Atrioventricular Canal Defect (AV Canal)
Basics of Atrioventricular Canal Defect
An atrioventricular canal defect (AV canal) is a congenital (present at birth) heart defect that is also known as an endocardial cushion defect or atrioventricular septal defect (AVSD). The defect occurs during the first eight weeks of pregnancy, when something disrupts the heart’s normal development, resulting in a complex heart problem with several abnormalities, including:
- Atrial septal defect, an opening in the interatrial septum, or dividing wall between the two upper chambers of the heart (right and left atria). This defect allows oxygen-rich (red) blood to pass from the left atrium, through the opening in the wall (or “septum”) between the two atria, and then mix with oxygen-poor (blue) blood in the right atrium.
- Ventricular septal defect, an opening in the interventricular septum, or dividing wall between the two lower chambers of the heart (right and left ventricles). This defect allows oxygen-rich (red) blood to pass from the left ventricle, through the opening in the wall (or “septum”) between the two ventricles, and then mix with oxygen-poor (blue) blood in the right ventricle.
- 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 (mitral valve): It has three cusps rather than the usual two. One of the normal cusps is divided into two cusps. The division between the two cusps is called the cleft (or a cut in the mitral valve). Abnormalities of 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. The end result is leakage of the mitral or tricuspid valves, known as regurgitation or insufficiency.
Fast Fact of Atrioventricular Canal Defects
- Atrioventricular canal defects occur in about 5 percent of all congenital heart disease patients and are more common in infants with Down syndrome.
Causes of Atrioventricular Canal Defects
The heart forms during the first eight weeks of pregnancy. The heart begins as a hollow tube. As the heart develops, it partitions, creating a wall (or “septum”) dividing the two sides of the heart. Atrial and ventricular septal defects occur when the partitioning process does not complete, leaving holes in the atrial and ventricular septa. The valves that separate the upper and lower heart chambers form during the latter portion of this eight-week period, and they do not develop properly either.
Genetics may play a role in 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.
- 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.
Symptoms of Atrioventricular Canal
The size of the septal openings affects the type and severity of symptoms, as well as the age at which they first occur. For larger ASDs and VSDs, there’s a greater volume of blood passing from the left side of the heart to the right, thus overloading the right heart and lungs.
Symptoms usually develop in infancy and can vary among children. Common signs include:
- Pale skin
- Cool skin
- Rapid breathing
- Heavy breathing
- Rapid heart rate
- Congested breathing
- Disinterest in feeding, or tiring while feeding
- Poor weight gain
Because symptoms of AVC may resemble other medical conditions or heart problems, it’s important to consult with a child's physician for a diagnosis.
Health Problems Associated with Atrioventricular Canal Defect
If left untreated, AV Canal can cause lung disease. As blood passes through the septal opening from the left side of the heart to the right, it forces the heart’s right side to handle a larger volume of blood than normal. In turn, this extra blood passes through the pulmonary artery into the lungs’ blood vessels, which now must handle a greater volume of blood at higher than normal blood pressure.
At first, a child’s lungs can cope with this extra volume of blood at high pressure, but the condition leads to eventual lung damage. Over time, the lungs’ blood vessels get thicker, a change that is reversible at first. Eventually the changes in the lungs become irreversible, resulting in irreversible damage to the lungs themselves.
Meanwhile, as the lungs’ arteries thicken, blood flow diminishes from the left side of the heart to the right side and on to the lungs. If lung disease develops, pressure in the right side of the heart will eventually exceed pressure in the left, making it easier for oxygen-poor blood to flow from the heart’s right side, through the ASD and VSD, into the heart’s left side and on to the body. When a child’s body receives oxygen-poor blood that does not meet its needs, the child’s skin, lips and nail beds will have a blue coloring (“cyanotic”).
Another health risk is bacteria in the bloodstream infecting the heart’s abnormal mitral and tricuspid valves, which are associated with AV canal defects, resulting in a serious illness known as bacterial endocarditis.
Tests, Procedures and Diagnosis of Atrioventricular Canal Defect
When a child's pediatrician hears a heart murmur during a physical examination, the pediatrician may refer the child to a pediatric cardiologist, who specializes in the diagnosis and treatment of congenital heart defects and heart problems that may develop later in childhood. To make a diagnosis, the cardiologist performs a physical examination, listens to the heart and lungs and makes other observations. Heart murmurs are evaluated on the basis of pitch, loudness and duration, which give the cardiologist an initial idea of which heart problem a child may have. Diagnostic testing for congenital heart disease varies by the child's age and clinical condition, and may include:
- Chest X-ray. This diagnostic test uses X-ray beams to produce images onto film of internal tissues, bones and organs.
- Electrocardiogram (ECG or EKG). The test records the electrical activity of the heart, shows abnormal rhythms (arrhythmias or dysrhythmias) and detects heart muscle stress.
- Echocardiogram (or “echo”). This procedure uses sound waves recorded on an electronic sensor to produce a moving picture of the heart and its valves. An echo can show the pattern of blood flowing through septal openings and determine the openings’ size and how much blood is passing through them. The vast majority of AV canal defects are diagnosed by echocardiography alone.
- Cardiac catheterization. This invasive procedure 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 gently guided to the inside of the heart. Blood pressure and oxygen measurements are taken in the four chambers of the heart, pulmonary artery and aorta. In addition, contrast dye is 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 age 6 months (in Down syndrome children) or before age 1 (in children without Down syndrome).
Treatment for Atrioventricular Canal Defect
A physician determines the best course of treatment for atrioventricular canal, based on:
- A child's age, overall health and medical history
- Severity of the disease
- A child's tolerance for specific medications, procedures or therapies
- Expectations for the course of the disease
- Parents’ opinions or preferences
AV canal is treated by surgical repair of the defects. Until the surgery can be performed, the condition may be managed by:
- Medications. Many children with an AV canal eventually need to take medications to help their heart and lungs work better. Medications that may be prescribed include:
- Digoxin. It 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. Diuretics help the kidneys remove excess fluid from the body.
- ACE (angiotensin-converting enzyme) inhibitors. This medication 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 to ensure a baby has adequate nutrition include:
- High-calorie formula or breast milk. Special nutritional supplements may be added to formula or pumped breast milk that increases the number of calories in each ounce, thereby allowing a 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 parents inform all medical personnel that their child has an atrioventricular canal defect so it can be determined if antibiotics are necessary before any major procedure.
To prevent damage to a child’s lungs from too much blood flow and pressure, it is necessary to surgically repair the septal openings and valves. A child's cardiologist recommends when the repair should be performed, based on echocardiogram and/or cardiac catheterization test results. Surgical techniques to repair atrioventricular canal have improved greatly over the past decade, giving the operation a high likelihood of success. While most children undergo surgery by age 6 months, children with Down syndrome may need the surgical repair at an earlier age to prevent lung problems.
During surgery to close a ventricular septal defect, a synthetic patch made from a polyester-like material known as Dacron is often used. For atrial septal defects, the opening is often closed with a pericardial patch made from the membrane that covers the outside surface of the heart. Valves are repaired by converting the abnormal three-leaflet mitral valve into a two-leaflet mitral valve by suturing the cleft (the cut in the valve leaflets) to recreate a two-leaflet (two-cusp) mitral valve. The operation is performed under general anesthesia.