Atrial Septal Defects ●The most common congenital lesion in adults after the bicuspid aortic valve. ●Atrial septation occurs during the fifth week of gestation. Septum primum grows first out of the endocardial cushion. Septum secundum grows caudally down towards the interventricular septum. ●In fetal circulation the Right sided pressure pushes blood through the valve formed by the two septums. At birth when the lungs expand, pulmonary pressures decrease while systemic pressures increase and the now left to right pressure closes the valve. ●In 70% of people the septums fuse, in the other 30% there is a probe patent foramen ovale