A physician monitoring a newborn infant’s heart sounds using a stethoscope hears the characteristic murmur of a patent ductus arteriosus. How soon after birth should this murmur normally disappear?

D. The ductus arteriosus functionally closes within 1–2 hours after birth via smooth mus-cle contraction of the tunica media. Before birth, the patency of the ductus arteriosus is controlled by the low oxygen content of the blood flowing through it, which in turn stimulates production of prostaglandins, which cause smooth muscle to relax. After birth, the high oxygen content of the blood due to lung ventilation inhibits production of prostaglandins, causing smooth muscle contraction. Premature infants can be treated with prostaglandin synthesis inhibitors (such as indomethacin) to promote closure of the ductus arteriosus.