Atrioventricular Canal Defect

What is atrioventricular canal defect?

Atrioventricular (AV) canal defects are a relatively common family of congenital heart defects—anatomical differences that are present at birth. Also known as atrioventricular septal defects or endocardial cushion defects, they account for about 5 percent of all congenital heart disease, and are most common in infants with Down syndrome.

Atrioventricular canal defect is a complex heart problem that involves several abnormalities of structures inside the heart, including the following:

• Atrial septal defect (ASD)—a hole in the septum (wall) between the two upper chambers of the heart, known as the right and left atria.

• Ventricular septal defect (VSD)—a hole in the septum (wall) between the two lower chambers of the heart, known as the right and left ventricles.

• Improperly formed AV valves (mitral and tricuspid)—these separate the upper heart chambers (atria) from the lower heart chambers (ventricles) and are not formed correctly. Specifically, there is an abnormality in the left-sided valve (the mitral valve), often resulting in one large “common” valve rather that two separate valves. This common valve between the upper and lower chambers doesn’t close properly, which allows blood to leak backward from the heart’s lower chambers to the upper chambers. This leak (called regurgitation or insufficiency) can occur on the right side, left side, or both sides of the heart. With a valve that leaks, the heart pumps an extra amount of blood, then becomes enlarged and overworked.
  
  

Types of atrioventricular canal defect

There are three types of atrioventricular canal defects: complete, partial and transitional.

• A complete atrioventricular canal defect is one in which there are defects in all structures formed by the endocardial cushions. Therefore, there are defects (holes) in the atrial septum and ventricular septum, and the AV valve remains undivided or "common."

• A partial or incomplete atrioventricular septal defect is one in which the part of the ventricular septum formed by the endocardial cushions has filled in, either by tissue from the AV valves or directly from the endocardial cushion tissue, and the tricuspid and mitral valves are divided into two distinct valves. The defect is, therefore, primarily in the atrial septum and mitral valve. This type of atrial septal defect is referred to as an ostium primum atrial septal defect, and is usually associated with a cleft in the mitral valve that may cause the valve to leak.

• The transitional type of defect looks similar to the complete form of atrioventricular septal defect, but the leaflets of the common AV valve are stuck to the ventricular septum thereby effectively dividing the valve into two valves and closing most of the hole between the ventricles. As a result of the effectively small defect between the ventricles, a transitional atrioventricular septal defect behaves more like a partial atrioventricular septal defect, even though it looks more like a complete atrioventricular septal defect.
  
  

Causes of atrioventricular canal defect

When the heart is forming during the first eight weeks of fetal development, it begins as a hollow tube. The partitions that form within the tube eventually become the walls dividing the right side of the heart from the left side. Sometimes, as the fetus is growing, something occurs to affect heart development during the first eight weeks of pregnancy and certain areas of the heart do not form properly.

Atrial and ventricular septal defects occur when the partitioning process doesn’t occur completely, leaving openings in the atrial and ventricular walls. The valves that separate the upper and lower heart chambers are formed toward the end of this eight-week period, and often they don’t develop properly either. Frequently, instead of two separate AV valves (tricuspid and mitral valve), there is a single large common valve that sits between the upper and lower chambers of the heart, allowing blood to flow freely between the chambers above and below the valve — mixing oxygen-rich and oxygen-poor blood.

Children with Down syndrome or other chromosomal abnormalities frequently also have a significantly higher risk for congenital heart disease. About half the children born with Down syndrome have some kind of congenital heart disease, and close to half of those cases are atrioventricular canal defects.

Because of this, your doctor may recommend genetic testing if your baby is diagnosed with a heart defect.

Diagnosing atrioventricular canal defects

Atrioventricular canal defects may be diagnosed within the first 20 weeks of pregnancy, during a scheduled ultrasound. If your doctor notices a concern consistent with this defect, they likely will order a fetal echocardiogram. The Fetal Health Center at Children’s Mercy can assist in the diagnosis of atrioventricular canal defect, along with further genetic testing that your doctor recommends.

Signs and symptoms of atrioventricular canal defects

Often, atrioventricular canal defects are diagnosed shortly after birth or within the first month of birth. Your pediatrician may notice some of the following signs with your baby:

• heart murmur
• disinterest in feeding, or tiring while feeding
• poor weight gain
• fatigue
• sweating
• pale skin
• cool skin
• rapid breathing
• heavy breathing
• rapid heart rate
• congested breathing
• blue color

If your child has any of these symptoms, your pediatrician will likely refer you to a pediatric cardiologist for testing and treatment.

Testing for atrioventricular canal defects

Several types of medical tests can help your doctor diagnose atrioventricular canal and its associated defects:

Basic Testing:

• Echocardiogram (echo)—an ultrasound of the heart that evaluates the structure and the function of the heart by using sound waves. Still and moving pictures of the heart structures, heart valves, and heart function are recorded for review by a cardiologist. This test is not painful.

• Electrocardiogram (ECG)— a visual representation of the heart's electrical activity captured via monitors placed on the skin. This test is not painful.
  
  

Advanced Testing—These studies may require sedation for completion:

• Cardiac MRI—a test that produces images (or pictures) of the body with the use of x-rays. The MRI uses a large magnet, radio waves, and a computer program to produce three-dimensional image of your chest that can show heart abnormalities. This test is not painful.

• Cardiac catheterization—a procedure where a catheter (small tube) is inserted into your baby’s heart through a large vein or artery in the leg to take pictures and pressure measurements. There may be some soreness at the insertion site following this procedure.