An atrial septal defect (ASD) is a hole in the atrial septum – the muscular wall that separates the heart’s two upper chambers, or atria. This defect makes your heart work harder than it should. ASD develops before birth, meaning it’s a congenital defect.
Because of the hole in the atrial septum, oxygen-rich blood from the left chamber mixes with oxygen-poor blood from the right chamber. This means some oxygen-rich blood is pumped to your lungs – where it has just been – instead of to your body.
ASDs can be small, medium or large. If the hole is small, it may repair itself during childhood. They can occur in the lower, middle or upper parts of the atrial septum. Holes in the middle part of the septum are the most common, affecting about 80% of those born with this condition. Holes in the lower and upper parts of the atrial septum are very rare.