An Atrioventricular Septal Defect (AVSD) is a complex congenital heart defect that involves abnormalities in the structure of the heart's atria, ventricles, and valves. This condition is sometimes also known as an atrioventricular canal defect or endocardial cushion defect. AVSD is characterized by a combination of several heart anomalies:

1. Atrial Septal Defect (ASD): There is a hole between the two upper chambers of the heart (the atria).
2. Ventricular Septal Defect (VSD): There is a hole between the two lower chambers of the heart (the ventricles).
3. Improperly Formed Atrioventricular Valves: The valves that control the flow of blood between the atria and ventricles (the mitral and tricuspid valves) may not be formed correctly. They might be fused together into a single, common valve that straddles the ASD and VSD.

AVSD results in a significant mixing of oxygen-rich and oxygen-poor blood. This mixing leads to inefficient blood circulation, as oxygen-rich blood gets recirculated to the lungs instead of being pumped out to the rest of the body, while oxygen-poor blood may be pumped to the body instead of the lungs.

The symptoms of AVSD can vary depending on the severity of the defect and the presence of associated heart problems. Common symptoms include difficulty breathing, frequent respiratory infections, poor growth, fatigue, and a bluish tint to the skin or lips (cyanosis) due to lack of oxygen.

Diagnosis of AVSD typically involves imaging tests like echocardiography, which provides detailed images of the heart's structure and function. Cardiac MRI and chest X-rays may also be used.

Treatment for AVSD usually involves surgery to correct the defects. The goals of surgery are to repair the holes in the septum, reconstruct the atrioventricular valves to function properly, and address any other associated heart problems. The timing of surgery depends on the severity of the symptoms and the overall health of the child. Early intervention is often necessary to prevent complications such as heart failure, pulmonary hypertension, and growth delays.

Post-surgical care and long-term monitoring are crucial for individuals with AVSD, as they may have an increased risk of arrhythmias, heart valve problems, and other cardiac issues as they age.