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). Patent foramen ovale (PFO) and atrial septal defect (ASD) are holes in the wall that separates the two upper chambers of your heart.Generally an ASD hole is larger than a PFO. The larger the hole, the more likely symptoms are. Why do these birth defects occur? ASD is a development issue, leaving a lasting hole. Patent foramen ovale (PFO) is a congenital cardiac lesion which is generally asymptomatic and affects up to a quarter of the population. ASD is a defect in the septum between the heart's two upper chambers. The septum is a wall that separates the heart's left and right sides.