It typically closes shortly after birth. But a PFO is not considered an ASD because no septal tissue is missing.An atrial septal defect (ASD) is a hole in the tissues separating these two chambers. Patent foramen ovale. Why do these birth defects occur? ASD is a development issue, leaving a lasting hole. Generally an ASD hole is larger than a PFO. The larger the hole, the more likely symptoms are. 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). A test called an echocardiogram is used to diagnose a PFO.