Atrioventricular Septal Defect

Atrioventricular septal defects (AVSD) are a relatively common family of congenital heart defects.

Also known as atrioventricular canal 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.

The primary defect is the failure of formation of the part of the heart that arises from an embryonic structure called the endocardial cushions. The structures that normally develop from the endocardial cushions include the lower part of the atrial septum (wall that divides the right atrium from the left atrium) and the ventricular septum (wall that divides the right ventricle from the left ventricle) just below the tricuspid and mitral valves. The endocardial cushions also complete the separation of the mitral and tricuspid valves by dividing the single valve between the embryonic atria and ventricles. An atrioventricular septal defect may involve failure of formation of any or all of these structures.

Most commonly, atrioventricular septal defects can be classified into one of three categories:

  • Complete atrioventricular septal defect – there are defects in all structures formed by the endocardial cushions. Therefore, there are defects (holes) in the atrial and ventricular septa, and the AV valve remains undivided.
  • Partial or incomplete atrioventricular septal defect – 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.
  • Transitional atrioventricular septal 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, a transitional atrioventricular septal defect behaves more like a partial atrioventricular septal defect, even though it looks more like a complete atrioventricular septal defect.

Atrioventricular septal defects can also occur with other types of congenital heart disease such as coarctation of the aorta or tetralogy of Fallot.

The specific type of defect strongly influences the symptoms that may develop and the timing and details of surgical repair.

A heart murmur is often the first clue that this heart defect exists. It is typically noted in the first week or two of life and it is not uncommon that no murmur is present at birth.

Symptomatic infants with atrioventricular septal defects may improve with medicine, but in all cases corrective heart surgery will be necessary, as these type of defects will never close on their own. Medical treatment of infants with atrioventricular septal defects is usually used to relieve symptoms and allow the baby to get big enough to undergo surgical repair with lower risks. Medicines commonly used to treat congestive heart failure from left-to-right shunts in infants include diuretics, angiotensin converting enzyme (ACE) inhibitors, and digoxin.

Surgery usually occurs at 3-6 months for infants with a complete atrioventricular septal defect and 6-18 months for infants with a partial atrioventricular septal defect. Surgical repair of either type of defect involves closure of the holes in the atrial and/or ventricular septa with patches, and reconstruction of the common atrioventricular valve. Reported surgical survival is greater than 97 percent.

Complete Atrioventricular Septal Defect

A complete atrioventricular septal defect allows oxygenated blood that has returned from the lungs to cross either the atrial or ventricular septum and go back out the pulmonary artery to the lungs. This re-circulation of blood to the lungs, called a left-to-right shunt, is inefficient because the left ventricle must pump a volume of already oxygenated blood back to the lungs while trying to meet the body’s usual demand for its own oxygenated blood. The amount of extra blood pumped by the left ventricle is often an additional 2-3 times that required of a left ventricle in an anatomically normal heart.

Because there is a large hole in the ventricular septum, the high pressure normally generated by the left ventricle to propel blood throughout the body is also transmitted to the lungs. Under normal circumstances, the lungs have a blood pressure much lower than that in the rest of the body. This cause the lungs to become engorged with blood, and causes fluid to leak from the bloodstream into the air spaces of the lungs. This condition is called pulmonary edema and makes it harder for a baby with this condition to move his or her lungs and breathe comfortably.

The combination of increased heart and lung work uses large amounts of calories and results in the constellation of symptoms referred to as congestive heart failure (CHF). Babies with congestive heart failure breathe fast and hard, often sweat and/or tire out while feeding, and grow slowly or sometimes even lose weight. These symptoms usually develop gradually over the first 1-2 months of life. The doctor will usually hear a loud heart murmur when this type of defect is present.

A small number of infants with a complete atrioventricular septal defect will not develop congestive heart failure. The muscle cells that line the small arteries to the lungs get bigger and constrict to try to protect the lungs from the extra flow and high pressure caused by the atrioventricular septal defect. Called increased pulmonary vascular resistance (PVR), this condition is more common in infants with Down syndrome.

The increase in PVR is very effective in preventing the signs and symptoms of congestive heart failure by minimizing the amount of left-to-right shunt, and may even cause blood with low oxygen to go from the right ventricle to the left ventricle and out to the body without picking up oxygen. This causes cyanosis, which is a bluish discoloration of the skin, fingernails and mouth and it may also cause the murmur to be softer.

The occurrence of increased pulmonary vascular resistance is an indication to proceed quickly with surgical correction of the defect. Repair of the atrioventricular septal defect lowers the pressure in the pulmonary artery and allows these muscles to relax before they become permanently constricted.

Partial or Transitional Atrioventricular Septal Defects

Infants with the partial or transitional forms of atrioventricular septal defects have more subtle signs and symptoms. Like children with a complete atrioventricular septal defect, they have an increased volume of blood passing through the pulmonary artery.

The main difference between a left-to-right shunt that occurs primarily between the atria rather than the ventricles is that the pressure in the pulmonary artery usually remains low despite the increase in flow. This causes less work for the heart and lungs and results in fewer breathing and growth problems. It also lessens the possibility that the pulmonary vascular resistance will increase. Nevertheless, there is an increased workload on the heart and growth may occur more slowly than infants and children with normal hearts. There is usually a heart murmur present, but it is softer than that which occurs with a complete atrioventricular septal defect.

These types of defects may not come to medical attention until the child is several months or even years old because of the subtlety of the signs and symptoms that may be associated with them.