It typically closes shortly after birth but, in some people, it will remain patent. Patent ductus arteriosus (PDA) is found in about 0.3% to 0.8% of term infants and is twice as common in females as males.S4.1.5-6 S4.1.5-8 The clinical and physiological manifestations of the PDA are dependent on the size of the vessel and the rel- ative systemic and pulmonary vascular resistances. The PDA can range from a small hemodynamically insignificant lesion that is not heard on auscultation to one that without intervention is large enough to cause congestive HF and pulmonary hypertension. Many PDAs are now closed in infancy or childhood with catheter-based or surgical approaches. For those whose ductus remains patent in adulthood, catheter- based or surgical intervention consideration depends on the symptoms and physiological expression of the lesion. Follow-up of these patients as adults is impor- tant for all, although timing and testing will vary a- mong individuals. See Section 3.3 for recommendations on who should perform surgeries, cardiac catheterization, and other procedures in these patients; Section 3.4 for recom- mendations on diagnostic evaluation; Section 4.4.6 for recommendations on severe PAH (4.4.6.1) and Eisen- menger syndrome (4.4.6.2) associated with PDA; and Table 16 for routine testing and follow-up intervals. Recommendation-Specific Supportive Text 1. Because cyanosis caused by right-to-left shunting in PDA may manifest predominantly downstream from the ductal insertion into the aorta, accurate assessment of oxygen saturation by oximetry and assessment of cyanosis should be done in the feet and both hands. As with other types of shunts, pulse oximetry with ambulation as well as at rest may identify patients with increased pulmonary arterial resistance and dynamic shunt reversal in- duced by exercise. 