Patent foramen ovale (PFO) and atrial septal defect (ASD) are holes in the wall that separates the two upper chambers of your heart. 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 PFO is smaller than an ASD, which can be bigger and cause more issues. Patent foramen ovale (PFO) refers to the nonclosure of the potential space between septum primum and secundum (analogous to a flap valve) Generally an ASD hole is larger than a PFO. The larger the hole, the more likely symptoms are. The presence of a patent foramen ovale (PFO) is implicated in the pathogenesis of a number of medical conditions. A PFO is usually smaller than an ASD. That's why it doesn't usually cause any problems or symptoms. The foramen ovale usually closes within 48 hours of birth due to pressure changes in the infant's chest.