An Atrioventricular Canal Defect (AVC), also known as an atrioventricular septal defect (AVSD) or endocardial cushion defect, is a serious type of congenital heart defect present at birth. This condition arises when the heart fails to develop correctly during the first eight weeks of fetal life. Specifically, it involves both a hole in the walls separating the heart’s chambers and abnormalities in the valves that control blood flow. The structural failure allows oxygen-rich and oxygen-poor blood to mix, forcing the heart and lungs to work significantly harder than they should. If not corrected, this defect can lead to serious complications, including heart failure and high blood pressure in the lungs, making timely diagnosis and intervention essential.
Understanding the Heart’s Structure and the Defect
The heart is divided into four chambers: two upper chambers called atria and two lower chambers called ventricles. Normally, a muscular wall, called the septum, completely separates the right and left sides of the heart, preventing oxygen-rich blood from mixing with oxygen-poor blood. The formation of this central partition and the heart’s valves depends on the proper development and fusion of structures called endocardial cushions early in gestation, typically between four and five weeks.
The defect is classified into two main types: partial and complete. In a partial AVC defect, the hole exists only between the two upper chambers (atrial septal defect, or ASD). The valves between the atria and ventricles are also malformed, often resulting in a leaky mitral valve.
The complete AVC defect is more severe, involving a large, central hole that connects all four heart chambers. Instead of the normal two separate valves—the mitral valve on the left and the tricuspid valve on the right—a single, large, common valve spans the central defect. This single valve is often incompetent, leaking blood backward. This abnormal flow forces excessive blood into the lungs, leading to pulmonary over-circulation and eventually causing the heart muscle to enlarge as it struggles to pump efficiently.
How AVC is Identified
Symptoms of a complete AVC defect usually become noticeable within the first few weeks or months of life, often presenting as signs of heart failure. Infants may exhibit difficulty feeding, leading to poor weight gain (failure to thrive). Other physical signs include rapid or labored breathing (tachypnea), excessive sweating, and sometimes a bluish or gray tint to the skin, lips, and nails (cyanosis) due to low oxygen levels.
A heart murmur, an abnormal sound caused by turbulent blood flow, is a common early finding upon physical examination. To confirm the diagnosis, a doctor will typically use non-invasive tests, with the echocardiogram being the primary tool. This test uses sound waves to create detailed images of the heart’s structure, allowing visualization of the septal defects, the common valve, and the direction and speed of blood flow.
A chest X-ray can reveal if the heart is enlarged or if there is fluid buildup in the lungs, both of which are signs of heart failure. An electrocardiogram (EKG) records the heart’s electrical activity, helping to identify abnormal rhythms or signs of stress on the heart chambers. In some cases, the defect may even be suspected before birth during a routine prenatal ultrasound or confirmed with a specialized fetal echocardiogram.
Who is at Risk
The exact cause of an Atrioventricular Canal Defect is often not known, but it is understood to result from a combination of genetic and environmental factors. The most significant association is with chromosomal abnormalities, particularly Down Syndrome (Trisomy 21). A high percentage of infants born with Down Syndrome also have an AVC defect, with endocardial cushion defects occurring in up to 40% of fetuses with the syndrome.
Genetic factors play a clear role, as congenital heart defects, including AVC, can run in families. Beyond genetic syndromes, certain maternal factors during pregnancy may increase the risk of the defect occurring. These environmental influences include uncontrolled diabetes, particularly in the early stages of pregnancy, and certain maternal infections like rubella.
Exposure to certain medications, alcohol use, or environmental toxins during the time the fetal heart is developing may also play a contributing role. These factors represent potential risks that can influence the heart’s formation.
Corrective Measures
Surgery is necessary to treat both complete and partial Atrioventricular Canal Defects, as the holes rarely close on their own. For infants with a complete defect, the procedure is typically scheduled within the first six months of life to prevent irreversible damage to the lung blood vessels from excessive blood flow. This timing is especially important if the baby is experiencing severe symptoms or is failing to gain weight.
The goal of the open-heart surgery is to reconstruct the center of the heart to create two separate, functional sides. The surgeon closes the holes in the atrial and ventricular septa by sewing one or two patches over the defects. These patches eventually become a permanent part of the heart wall as the body’s own tissue grows over them.
For a complete AVC, the single common valve must also be divided and reconstructed to form two separate valves, a mitral and a tricuspid valve. In a partial defect, the repair is less complex, focusing on closing the atrial defect and repairing the often-leaky mitral valve. Children who undergo successful repair will require lifelong follow-up care with a cardiologist experienced in congenital heart conditions to monitor for potential long-term issues, such as valve leakage or heart rhythm irregularities.