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What is an atrioventricular septal defect (AVSD)?
Atrioventricular septal defect was previously known as “common atrioventricular canal” (CAVC) or “endocardial cushion defect” and are a relatively common family of congenital heart defects.
Antrioventricular septal defects is characterised by a deficiency of the atrioventricular septum of the heart. They account for about 5 percent of all congenital heart disease, and are most common in infants with Down syndrome. (About 15 percent to 20 percent of newborns with Down syndrome have an atrioventricular septal defects). Forty-five percent of children with Down syndrome have congenital heart disease. Of these, 35–40% have AV septal defects. (1)
Formation of Atrioventricular Septal Defect
A normal heart during development develops a septum or partition and two valves that separate the upper atrium from the lower ventricle and also separates the heart into four chambers but with atrioventricular septal defect this does not occur because the septum has not developed properly. Hence, the primary defect being the failure of formation of the part of the heart that arises from an embryonic structure known as the endocardial cushions. The endocardial cushions are responsible for separating the central parts of the heart near the tricuspid and mitral valves (AV valves), which separate the atria from the ventricles. The structures that 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.
There are several types of AV canal defects, the most severe being the Complete AV Septal Defect, where there is only one common AV valve over a large hole between all four chambers of the heart. All four chambers mix blood and there is typically a large amount of blood going to the lungs. Both ventricles have to pump harder and pulmonary hypertension (high blood pressure in the lungs) develops over time. Heart failure and poor growth are common findings.
Classification of forms
The most common form of atrioventricular septal defects are be classified into one of three categories:
- complete (a large hole exists in the centre of the heart where the walls between the upper and lower chambers meet instead of two separate valves on the right and left, one large common valve sits between the upper and lower chambers. Often, this valve doesn’t close tightly). Thus a complete atrioventricular septal defect is one in which there are defects in all structures formed by the endocardial cushions. Therefore, there are defects (holes) in the atrial and ventricular septal, and the AV valve remains undivided or “common.”
- partial (or incomplete) (a hole exists in the wall between the heart’s upper chambers, and the valve between the left chambers does not close completely). Thus a partial or incomplete atrioventricular septal defect is one in which 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. This type of atrial septal defect is referred to as an ostium primum atrial septal defect, and is usually associated with a cleft in the mitral valve that may cause the valve to leak.
- transitional (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). The transitional type of 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 defect is also a common part of more complex heart disease that occurs in heterotaxy syndromes.
These forms of atrioventricular septal defects usually result in the common AV valve opening predominantly into only one of the ventricles with the other ventricle being underdeveloped. These situations are more accurately described as single ventricle lesions such as hypoplastic left heart syndrome or tricuspid atresia.
In a heart with AVSD, several defects prevent the heart from working normally:
- Atrial septal defect: A hole that allows oxygen-rich (red) blood from the left atrium to mix with oxygen-poor (blue) blood in the right atrium.
- Ventricular septal defect: Another hole, which allows oxygen-rich (red) blood from the left ventricle to mix with oxygen-poor (blue) blood in the right ventricle.
- Abnormalities of the mitral or tricuspid valves: These defects allow blood that should flow forward through the system to instead flow backward.
Either way, the AVSD allows oxygen-rich and oxygen-poor blood to mix through the hole in the wall of the heart and the abnormal valves leak blood into the heart’s lower chambers. These problems make the heart work harder, causing it to enlarge.
As a result, infants with AVSD may have trouble breathing and they may not grow normally. Left untreated, the potential complications of AVSD also include:
- Pneumonia: Untreated AVSD may lead to repeated problems with this lung infection.
- Enlargement of the heart (cardiomegaly): Increased blood flow through the heart forces it to work harder, which causes it to grow larger.
- Congestive heart failure: Left untreated, AVSD causes congestive heart failure, a condition in which the heart cannot pump enough blood to meet the body’s needs.
- High blood pressure in the lungs (pulmonary hypertension): When the heart weakens and can’t pump enough blood, this increases blood pressure in the heart and lungs. High blood pressure in the blood vessels in the lungs can cause lung damage.
- Bacterial endocarditis: A serious infection of the lining of the heart.
Atrioventricular septal defects can also occur with other types of congenital heart disease such as coarctation of the aorta or tetralogy of Fallot.
What Causes Atrioventricular septal defect?
AVSD is caused by an abnormal or inadequate fusion of the superior and inferior endocardial cushions with the mid portion of the atrial septum and the muscular portion of the ventricular septum.
What are the symptoms of Atrioventricular septal defect?
Symptoms of babies with congestive heart failure usually develop gradually over the first 1-2 months of life are:
- breathe fast and hard,
- often sweat and / or tire out while feeding, and
- grow slowly or sometimes even lose weight.
