Atrioventricular septal defect
|Atrioventricular septal defect|
|Classification and external resources|
Ultrasound showing a complete atrioventricular septal defect
Atrioventricular septal defect (AVSD) or atrioventricular canal defect (AVCD), previously known as "common atrioventricular canal" (CAVC) or "endocardial cushion defect", is characterized by a deficiency of the atrioventricular septum of the heart. It 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.
A variety of different classifications have been used, but the defects are usefully divided into "partial" and "complete" forms.
- In the partial AVSD, there is a defect in the primum or inferior part of the atrial septum but no direct intraventricular communication (ostium primum defect).
- In the complete AVSD (CAVSD), there is a large ventricular component beneath either or both the superior or inferior bridging leaflets of the AV valve. The defect involves the whole area of the junction of the upper and lower chambers of the heart, i.e. where the atria join the ventricles. There is a large hole between the lower portion of the atria and the upper or `inlet' portion of the ventricles and this is associated with a significant abnormality of the valves separating the atria from the ventricles. The valves in effect become a common atrio-ventricular valve, and the severity of the defect depends largely on the supporting attachments of the valve to the ventricles and whether the valve allows dominant flow from the right atrium to right ventricle and from left atrium to left ventricle ("unbalanced" flow). The overall problems are similar to those of VSD but are more complicated. There is an increased flow of blood to the lungs through both the ventricular and atrial components of the defect. In addition, the abnormal atrio-ventricular valve invariably leaks, so that when the ventricles contract, blood flows not only forwards to the body and the lungs, but also backwards into the atria. The back-pressure effect on the atria causes congestion of blood in the left atrium in particular, and this in turn causes congestion in the veins draining the lungs. The effect on the baby is to worsen the heart failure that is associated with an isolated VSD and to hasten the onset of pulmonary hypertension. It should be mentioned that CAVSD is found in approximately one-third of babies who have Down syndrome, but it also occurs as an isolated abnormality.
If there is a defect in the septum, it is possible for blood to travel from the left side of the heart to the right side of the heart, or the other way around. Since the right side of the heart contains venous blood with a low oxygen content, and the left side of the heart contains arterial blood with a high oxygen content, it is beneficial to prevent any communication between the two sides of the heart and prevent the blood from the two sides of the heart from mixing with each other.
Associated conditions 
This type of congenital heart defect is associated with patients with Down syndrome (trisomy 21) or heterotaxy syndromes. Forty-five percent of children with Down syndrome have congenital heart disease. Of these, 35–40% have AV septal defects. Similarly, one-third of all children born with AVSDs also have Down syndrome.
AVSDs can be detected by cardiac auscultation; they cause atypical murmurs and loud heart tones. Confirmation of findings from cardiac auscultation can be obtained with a cardiac ultrasound (echocardiography - less invasive) and cardiac catheterization (more invasive).
Tentative diagnosis can also be made in utero via fetal echocardiogram. An AVSD diagnosis made before birth is a marker for Down syndrome, although other signs and further testing are required before any definitive confirmation of either can be made.
Treatment is surgical and involves closure of the atrial and ventricular septal defects and restoration of a competent left AV valve as far as is possible. Open surgical procedures require a heart-lung machine and are done with a median sternotomy. Surgical mortality for uncomplicated ostium primum defects in experienced centers is 2%; for uncomplicated cases of complete atrioventricular canal, 4% or less. Certain complications such as tetralogy of Fallot or highly unbalanced flow across the common AV valve can increase risk significantly.
Infants born with AVSD are generally in sufficient health to not require immediate corrective surgery. If surgery is not required immediately after birth, the newborn will be closely monitored for the next several months, and the operation held-off until the first signs of lung distress or heart failure. This gives the infant time to grow, increasing the size of, and thereby the ease of operation on, the heart, as well as the ease of recovery. Infants will generally require surgery within three to six months, however, they may be able to go up to two years before the operation becomes necessary, depending on the severity of the defect.
See also 
- Atrial septal defect
- Congenital heart disease
- Heart sounds
- Pulmonary hypertension
- Ventricular septal defect
- Report of the New England Regional Infant Cardiac Program. Pediatrics 1980;65(suppl):441–444.
- Al-Hay AA et al.: Complete atrioventricular septal defect, Down syndrome and surgical outcome: Risk factors. Ann Thorac Surg 2003;75:412.
- Atrioventricular Canal Defect at Children's National
- Atrioventricular septal defect at Mount Sinai Hospital
- Kirklin J, Barratt-Boyes B, ed. Cardiac Surgery, New York: Wiley, 1986:463–497.
- Marx, GR, Fyler DC, "Endiocardial Cusion Defects", in Keane, JF, Lock, JE, Fyler DC ed. Nadas' Pediatric Cardiology 2ed., Philadelphia: Saunders-Elsevier, 2006:663-674..
- Hay, WW, et al. 2007. Lange Current Pediatric Diagnosis and Treatment, 18th ed. New York: McGraw-Hill.
- Atrioventricular Septal Defect information from Seattle Children's Hospital Heart Center
- AVSD information, diagram and flash animation from Cincinnati Children's Hospital Medical Center
- Overview and diagram at umich.edu
- Diagram at fairview.org
- Atrioventricular septal defect information for parents.