AV Canal (Atrio-ventricular septal defect, endocardial cushion defect, atrial ventricular canal defect)
AV canal is a complex congenital heart defect that results in a large hole in the center portion of the heart. This defect may involve several abnormalities of structures inside the heart:
Atrial septal defect (ASD), which is a hole between the two upper heart chambers.
Ventricular septal defect (VSD), which is a hole between the two lower heart chambers.
Maldevelopment of the tricuspid and mitral valves. Most patients have a single (common) large and leaky AV valve instead.
The end result of these heart defects is “volume overload.” The heart is working harder trying to compensate for the excessive amount of blood being retained inside the heart plus a large amount of blood passing through the heart and lungs.
The primary defect in this malformation is failure of formation of the part of the heart that arises from an embryonic structure called the endocardial cushions. These cushions are responsible for separating the central parts of the heart at the level of the tricuspid and mitral valves. The structures that are supposed to develop from this endocardial cushions include the lower part of the atrial septum (the wall that divides the two upper heart chambers) and the ventricular septum (the middle wall that divides the two lower heart chambers). These cushions also complete the separation of the mitral and tricuspid valve by dividing the embryonic single AV valve into two. In the complete form of AV canal there is failure of formation of the mitral and tricuspid valves and a great part of the middle walls of the heart.
The AV canal may be associated with Down syndrome, other chromosomal abnormalities and heterotaxia (splenic syndromes). Other conditions may be associated with development of AV canal.
Many times patients with AV canal have other congenital heart defects, some of them quite complex in themselves
The doctor will usually hear a heart murmur or may refer an asymptomatic child who has Down syndrome. These patients may present with no murmur at all. Symptoms may occur at any time from birth to a few months of age. Most infants with AV canal do not grow normally and they become undernourished because of the large amount of blood flowing through the heart and lungs. Heart failure (CHF) and pulmonary hypertension (high blood pressure in the lungs) may develop during early infancy. In addition, if left untreated this heart defect can cause irreversible lung damage. The symptoms depend on the size of the holes and leakage of the common AV valve as well as the amount of volume overload (increased amount of blood passing through the heart and lungs) and if there are associated heart defects. Symptoms may include:
Poor feeding or tiring out
Poor weight gain
Rapid or heavy breathing
Rapid heart rate
Pale, cold or dusky skin
Electrocardiogram, Echocardiogram, chest x-ray, blood tests and, in some cases, cardiac catheterization. The echocardiogram is an essential part of the cardiovascular evaluation as it helps determine how much the common AV valve is leaking, the amount of holes and how large the holes are. It also helps in identifying any associated heart defects and any underdeveloped heart chambers.
Most patients with AV canal will require cardiac medications, which may include diuretics (water pill), Digoxin and ACE-inhibitors (Enalapril, Lisinopril). The above treatment helps alleviate symptoms and allows the baby to get big enough to undergo surgical repair with lower risks. Most patients have heart surgery by mid-infancy. Children with a transitional or partial AV canal may undergo surgery between 6 to 24 months of age.
Surgery involves closure of the upper and lower holes with a patch or patches, and reconstruction of the leaky valve. Surgery is considered a palliative procedure (it is not a cure or fix). Many patients will remain with leaky valves or obstruction (mitral stenosis). In addition, the patch may have a residual leak.
The conduction system that conducts the electrical impulse through the heart is very close where the surgical stitches are placed and there is a small chance of creating an electrical heart block, which usually requires implantation of a pacemaker. In any event, the heart surgery is considered low-risk, unless it is a complex type of AV canal with other associated heart defects. In those patients with more complex heart lesions the surgical risk would be more significant. About 10% of patients may require more than one heart surgery during their lifetime. Children with Down syndrome will benefit from special programs that enhance their mental and physical development.