Atrioventricular Canal (AV canal or AVC)

What is an atrioventricular canal defect?

Atrioventricular canal defect (AV canal) is a congenital (present at birth) heart defect. Other terms used to describe this defect are endocardial cushion defect and atrioventricular septal defect (AVSD). 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. AV canal is a complex heart problem that involves several abnormalities of structures inside the heart, including the following:

Anatomy of the heart, normal

 

  • Atrial septal defect. An opening in the interatrial septum, or dividing wall, between the two upper chambers of the heart, known as the right and left atria.
  • Ventricular septal defect. An opening in the interventricular septum, or dividing wall, between the two lower chambers of the heart, known as the right and left ventricles.
  • Improperly formed mitral and/or tricuspid valves. The valves that separate the upper heart chambers (atria) from the lower heart chambers (ventricles) are improperly formed. Specifically, there is an abnormality in the left-sided valve (the mitral valve): it has three cusps, rather than the two cusps that normally form the valve. One of the normal cusps is divided into two cusps. This division between these two cusps is called the cleft (or a cut in the mitral valve).

Normally, oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, and then is pumped out to the body through the aorta.

Anatomy of a heart with an atrioventricular canal defect

An atrial septal defect allows oxygen-rich (red) blood to pass from the left atrium, through the opening in the septum (the wall) between the two atria, and then mix with oxygen-poor (blue) blood in the right atrium.

A ventricular septal defect allows oxygen-rich (red) blood to pass from the left ventricle, through the opening in the septum (the wall) between the two ventricles, and then mix with oxygen-poor (blue) blood in the right ventricle.

Abnormalities of the mitral or tricuspid valves allow blood that should be moving forward from the ventricle into either the pulmonary artery or the aorta to instead flow backward into the atria. This results in leakage of the mitral or tricuspid valves, knows as regurgitation or insufficiency.

Atrioventricular canal defects occur in a small percentage of congenital heart disease cases and are more common in infants with Down syndrome.

Prevention & Risk Treatment

Prevention & Risk Treatment

What causes atrioventricular canal?

The heart is forming during the first eight weeks of fetal development. It begins as a hollow tube, then partitions within the tube develop that eventually become the septa (or walls) dividing the right side of the heart from the left. Atrial and ventricular septal defects occur when the partitioning process does not occur completely, leaving openings in the atrial and ventricular septum. The valves that separate the upper and lower heart chambers are being formed in the latter portion of this eight-week period, and they too do not develop properly.

There may be genetic influences on the development of atrioventricular canal. Consider the following:

  • Congenital heart disease (CHD) is present in half of children born with Down syndrome, and close to half of these cases have AV canal; Down syndrome is caused by the presence of three #21 chromosomes in the cells of the body, rather than the usual pair #21 chromosomes.
  • Similarly, about one-third of all children born with AV canal defect also have Down syndrome.
  • About 15 percent of mothers with an AV canal defect give birth to a child with the disease.

Other chromosome abnormalities (in addition to Down syndrome) are linked to the development of atrioventricular canal. Maternal age can have an effect on the prevalence of AV canal, which may be related to the increased probability of a woman giving birth to a child with Down syndrome as she gets older.

Why is atrioventricular canal a concern?

If not treated, this heart defect can cause lung disease. When blood passes through both the ASD and VSD from the left side of the heart to the right side, then a larger volume of blood than normal must be handled by the right side of the heart. This extra volume of blood is also under high pressure, since the pressure in the left ventricle is high. This blood then passes through the pulmonary artery into the lungs, causing higher volume than normal and higher pressure than normal in the blood vessels in the lungs.

The lungs are able to cope with this extra volume of blood at high pressure for a while. The child will be breathing at a faster rate than normal since the lungs have a lot of extra blood at high pressure compared to normals. After a while, however, the blood vessels in the lungs become damaged by this extra volume of blood at high pressure. The blood vessels in the lungs get thicker. Those changes are reversible at first. With time, these changes in the lungs become irreversible, resulting in irreversible damage to the lungs themselves.

