Patent Ductus Arteriosus
- Patent Ductus Arteriosus (PDA) refers to inappropriate patency of the fetal ductus arteriosus after birth which allows right-to-left shunting of blood following delivery.
- The Ductus Arteriosus is a communication between the pulmonary artery and the aorta which exists during fetal life and allows blood to bypass the fetal lung during development. In normal individuals, rising arterial oxygen tension (PaO2) and reductions in PGE2 following birth induce vascular smooth muscle surrounding the ductus arteriosus to constrict and thus close the communication. Over time, the ductus lumen is eliminated completely through scarring and atrophy, leaving its remnant termed the "Ligamentum Arteriosum".
- The clinical consequences of a PDA largely depend on the resistance to blood flow offered by the abnormally patent ductus. PDAs typically result in a characteristic continuous, machine-like murmur reflecting continuous turbulent flow of blood from the high-pressure aorta into the low-pressure pulmonary artery.
- Small PDAs
- In patients where the resistance of the patency is very high, a PDA can be well-tolerated and provide few if any clinical consequences.
- Large PDAs
- In patients where the patency is quite large and thus offers little resistance to blood flow, large volumes of blood can undergo left-right shunting from the aorta to the pulmonary artery. Because shunted blood must ultimately return to the left heart, over time the left heart undergoes volume overload and enters a spiral of left heart failure. Additionally, through poorly-understood processes chronically increased blood flow through the pulmonary circulation due to left-right shunting results in pulmonary hypertension which can increase the pulmonary vascular resistance beyond that of systemic vascular resistance] and ultimately lead to reversal of the shunt. This physiological scenario, termed Eisenmenger's Syndrome, yields a right-left shunt can result in hypoxemia and symptomology of cyanosis at this late stage.
- Because circulating PGE2 is critical for maintaining patency of the ductus arteriosus, treatment with the PGE inhibitor indomethicin can be used to postnatally close a PDA.