An atrial septal defect (ASD) is a hole in the tissues separating these two chambers. Patent foramen ovale.Generally an ASD hole is larger than a PFO. The larger the hole, the more likely symptoms are. A test called an echocardiogram is used to diagnose a PFO. The test uses sound waves to create pictures of the beating heart. It typically closes shortly after birth. But a PFO is not considered an ASD because no septal tissue is missing. A patent foramen ovale is a small, flaplike opening in the wall between the upper chambers of the heart. ASD is a birth defect with a gap in the heart's upper wall.