Which congenital heart defect is described as the incomplete fusion of the endocardial cushions?

What is an Atrioventricular Septal Defect or an AV Canal Defect?

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

Atrioventricular septal defects make up about five percent of all congenital heart disease. They are most common in infants with Down syndrome.

The main defect is that part of the heart called the endocardial cushions doesn’t form during the baby’s development before birth. The endocardial cushions separate the parts of the heart.

One structure that is made from the endocardial cushions is part of the atrial septum (the wall that divides the right atrium from the left atrium). The ventricular septum (the wall that divides the right ventricle from the left ventricle) is also made from the endocardial cushions.

The endocardial cushions separate the mitral and tricuspid valves. An atrioventricular septal defect can include the heart not having any of these structures formed.

AVSD Categories

Atrioventricular septal defects can be put into one of three categories. These are called complete, partial (or incomplete) or transitional.

A complete atrioventricular septal defect is one in which there are defects in all structures made by the endocardial cushions. There are defects (holes) in the atrial and ventricular septa. The AV valve stays undivided or "common."

A partial or incomplete atrioventricular septal defect is one in which the part of the ventricular septum made by the endocardial cushions has filled in. It is filled in by tissue from the AV valves or directly from the endocardial cushion tissue. The tricuspid and mitral valves are two different valves.

The defect is mostly in the atrial septum and mitral valve. This type of atrial septal defect is called an ostium primum atrial septal defect. It comes with a cleft in the mitral valve causing the valve to leak.

The transitional type of defect looks like the complete form of atrioventricular septal defect. But the leaflets of the AV valve are stuck to the ventricular septum. This divides the valve into two valves. It closes the hole between the ventricles.

As a result of the small defect between the ventricles, a transitional atrioventricular septal defect acts more like a partial atrioventricular septal defect. It even looks more like a complete atrioventricular septal defect.

Atrioventricular septal defect is a common part of more complex heart disease that occurs in heterotaxy syndromes (when the heart and other organs are in the wrong place in the chest and abdomen).

These forms of atrioventricular septal defects lead to the common AV valve opening into one of the ventricles. The other ventricle is underdeveloped. These situations are better described as single ventricle lesions such as hypoplastic left heart syndrome or tricuspid atresia.

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

Problems With AVSD

The specific type of defect impacts the symptoms that may develop. It also impacts the timing and details of surgical repair.

A complete atrioventricular septal defect allows oxygenated blood that has come back from the lungs to the left atrium and ventricle to cross the atrial or ventricular septum. The blood then goes back out the pulmonary artery to the lungs.

This re-circulation of blood to the lungs, called a left-to-right shunt, doesn't work well. The left ventricle must pump already oxygenated blood back to the lungs. At the same time, the left ventricle is 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 two to three times more than what is needed of a left ventricle in a normal heart.

Because there is a large hole in the ventricular septum, the high pressure normally created by the left ventricle to send blood throughout the body is also sent to the lungs.

Having a large left-to-right shunt, the higher workload on the left ventricle, and the high pulmonary artery pressure causes the lungs to be filled with blood. This causes fluid to leak from the bloodstream into the air spaces of the lungs.

This condition is called pulmonary edema. Pulmonary edema makes it harder for a baby to move their lungs and breathe comfortably. The combination of increased heart and lung work uses large amounts of calories. This results in congestive heart failure (CHF).

Signs and Symptoms of AVSD

Babies with congestive heart failure breathe fast and hard. They often sweat and / or tire out while feeding. They also grow slowly or lose weight. These symptoms develop slowly over the first one to two months of life.

The doctor will hear a heart murmur when this type of defect is present. The murmur is caused by the blood passing from the left ventricle to the right ventricle and out the pulmonary artery.

A small number of infants with a complete atrioventricular septal defect will not develop congestive heart failure. This occurs because the muscle cells that line the small arteries to the lungs get bigger. They will get tighter to protect the lungs from the extra flow and high pressure from the atrioventricular septal defect.

This is called increased pulmonary vascular resistance (PVR), or pulmonary vascular disease. This condition is more common in infants with Down syndrome.

The increase in pulmonary vascular resistance is effective in preventing the signs and symptoms of congestive heart failure. Pulmonary vascular resistance reduces the amount of left-to-right shunt. It may 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. It may also cause the murmur to be softer.

Infants with a complete atrioventricular septal defect and elevated pulmonary vascular resistance often grow better and look healthier than those with low pulmonary vascular resistance and congestive heart failure. Having pulmonary vascular resistance may prompt early surgical correction of the defect.

Repair of the atrioventricular septal defect lowers the pressure in the pulmonary artery. It allows these muscles to relax before they become permanently narrow.

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

There is more work on the heart. Growth may occur more slowly than in infants and children with normal hearts. There is a heart murmur present. It is softer than a murmur 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.

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.

Diagnosis of AVSD

A heart murmur is often the first clue that this heart defect exists. It is noted in the first week or two of life. It is common that no murmur is present at birth.

The diagnosis of atrioventricular septal defect in any form is made by echocardiography. Chest X-ray and an electrocardiogram may be used to help with the assessment.

There is high chance of atrioventricular septal defects in infants with Down syndrome. All infants with Down syndrome should have an echocardiogram. This should happen if there is not a heart murmur. It should happen if the child doesn’t have any signs or symptoms.

Treatment for AVSD

Infants that have symptoms of atrioventricular septal defects may improve with medicine. In all cases corrective heart surgery will be necessary.

Medicines used to treat congestive heart failure from left-to-right shunts in infants include diuretics (helping to rid the body of sodium and water) such as Lasix (furosemide).

Medical treatment of infants with atrioventricular septal defects relieve symptoms. It also allows the baby to get big enough to have surgery with lower risks.

Surgery occurs at 3-6 months for infants with a complete atrioventricular septal defect. Surgery happens around 6-18 months for infants with a partial atrioventricular septal defect.

Surgical repair of either type of defect involves closing of the holes in the atrial and / or ventricular septa with a patch. It also includes reconstruction of the common atrioventricular valve.

Complications after surgery can happen if the opening in the mitral valve is too narrow or leaky.

The tissue that starts the impulse for the heart to beat runs near the area where the stitches for the ventricular patch need to be placed. If this impulse is interrupted, a pacemaker may be needed.

Atrioventricular Septal Defects Treatment Outcomes

The recovery period after repair of a partial atrioventricular septal defect is usually short. Most patients are out of the intensive care unit (ICU) in one to two days. They are home in four to five days after surgery.

Reported surgical survival is greater than 97 percent but is close to 100 percent.

Repair of a complete atrioventricular septal defect is often complex. It may be associated with other factors that can make recovery after surgery longer.

Having higher pulmonary vascular resistance (PVR) before surgery can lead to a long time on a mechanical ventilator. The child may need higher amounts of medicine to help the heart work well after surgery.

Problems with the mitral valve being leaky, the path out of the left ventricle being narrow or issues with the electrical system of the heart are more common after this type of surgery.

Most patients need two to four days in the intensive care unit after repair of a complete atrioventricular septal defect. They will stay in the hospital for five to seven days after surgery. The surgery survival is 97% for this type of surgery.

The most common problem that is seen after surgery is a leaky mitral valve. This may need another surgery in 10 percent of patients. Most become medicine-free and free of heart symptoms.

Follow-up visits with the cardiologist are important to look at valve and heart muscle function. Continued antibiotic prophylaxis for endocarditis is recommended.

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