I I. P A T E N T D U C T U S A R T E R I O S U S ( P D A )
PDA is the most commonly diagnosed cardiac condition in the NICU. It is often
associated with prematurity &/or respiratory distress.
In the fetus, the ductus arteriosus is a direct connection between the main pulmonary
artery and the descending aorta. From six weeks gestation to delivery, it is the main
outlet of blood flow from the right ventricle allowing blood to bypass the fetal lungs. It
carries ~60% of combined ventricular output. The ductus media contains primarily
muscular cells in contrast to the aorta and pulmonary artery which are comprised of
elastic tissues.
In term infants, the breath taken at birth opens the lungs and rapidly decreases
pulmonary vascular resistance. Less blood is diverted from the lung increasing the
PaO2.
Closure of PDA occurs in two stages. First, there is the rapid constriction of the muscle
cells in the media layer occurring shortly after birth. Second, there is fibrous and
anatomic obliteration over a period of weeks to months. Shunting of blood may be bi-
directional during the 1st few hours of life, but subsequently becomes left to right. By
24 - 72 hours of life, in most infants, the PDA is no longer physiologically significant.
Sensitivity of ductal closure to increased PaO2 increases with gestational age.