IIa C-EO In addition to the standard diagnostic tools, cardiac catheterization can be useful in patients with PDA and suspected pulmonary hypertension (Section 3.5). Therapeutic I C-LD PDA closure in adults is recommended if left atrial or LV enlargement is present and attributable to PDA with net left-to-right shunt, PA systolic pressure less than 50% systemic and pulmonary vascular resistance less than one third systemic.S4.1.5-1 S4.1.5-3 IIb B-NR PDA closure in adults may be considered in the presence of a net left-to-right shunt if PA systolic pressure is 50% or greater systemic, and/or pulmonary vascular resistance is greater than one third systemic.S4.1.5-3,S4.1.5-4 III: Harm C-LD PDA closure should not be performed in adults with a net right-to-left shunt and PA systolic pressure greater than two thirds systemic or pulmonary vascular resistance greater than two thirds systemic.S4.1.5-5 Table 15. AVSD: Routine Follow-Up and Testing Intervals Frequency of Routine Follow-Up and Testing Physiological Stage A* (mo) Physiological Stage B* (mo) Physiological Stage C* (mo) Physiological Stage D* (mo) Pulse oximetry As needed As needed Each visit Each visit Exercise test As needed As needed 12 24 6 12 *See Tables 3 and 4 for details on the ACHD AP classification system. 6-minute walk test or CPET, depending on the clinical indication. ACHD indicates adult congenital heart disease; AVSD, atrioventricular septal defect; CPET, cardiopulmonary exercise testing; ECG, electrocardiogram; and TTE, transthoracic echocardiogram. CLINICAL STATEMENTS AND GUIDELINES Synopsis The ductus arteriosus is a vascular connection be- tween the aorta and PA that is present in fetal life. It typically closes shortly after birth but, in some people, it will remain patent. 