Why is atrial septal defect dangerous?

An atrial septal defect (ASD) is an opening between the upper two chambers of the heart.

After birth, an ASD causes blood from the left atrium to move into the right atrium, causing excessive blood to flow through the lungs. If this flow, otherwise known as "shunting" is significant enough, resistance to flow develops in the lung. This results in a gradual reversal of deoxygenated blood from the right to the left atrium, and subsequently through the body. This further causes cyanosis, where the body doesn't get properly oxygenated blood. The result is early fatigue and congestive heart failure.