Endocardial cushion defect (ECD) is an abnormal heart condition in which there is no separation between the chambers of the heart. Essentially, the middle part of the heart is missing. It is a congenital heart disease, which means it is present from birth.

Endocardial cushion defect occurs while a baby is still growing in the womb. The endocardial cushions are two areas of thickening that eventually develop into the wall (septum) that separates the four chambers of the heart. They also form the mitral and tricuspid valves.

The lack of separation between the two sides of the heart causes several problems:

• Increased blood pressure in the lungs. In persons with this condition, blood flows through the abnormal openings from the left to the right side of the heart, then to the lungs. The increased blood flow into the lungs leads to a rise in blood pressure in the lungs.
• Lung irritation and inflammation. Increased blood flow into the lungs causes irritation and swelling.
• Heart failure. Because the heart has to pump more blood to the lungs, it has to work much harder than normal. The heart may enlarge and weaken.
• Lack of oxygen. As the blood pressure increases in the lungs, blood flow starts to move from the right side of the heart to the left. The heart starts pumping oxygen-poor blood throughout the body, and will have to work harder at it. The body will not get enough oxygen.

There are two types of ECD:

• Complete ECD: A complete ECD involves an atrial septal defect (ASD) and a ventricular septal defect (VSD). Persons with a complete ECD have only one large heart valve (common AV valve) instead of two distinct valves (mitral and tricuspid).
• Partial (or incomplete) ECD: Only an ASD is present. There are two distinct valves, but one of them (the mitral valve) is often abnormal with an opening ("cleft") in it, letting blood leak between the two left chambers of the heart.

ECD is strongly associated with Down syndrome. Several gene changes are also connected to ECD. However, the exact cause of ECD is unknown.

ECD may be associated with other congenital heart defects such as:

• Double outlet right ventricle
• Single ventricle
• Transposition of the great vessels
• Tetralogy of Fallot