Atrioventricular Septal Defect allows the blood to move freely between the four chambers of the heart.

With this physiology, two different scenarios can occur. In some infants, resistance in the pulmonary arteries remains high. Therefore, even though there are large defects in the heart that allow the mixing of blood, the infants will have very little murmur and no symptoms.

In the other scenario, resistance in the lung arteries falls normally after birth. This allows an excess of blood to pass to the lungs and can cause heart failure, rapid breathing, and poor feeding.

This defect almost invariably requires surgical correction within a few months after birth.
Genetic syndromes are involved in more than three quarters of all AVC (also known as Atrioventricular Septal Defect) cases, with 60% occurring in people with Down Syndrome. There are complete and partial versions of the anomaly - complete AVC being more common in Down patients.

Ideally, surgical correction takes place during the first 4-5 months of life - otherwise, pulmonary vascular obstructive disease is likely to develop. The operation may be safely delayed, however, if there are no adverse symptoms, the mitral and triscupid valves (the atrioventricular, or AV, valves) are functioning adequately, and pulmonary blood pressure is near normal.

After surgery, a major concern is dysfunction of the mitral valve, which connects the left atrium and left ventricle. Repair of the mitral valve can often result in a combination of both mitral insufficiency (leakiness) and stenosis. This valve will be closely monitored through echocardiography and may require reoperation or replacement if stenosis or insufficiency is progressive at some point. Also, approximately 10% of postoperative AVC patients will develop some degree of subvalvar aortic stenosis. Surgery to correct this problem is not uncommon and usually successful, though occasionally re-operations are necessary.

In rare cases, heart block may develop in the AVC patient, either spontaneously or as a result of surgery. Heart block is a condition in which the impulses from the heart's natural pacemaker (sinoatrial node) are delayed (1st degree heart block), partially blocked (2nd degree), or completely blocked (3rd degree) as they make their way to the pumping chambers (the ventricles). If this condition occurs, then a pacemaker will be implanted.

Arrhythmias of either the atria or ventricles as well as sinoatrial dysfunction sometimes develop in the AVC patient in the years after surgery. Therefore, regular checkups involving electrocardiography are necessary.