As the arteries in the lungs get thicker, the flow of blood from the left side of the heart to the right side and on to the lungs will diminish. Blood flow within the heart goes from areas where the pressure is high to areas where the pressure is low. If the septal defects are not repaired, and lung disease begins to occur, pressure in the right side of the heart will eventually exceed pressure in the left. In this instance, it will be easier for oxygen-poor (blue) blood to flow from the right side of the heart, through the ASD and VSD, into the left side of the heart , and on to the body. When this happens, the body does not receive enough oxygen in the bloodstream to meet its needs, and patients become cyanotic, or exhibit a blue coloring in their skin, lips, and nail beds.

Because blood is pumped at high pressure through the septal openings, the right ventricle will remain thick. Bacteria in the bloodstream can occasionally infect the abnormal valves in the heart (the abnormal mitral and tricuspid valves associated with AV canal defects), causing a serious illness known as bacterial endocarditis.

What are the symptoms of an atrioventricular canal defect?

The size of the septal openings will affect the type of symptoms noted, the severity of symptoms, and the age at which they first occur. The larger the openings, the greater the amount of blood that passes through from the left side of the heart to the right and overloads the right heart and the lungs.

Symptoms occur in infancy. The following are the most common symptoms of AVC. However, each child may experience symptoms differently. Symptoms may include:

  • Fatigue
  • Sweating
  • Pale skin
  • Cool skin
  • Rapid breathing
  • Heavy breathing
  • Rapid heart rate
  • Congested breathing
  • Disinterest in feeding, or tiring while feeding
  • Poor weight gain

As the pressure in the lungs rises, after years blood within the heart will "shunt" through the septal openings from right heart to the left. This allows oxygen-poor (blue) blood to reach the body, and cyanosis will be noted. Cyanosis gives a blue color to the lips, nail beds, and skin. The symptoms of AVC may resemble other medical conditions or heart problems. Always consult your child's physician for a diagnosis.

Diagnosis

Diagnosis

How is AV canal diagnosed?

Your child's pediatrician may have heard a heart murmur during a physical examination, and referred your child to a pediatric cardiologist for a diagnosis. In this case, a heart murmur is a noise caused by the turbulence of blood flowing through the opening from the left side of the heart to the right. Symptoms your child exhibits will also help with the diagnosis.

A pediatric cardiologist specializes in the diagnosis and medical management of congenital heart defects, as well as heart problems that may develop later in childhood. The cardiologist will perform a physical examination, listening to the heart and lungs, and make other observations that help in the diagnosis. The location within the chest that the murmur is heard best, as well as the loudness and quality of the murmur (harsh, blowing, etc.) will give the cardiologist an initial idea of which heart problem your child may have. Diagnostic testing for congenital heart disease varies by the child's age, clinical condition, and institutional preferences. Some tests that may be recommended include the following:

  • Chest X-ray. A diagnostic test which uses invisible X-ray beams to produce images of internal tissues, bones, and organs onto film.
  • Electrocardiogram (ECG or EKG). A test that records the electrical activity of the heart, shows abnormal rhythms (arrhythmias or dysrhythmias), and detects heart muscle stress.
  • Echocardiogram (echo). A procedure that evaluates the structure and function of the heart by using sound waves recorded on an electronic sensor that produce a moving picture of the heart and heart valves. An echo can show the pattern of blood flow through the septal openings, and determine how large the openings are, as well as how much blood is passing through them. The vast majority of AV canal defects are diagnosed by echocardiography alone.
  • Cardiac catheterization. A cardiac catheterization is an invasive procedure that gives very detailed information about the structures inside the heart. Under sedation, a small, thin, flexible tube (catheter) is inserted into a blood vessel in the groin, and guided to the inside of the heart. Blood pressure and oxygen measurements are taken in the four chambers of the heart, as well as the pulmonary artery and aorta. Contrast dye is also injected to more clearly visualize the structures inside the heart. Cardiac catheterization is recommended prior to AV canal repair if pulmonary hypertension (high blood pressure in the vessels of the lungs) is suspected or if other information is desired. Cardiac catheterization is typically not necessary if surgical repair of AV canal defects occurs at a young age.
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