Atrioventricular canal defect

Atrioventricular (AV) canal defect is a large hole in the centre of the heart. The defect is situated where the septal wall between the upper chambers (atria) joins the septal wall between the lower chambers (ventricles). In addition, the tricuspid and mitral valves (the atrioventricular valves), which normally separate the heart's upper and lower chambers, are not formed as individual valves. Instead, one large valve bridges the defect.

Diagram 2.8 Atrioventricular (AV) canal defect 1 - atrial septal defect 2 - abnormal tricuspid valve 3 - abnormal mitral valve 4 - ventricular septal defect The shunt or abnormal flow is from left atrium to right atrium, left ventricle to right ventricle (as indicated by the shaded red arrows). Tricuspid and mitral valve regurgitation occurs as a result of the abnormal tricuspid and mitral valves.

Complete AV canal defect, also known as an atrioventricular septal defect or endocardial cushion defect, is often associated with other cardiac defects, such as Tetralogy of Fallot. The complete AV canal defect is commonly found in children with Down's syndrome. An incomplete or partial form of AV canal defect, known as an ostium primum atrial septal defect, involves only the atrial septum.

The AV canal defect lets some of the oxygen-rich blood from the heart's left side pass (or shunt) back into the heart's right side. There, the oxygen-rich blood mixes with the oxygen-poor venous blood from the body and is sent back to the lungs. This results in the heart pumping an extra amount of blood and working harder than it should. In time, the extra work causes the heart to enlarge. There is also an associated increase in the pressure in the pulmonary artery, which may cause problems.

With complete AV canal defects, signs and symptoms occur early in infancy. These usually include abnormal heart sounds, congestive heart failure, intermittent cyanosis, respiratory infections, poor feeding and inadequate weight gain.

In a baby with severe symptoms or high pulmonary artery pressure, surgery must usually be done in infancy. The surgeon closes the hole with one or two patches. The single valve is then divided between the atria and the ventricles to make two separate valves. Again, care must be taken not to damage the heart's electrical conduction system.