Atrioventricular Canal (AV Canal or AVC)

What is 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:

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• 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.

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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 about 5 percent of all congenital heart disease cases and are more common in infants with Down syndrome.

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 statistics:

• Congenital Heart Disease (CHD) is present in half of children born with Down syndrome, and 45 percent 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 (two) of #21 chromosomes.

• Similarly, one-third of all children born with AV canal defect also have Down syndrome.

• Fourteen 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 children become cyanotic, or exhibit a blue coloring in their skin, lips, and nailbeds.

Because blood is pumped at high pressure through the septal openings, the right and left ventricles 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, blood within the heart will eventually "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, nailbeds, and skin. The symptoms of AVC may resemble other medical conditions or heart problems. Always consult your child's physician for a 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. A heart murmur is simply 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 before six months of age (in Down syndrome children) or before one year of age (in non-Down syndrome children).

Treatment for atrioventricular canal

Specific treatment for atrioventricular canal will be determined by your child's physician based on:

• Your child's age, overall health, and medical history

• Extent of the disease

• Your child's tolerance for specific medications, procedures, or therapies

• Expectations for the course of the disease

• Your opinion or preference

AV canal is treated by surgical repair of the defects. However, medical support (i.e., medications) may be necessary until the operation is performed. Treatment may include:

• Medical management. Many children will eventually need to take medications to help the heart and lungs work better, due to strain from the extra blood passing through the septal defects. Medications that may be prescribed include the following:

  • Digoxin. Helps strengthen the heart muscle, enabling it to pump more efficiently.

  • Diuretics. The body's water balance can be affected when the heart is not working as well as it could. These medications help the kidneys remove excess fluid from the body.

  • ACE (angiotensin-converting enzyme) inhibitors. Dilates the blood vessels, making it easier for the heart to pump blood forward into the body.

• Adequate nutrition. Infants may become tired when feeding, and may not be able to eat enough to gain weight. Options that can be used to ensure your baby will have adequate nutrition include:

  • High-calorie formula or breast milk. Special nutritional supplements may be added to formula or pumped breast milk that increase the number of calories in each ounce, thereby allowing your baby to drink less and still consume enough calories to grow properly.

  • Supplemental tube feedings. Feedings given through a small, flexible tube that passes through the nose, down the esophagus, and into the stomach, can either supplement or take the place of bottle feedings. Infants who can drink part of their bottle, but not all, may be fed the remainder through the feeding tube. Infants who are too tired to bottle feed may receive their formula or breast milk through the feeding tube alone.

• Infection control. Children with certain heart defects are at risk for developing an infection of the valves of the heart known as bacterial endocarditis. It is important that you inform all medical personnel that your child has a an atrioventricular canal defect so they may determine if the antibiotics are necessary before any major procedure.

• Surgical repair. The goal is to repair the septal openings and repair the valves before the lungs become damaged from too much blood flow and pressure. Your child's cardiologist will recommend when the repair should be performed based on results from the echocardiogram and/or cardiac catheterization.

  The operative methods used to repair atrioventricular canal have improved greatly in the past decade, and the operation has a high likelihood of success. Most children undergo surgery by the age of 6 months. Children with Down syndrome may develop lung problems earlier than other children, and may need to have surgical repair at an earlier age.

  The operation is performed under general anesthesia.

  At surgery, the ventricular septal defect is often closed with a synthetic patch made from a polyester-like material known as Dacron. The atrial septal defect is often closed with a pericardial patch made from the membrane that covers the outside surface of the heart. The valve repair technique consists of converting the abnormal three-leaflet mitral valve into a two-leaflet mitral valve. This is accomplished by suturing the cleft (the cut in the valve leaflets) to recreate a two-leaflet (two-cusp) mitral valve.

Postoperative care for your child

Children will spend time in the intensive care unit (ICU) after an AV canal repair. During the first several hours after surgery, your child will be very drowsy from the anesthesia that was used during the operation, and from medications given to relax him or her and to help with pain. To help your child rest, these medications might be continued overnight or longer. As time goes by, your child will become more alert.

While your child is in the ICU, special equipment will be used to help him or her recover, and may include the following:

• Ventilator. Machine that helps your child breathe while he or she is under anesthesia during the operation. A small, plastic tube is guided into the windpipe and attached to the ventilator, which breathes for your child while he or she is too sleepy to breathe effectively on his or her own. After a AV canal repair, children will benefit from remaining on the ventilator overnight or even longer so they can rest. Children with Down syndrome often will remain on the ventilator a bit longer than children without Down syndrome.

• Intravenous (IV) catheters. Small, plastic tubes inserted through the skin into blood vessels to provide IV fluids and important medicines that help your child recover from the operation.

• Arterial line. A specialized IV placed in the wrist or other area of the body where a pulse can be felt, that measures blood pressure continuously during surgery and while your child is in the ICU.

• Nasogastric (NG) tube. A small, flexible tube that keeps the stomach drained of acid and gas bubbles that may build up during surgery.

• Urinary catheter. A small, flexible tube that allows urine to drain out of the bladder and accurately measures how much urine the body makes, which helps determine how well the heart is functioning. After surgery, the heart will be a little weaker than it was before, and, therefore, the body may start to hold onto fluid, causing swelling and puffiness. Diuretics may be given to help the kidneys remove excess fluids from the body.

• Chest tube. A drainage tube may be inserted to keep the chest free of blood that would otherwise accumulate after the incision is closed. Bleeding may occur for several hours, or even a few days after surgery.

• Heart monitor. A machine that constantly displays a picture of your child's heart rhythm, and monitors heart rate, arterial blood pressure, and other values.

Your child may need other equipment not mentioned here to provide support while in the ICU, or afterwards. The hospital staff will explain all of the necessary equipment to you.

Your child will be kept as comfortable as possible with several different medications; some of which relieve pain, and some of which relieve anxiety. The staff will also be asking for your input as to how best to soothe and comfort your child.

After discharge from the ICU, your child will recuperate on another hospital unit for a few days before going home. You will learn how to care for your child at home before your child is discharged. Your child may need to take medications for a while at home, and these will be explained to you. The staff will give you written instructions regarding medications, activity limitations, and follow-up appointments before your child is discharged.

Caring for your child at home following AV canal repair

Most infants and older children feel fairly comfortable when they go home. Pain medications, such as acetaminophen or ibuprofen, may be recommended to keep your child comfortable. Your child's physician will discuss pain control before your child is discharged from the hospital.

Often, infants who fed poorly prior to surgery have more energy after the recuperation period, and begin to eat better and gain weight faster. However, high-calorie formulas may be needed for several weeks or months after surgery to help your child catch up growth-wise. Tube feedings may also be helpful until your child is able to feed better.

After surgery, older children usually have a fair tolerance for activity. Your child may become tired easily, and sleep more right after surgery, but, within a few weeks, your child may be fully recovered.

You may receive additional instructions from your child's physicians and the hospital staff.

Long-term outlook after AV canal surgical repair

Many children who have had an AV canal defect repair will live healthy lives. Activity levels, appetite, and growth will eventually return to normal in most children. Your child's cardiologist may recommend that antibiotics be given to prevent bacterial endocarditis for a specific time period after discharge from the hospital.

Some children will still have some degree of mitral or tricuspid valve abnormality after AV canal repair surgery. This may require another operation in the future to repair the leaky valve(s).

Children with Down syndrome will benefit from special programs that enhance their mental and physical development. Your child's physician can assist you in locating such programs in your community.

Consult your child's physician regarding the specific outlook for your child.