Congestion of the lungs will interfere in its job to transfer oxygen from the air to the blood and carbon dioxide from blood to air. Consequently the lungs will have to work harder to do its job and the child will become short of breath, particularly with exertion such as when feeding. Whenever a communication is made between the right and the left sides of the heart, blood will tend to flow from left to right. Congestion of the lungs, leading to difficulty in breathing. Many atrial septal defects close by themselves.
Smaller defects tend to do so more readily.
Larger defects will persist. Since there is a chance that atrial septal defects can close spontaneously, then cardiologists will tend to wait and hope that this will happen. If the holes are small the amount of increased blood flow in the right heart and lungs is not significantly increased and the child will not be notably affected. On the other hands larger holes will allow much more blood flow to the right heart and lungs causing congestive heart failure. This is an open heart surgery.