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Normal heart (click to enlarge)

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.

See About the Heart and Blood Vessels for more information on how the heart normally works.

Hypoplastic left heart syndrome (click to enlarge)

A pediatric cardiologist specializes in the diagnosis and medical management of congenital heart defects, as well as heart problems that may develop later in childhood. The cardiologist will perform a physical examination, listening to the heart and lungs, and make other observations that help in the diagnosis. Other tests are needed to help with the diagnosis.

The Fontan procedure takes advantage of the fact that blood flows so easily through the mature lung that it does not need the force of the right ventricle to push it through. In fact, the great veins bringing "blue" blood back to the heart, called the superior vena cava and inferior vena cava, can be channeled directly to the lungs through the pulmonary artery, bypassing the right ventricle altogether. When this is achieved, the right ventricle (in HLHS, the only ventricle) can be devoted to pumping "red" blood to the rest of the body. In this way, "blue" blood pathways and "red" blood pathways are separate, with "blue" blood reaching the lungs passively, where it is oxygenated before returning to the single ventricle as "red" blood, then pumped to the body.

The blood vessels of the lungs change over the first year of life. The Fontan procedure depends on sufficiently easy flow through the lungs to not require the force of a ventricle to conduct blood through the pulmonary circuit. This is not possible in the first months of life, and two operations must precede the Fontan procedure until the lungs are ready.

The Stage I operation (Norwood Procedure) connects the right ventricle to the aorta so that its flow, which will eventually be "red" blood, will be delivered to the body through branches of the aorta. This usually involves rebuilding a small aorta and connecting it to the ventricle. To survive the newborn period, blood flow to the immature lungs must be forceful. Blood flow to the lungs is provided through a tube from a branch of the aorta to the pulmonary artery called the Modified Blalock-Taussig Shunt.

Usually, at between three and eight months of age, the lungs mature to the point that blood flows more easily through, and ventricular force is no longer necessary. The Stage II, or Bi-directional Glenn Shunt, is a direct connection between the superior vena cava and the pulmonary artery, diverting half of the "blue" blood directly to the lungs without the assistance of the ventricle.

Finally, with still further lung maturity, the vessels of the lungs can accommodate all the body's "blue" blood and the Stage III, or Fontan procedure, can be done. The Fontan procedure connects the inferior vena cava to the pulmonary artery by creating a channel, or baffle, through the heart to direct its flow to the pulmonary artery. Now all the "blue" blood flows passively to the lungs, and the single ventricle pumps exclusively "red" blood to the body.