A small number of infants with a complete atrioventricular septal defect will not develop congestive heart failure. This occurs because in some cases, 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. This is known as increased pulmonary vascular resistance (PVR) or pulmonary vascular disease and this condition is more common in infants with Down syndrome.
The increase in pulmonary vascular resistance 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.
While infants with a complete atrioventricular septal defect and elevated pulmonary vascular resistance often grow better and appear healthier those with low pulmonary vascular resistance and congestive heart failure, the occurrence of increased pulmonary vascular resistance is an indication to proceed quickly with surgical correction of the defect.
How is Atriventricular septal defect diagnosed?
A doctor’s first clue that this type of heart defect exists is that he will hear a heart murmur. A murmer is caused by the blood passing from the left ventricle to the right ventricle and out the pulmonary artery. It is typically noted in the first week or two of life and it is not uncommon that no murmur is present at birth.
- The diagnosis of atrioventricular septal defect in any form is easily made by echocardiography.
- Other useful tests include chest X-ray and an electrocardiogram.
- Both may show characteristic findings in atrioventricular septal defects.
Because of the high incidence of atrioventricular septal defects in infants with Down syndrome, all infants with Down syndrome should have an echocardiogram, even if they do not have a heart murmur or any of the signs or symptoms discussed above.
With the repair of the atrioventricular septal defect this lowers the pressure in the pulmonary artery and allows these muscles to relax before they become permanently constricted.
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 and 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. Significant congestive heart failure, growth failure or a very loud murmur in a child with a partial atrioventricular septal defect can occur when the defect in the mitral valve leaflet causes this valve to be very leaky.
What are the treatments for Atrioventricular septal defects?
Symptomatic infants with atrioventricular septal defects may improve with medicine, but in all cases corrective heart surgery will be necessary. Infants maybe prescribed medicines which are commonly used to treat congestive heart failure from left-to-right shunts include diuretics such as lasix (furosimide), angiotensin converting enzyme (ACE) inhibitors such as captopril, and digoxin.
These type of defects will never close on their own and will always require corrective surgery for treatment.
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. This 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. With surgical repair of either type of defect this involves closure of the holes in the atrial and / or ventricular septa with a patch or patches, and reconstruction of the common atrioventricular valve. However, a particularly challenging aspect of the repair of a complete atrioventricular septal defect is dividing the common AV valve found in this condition.
Complications following surgery can arise if the opening in the mitral valve is now too narrow or it is still very leaky. Other problems to be avoided include narrowing the path for blood to pass from the left ventricle to the aorta, or disturbances of the electrical system of the heart.
The specialised tissue that conducts the impulse for the heart to beat runs very near the area where the stitches for the ventricular patch need to be placed. If this is disrupted, placement of a pacemaker may be necessary.
What are the treatment outcomes of Atrioventricular septal defects?
Partial Atrioventricluar septal defect: The usual recovery period following repair of a partial atrioventricular septal defect is relatively brief. Most patients are out of the Intensive Care Unit (ICU) in 1-2 days and home in 4-5 days following surgery. It is reported that surgical survival is greater than 97 percent but it is probably close to 100 percent in the current era.
Complete atrioventricular septal defect: For the repair of a complete atrioventricular septal defect this is often more complex and may be associated with other factors that can prolong the post-operative course. In particular, the presence of elevated PVR pre-operatively can necessitate a prolonged time on a mechanical ventilator and the need for higher amounts of medication to help the heart work well after surgery. Additionally, problems with the mitral valve being too leaky, the path out of the left ventricle being too narrow or with the electrical system of the heart are more common after this type of surgery. Most patients require 2-4 days in the Intensive Care Unit after repair of a complete atrioventricular septal defect, and a 5-7 day hospital stay. Several reports suggest about a 90 percent survival after this type of surgery, but more recent experience is in the range of 97 percent.
The most common later problem that is seen following surgery is a leaky mitral valve which may require reoperation in up to 10 percent of patients, but most become medication-free and are able to lead essentially normal lives.
Follow-up visits with the cardiologist are important to assess valve and heart muscle function and continued antibiotic prophylaxis for endocarditis is recommended.
What problems are caused by atrioventricular septal defects?
The specific type of defect strongly influences the symptoms that may develop and the timing and details of surgical repair. A complete atrioventricular septal defect allows oxygenated blood that has returned from the lungs to the left atrium and ventricle 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 is required of a left ventricle in an anatomically normal heart. Due to the 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.
The presence of a large left-to-right shunt and the associated increased workload on the left ventricle and high pulmonary artery pressure 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).
(1) Al-Hay AA et al: Complete atrioventricular septal defect, Down syndrome and surgical outcome: Risk factors. Ann Thorac Surg 2003;75:412.
Disclaimer: The facts and opinions shown in this article are as accurate and up to date as possible, but are provided as general “information resources”, which may not be relevant to individual persons. This article is not a substitute for individual assessment and always take advice from a doctor who is familiar with the particular person..