The proper development of the atrioventricular canal into its prospective components (The heart septum and associated valves) to create a clear division between the four compartments of the heart and ensure proper blood movement through the heart, are essential for proper heart function. When this process does not happen correctly, a child will develop atrioventricular canal defect which occurs in 2 out of every 10,000 births. It also has a correlation with Down Syndrome because 20% of children with Down Syndrome suffer from atrioventricular canal disease as well. This is a very serious condition and surgery is necessary within the first six months of life for a child.  Half of the children who are untreated with this condition die during their first year due to heart failure or pneumonia.
Atrioventricular canal defect is a combination of abnormalities of the heart and is present at birth. There is a problem when there are holes present in the walls that separate chambers (septa),as well as when valves are incorrectly constructed. There are other names for these heart abnormalities such as endocardial cushion defects or atrioventricular septal defect
In a normal healthy heart there are four chambers. This consists of right and left atria, and right and left ventricles. The right atrium and right ventricle function to pump blood to the lungs while the left atrium and left ventricle pump blood to the rest of the body. There are valves in place that inhibit back-flow between these chambers.
Atrioventricular canal defect
This defect is developed because of the improper formation of the endocardial cushions, and their job is to separate the different parts of the heart during development when they fuse. It is strongly associated with Down's syndrome. The structures that develop from the fusion of the endocardial cushions are:
- The Atrial septum- this structure divides the left and right atrium
- The Ventricular septum-this structure divides the left and right ventricles.
- The Mitral Valve and tricuspid valve - these valves are formed by the proper division of an early common valve being separated into two.
Atrioventricular canal defect may be divided into partial or complete forms. In the partial form, openings between the left and right atria and improper formation of the mitral valve exist. In the complete form, there is free movement in all chambers because there is a large hole where the atria and ventricles meet, and instead of there being two valves there is one common valve. 
Surgery is usually conducted in-between the 3rd and 6th month of life, and with this condition the earlier the better. Usually, intracardiac repair is completed which is when they close the holes in the septum and they make two new atrioventricular valves that is available from the underdeveloped common valve leaflet.
- Brown, David MD "Atrioventricular Canal Defect" Children's Hospital Boston 20120 http://www.childrenshospital.org/az/Site521/mainpageS521P0.html
- Calabro, Raffaele & Limongelli, Giuseppe "Complete Atrioventricular Canal" 2006 April 5, US National Library of Medicine National Institutes of Health http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1459121/
- Mayo Clinic September 12, 2012"Atrioventricular Canal Defect" http://www.mayoclinic.com/health/atrioventricular-canal-defect/DS00745
- Cincinnati Children's Hospital "Atrioventricular Septal Defect" April 2010 http://www.cincinnatichildrens.org/health/a/avsd/