The Fontan is an open-heart surgery used to treat a number of congenital heart defects (conditions that are present at birth). It is typically used on children who have complex cardiac defects in which the heart has only one working chamber (ventricle) or has no wall separating the right and left ventricles.

In a normal heart, oxygen-poor (blue) blood flows into the right atrium (upper chamber) from large veins. Then the blood flows through the tricuspid valve and into the right ventricle. From there, the pulmonary arteries carry the blood into the lungs, where it receives oxygen. Once it has been oxygenated, the blood is carried back to the left side of the heart through the pulmonary veins. It first empties into the left atrium, then travels through the mitral valve into the left ventricle. Finally, the left ventricle pumps the oxygen-rich blood through the aortic valve into the aorta and out to the body.

If, however, a child is born with only one operating ventricle, or with a defect that prevents the normal flow of blood through the heart, physicians may choose to perform a Fontan procedure.

 During this operation, surgeons create a passageway that allows oxygen-poor blood from the body to bypass the heart and go directly to the pulmonary arteries. By routing blood around the heart, surgeons are able to adapt the right ventricle to pump oxygen-rich blood to the body in the event the left ventricle is unable to. In this way, the right ventricle is no longer used to pump blood to the lungs. Instead, the circulation to the lungs is considered passive.

The same principle applies if the left ventricle is healthy and the right ventricle is inadequate.  The basic process is that the oxygen-poor blood is delivered to the pulmonary artery circuit without going through the heart. The oxygen-rich blood returns to the heart to be delivered to the body with only a small amount of oxygen-poor blood in the mixture.

The Fontan procedure was first performed in 1968 by Dr. Francis Fontan to treat tricuspid atresia. Currently, it is performed worldwide to treat a variety of congenital heart defects, including:

The Fontan is also performed as part of the Norwood procedure, which is used to treat hypoplastic left heart syndrome.

Typically, the Fontan procedure is part of a series of surgeries meant to gradually reroute the flow of blood through the heart while also maintaining healthy circulation. These procedures are usually performed over the course of years or months, allowing the child to recover between operations. Other operations that are performed as part of the operations associated with a Fontan procedure include:

• Bi-directional Glenn procedure
  
• Glenn shunt operation (or modified Glenn shunt operation)
  
• Glenn anastomosis
  
• Hemi-Fontan procedure

In a Glenn shunt operation or its modifications, the superior vena cava (which collects oxygen-poor blood from the upper part of the body) is connected directly to the pulmonary artery (which travels from the right ventricle to the lungs).

This new pathway allows oxygen-poor blood from the head and upper body to flow directly to the lungs, bypassing the heart. At this point, the baby will be better able to handle infection and other problems. However, the treatment is only half-finished because the oxygen-poor blood returning from the lower part of the body has not yet been rerouted

The second stage of the Fontan is usually done when the patient is between two and three years old. During this stage, the inferior vena cava (which collects oxygen-poor blood from the lower part of the body) is connected directly to the pulmonary artery. Once both stages of the surgery are complete, oxygen-poor blood from both the upper and lower parts of the body can travel to the lungs for fresh oxygen.

As with most complex congenital heart surgeries, there are a number of variations that might arise in particular circumstances. A fenestrated Fontan procedure is one in which surgeons have created a tiny hole in the conduit that carries blood to the pulmonary arteries as it travels through the right atrium. This hole acts as an "escape valve" if the blood pressure rises too high. In some cases, this hole will be closed later, in a catheter laboratory. In other cases, the hole will close naturally. Fenestrated Fontan procedures are used among children who may have elevated blood pressure in the lungs and who previously had higher rates of complications with a regular Fontan procedure. Besides the fenestrated Fontan, there are other variations of this technique, depending on the unique condition of each patient and the surgeon's preference.