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). 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. Generally an ASD hole is larger than a PFO. The larger the hole, the more likely symptoms are. A PFO is similar in location to atrial septal defect (ASD), but it is smaller and rarely causes problems on its own. A Patent Foramen Ovale (PFO) is a small opening between the two upper chambers of the heart, the right and the left atrium.