Fontan procedure

# Overview
The Fontan procedure is a palliative surgical procedure used in children with complex congenital heart defects. It involves diverting the venous blood from the right atrium to the pulmonary arteries without passing through the morphologic right ventricle.

# Etymology
It was initially described in 1971 as a surgical treatment for tricuspid atresia. 

# Indications
The Fontan procedure is nowadays used where a child only has a single effective ventricle, either due to defects of the heart valves (e.g. tricuspid atresia or pulmonary atresia) or an abnormality of the pumping ability of the heart (e.g. hypoplastic left heart syndrome), or has complex congenital heart disease where a bi-ventricular repair is impossible or inadvisable.
These children have a single effective ventricle supplying blood to the lungs and the body (either from birth or after an initial surgery e.g. Norwood procedure).  They are delicately balanced between inadequate blood supply to the lungs (causing cyanosis) and oversupply to the lungs (causing pulmonary edema).  In addition, the single ventricle is doing nearly twice the expected amount of work (because it has to pump blood for both lungs and body).  As a result, these children can have trouble gaining weight, and are also vulnerable to decompensation in the face of otherwise minor illnesses (even a common cold).  Sometimes medications (e.g. diuretics) can help them through this stage.
Therefore, when either they are large enough, or when they decompensate more than can be managed medically, these children are referred for Fontan procedure.

# Contraindications
After Fontan, 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 pumonary vascular resistance is high in utero and takes months to drop.)

# Approach
The Fontan is usually done as a two staged repair.
The first stage, also called a Bidirectional Glenn procedure or Hemi-Fontan (see also Kawashima procedure), involves redirecting oxygen-poor blood from the top of the body to the lungs.  That is, the pulmonary arteries are disconnected from their existing blood supply (e.g. a shunt created during a Norwood procedure, a patent ductus arteriosus, etc).  The superior vena cava (SVC), which carries blood returning from the upper body, is disconnected from the heart and instead redirected into the pulmonary arteries.  The inferior vena cava (IVC), which carries blood returning from the lower body, continues to connect to the heart.
At this point, patients are no longer in that delicate balance, and the single ventricle is doing much less work.  They usually can grow adequately, and are less fragile.  However, they still have marked hypoxia (because of the IVC blood that is not fed into the lungs to be oxygenated), and the ventricle is still doing more work than expected.  Therefore most patients are referred for another surgery.
The second stage, also called Fontan completion, involves redirecting the blood from the IVC to the lungs as well.  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).  This corrects the hypoxia, and leaves the single ventricle responsible only for supplying blood to the body.

# Post-operative complications
In the short term, children can have trouble with pleural effusions, fluid building up around the lungs. This can require a longer stay in the hospital for drainage with chest tubes. To address this risk, some surgeons make a fenestration (a small hole) from the venous circulation into the atrium. When the pressure in the veins is high, some of the oxygen-poor blood can escape through the fenestration to relieve the pressure. However, this results in hypoxia, so the fenestration may eventually need to be closed by an interventional cardiologist.
In the long term, children can have trouble with atrial arrhythmias, most often micro or macro reentry tachy cardias. There are rare reports of AV nodal reentry tachycardias.  Arrhythmias are more frequent if the connection of IVC to pulmonary arteries involved an intracardiac baffle (instead of an extracardiac conduit). This sometimes requires treatment such as radiofrequency ablation. There are other long-term risks, including protein-losing enteropathy and chronic renal insufficiency, although understanding of these risks is still incomplete. Some patients require long-term blood thinners.
The Fontan procedure is palliative, not curative. But in many cases it can result in normal or near-normal growth, development, and exercise tolerance. In some cases, patients will eventually require heart transplantation.