ASD is a birth defect with a gap in the heart's upper wall. This gap happens when the wall does not form right before birth.There are two types of holes which can be present between the upper chambers of the heart: an Atrial septal defect (ASD) and a patent foramen ovale (PFO). An atrial septal defect (ASD) is a hole in the tissues separating these two chambers. Patent foramen ovale. Generally an ASD hole is larger than a PFO. The larger the hole, the more likely symptoms are. A test called an echocardiogram is used to diagnose a PFO. The test uses sound waves to create pictures of the beating heart.