The Fontan procedure or Fontan–Kreutzer procedure is a palliative surgical procedure used in children with univentricular hearts. It involves diverting the venous blood from the inferior vena cava (IVC) and superior vena cava (SVC) to the pulmonary arteries. The procedure varies for differing congenital heart pathologies. For example, in tricuspid atresia, the procedure can be done where the blood does not pass through the morphologic right ventricle; i.e., the systemic and pulmonary circulations are placed in series with the functional single ventricle. By contrast, in hypoplastic left heart syndrome, the heart is more reliant on the more functional right ventricle to provide blood flow to the systemic circulation. The procedure was initially performed in 1968 by Francis Fontan and Eugene Baudet from Bordeaux, France, published in 1971, simultaneously described in July 1971 by Guillermo Kreutzer from Buenos Aires, Argentina, presented at the Argentinean National Cardilogy meeting of that year and finally published in 1973.

The Fontan Kreutzer procedure is used in pediatric patients who possess only a single functional ventricle, either due to lack of a heart valve (e.g. tricuspid or mitral atresia), an abnormality of the pumping ability of the heart (e.g. hypoplastic left heart syndrome or hypoplastic right heart syndrome), or a complex congenital heart disease where a bi-ventricular repair is impossible or inadvisable. The surgery allows blood to be delivered to the lungs via central venous pressure rather than via the right ventricle. Patients typically present as neonates with cyanosis or congestive heart failure. Fontan completion is usually carried out when the patient is 2–5 years of age, but is also performed before 2 years of age.

There are four variations of the Fontan procedure:

The Fontan-Kreutzer procedure is the third procedure in the staged surgical palliation. It is performed in children born with congenital heart disease without two functional ventricles and an effective parallel blood flow circuit.

The first stage is known as the Norwood procedure. This stage generally involves combining the pulmonary artery and aorta to form a larger vessel for blood to get to the body. An artificial tube or shunt can be placed from this larger vessel to the pulmonary arteries so that blood can get from the heart to the lungs. The wall between the left and right atrium can be removed to allow the mixing of oxygenated and de-oxygenated blood.

The second stage is called the hemi-Fontan or the Bidirectional Glenn procedure. This intermediary stage incorporates the shifting of oxygen-poor blood from the top of the body to the lungs. The superior vena cava (SVC), which carries blood returning from the upper parts of the body, is disconnected from the heart and instead redirects the blood into the pulmonary arteries. The inferior vena cava (IVC), which carries blood returning from the lower body, continues to connect to the right atrium.

The third stage is called the Fontan-Kreutzer procedure which involves redirecting the blood from the inferior vena cava to the lungs. At this point, the oxygen-poor blood from upper and lower body flows through the lungs without being pumped (driven only by the pressure that builds up in the veins or central venous pressure). This improves the lower than normal oxygen levels and results in one functional ventricle that is responsible for supplying blood to the rest of the body. There are currently three various modern techniques for the Fontan procedure which include: Atriopulmonary connection, lateral tunnel total cavopulmonary connection, and extracardiac conduit.

After Fontan Kreutzer completion, blood must flow through the lungs without being pumped by the heart. Therefore, children with high pulmonary vascular resistance may not tolerate a Fontan procedure. Often, cardiac catheterization is performed to check the resistance before proceeding with the surgery. This is also the reason a Fontan procedure cannot be done immediately after birth; the pulmonary vascular resistance is high in utero and takes months to drop. Fontan procedure is also contraindicated in those with pulmonary artery hypoplasia and significant mitral insufficiency.^{[citation needed]}