controls blood flow between the left atrium and left ventricle in the heart.
  • Left ventricle. The lower left-hand chamber of the heart. It receives oxygen-rich (red) blood from the left atrium and pumps it into the aorta, which takes the blood to the body. The left ventricle must be strong and muscular in order to pump enough blood to the body to meet its requirements.
  • Aortic valve. The valve that regulates blood flow from the heart into the aorta.
  • Aorta. The largest artery in the body and the primary blood vessel leading from the heart to the body.

Perhaps the most critical defect in HLHS is the small, underdeveloped left ventricle. This chamber is normally very strong and muscular so it can pump blood to the body. When the chamber is small and poorly developed, it will not function effectively and cannot provide enough blood flow to meet the body's needs. For this reason, an infant with hypoplastic left heart syndrome will not live long without surgical intervention.

  • Hypoplastic left heart syndrome occurs in 2 to 3 out of every 10,000 live births or 1 in every 4,344 babies born in the U.S. each year.
  • The syndrome comprises 2 to 3 percent of all cases of congenital heart disease.
  • HLHS occurs slightly more often in boys than in girls.