Atrio-ventricular septal defect (AVSD) is a defect in which there are holes between the chambers of the right and left sides of the heart, and the valves that control the flow of blood between these chambers may not be formed correctly.
In AVSD, blood flows where it normally should not go. The blood may also have a lower than normal amount of oxygen, and extra blood can flow to the lungs.
This extra blood being pumped into the lungs forces the heart and lungs to work hard and may lead to congestive heart failure.
Early surgical repair is needed in most cases (in the first 4 - 6 months).
This video explains AVSD: