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). Both PFO and ASD present as a hole between the top two heart chambers.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. An ASD is a congenital heart defect, a condition that you are born with. PFOs can occur only after birth when the foramen ovale fails to close. A PFO is similar in location to atrial septal defect (ASD), but it is smaller and rarely causes problems on its own. Patent foramen ovale (PFO) and atrial septal defect (ASD) are holes in the wall that separates the two upper chambers of your heart.