In the womb, the oxygen comes from the mother, passes through the placenta and the umbilical cord, and then travels to the fetal heart through a blood vessel called the ductus venosus. Once it reaches the heart, the blood is pumped to the rest of the body. As there is no need for blood to pass through the lungs, the blood is diverted away form the lungs through an artery called the ductus arteriosus. After it has delivered the oxygen, the blood returns to the heart and is pumped back to the umbilical cord and placenta to be re-oxygenated.
About 40% of premature babies under 34 weeks gestational age will have some complication resulting from a patent ductus arteriosus (PDA). As described above, under normal circumstances, the ductus arteriosus will close and disappear rapidly following birth. Once the baby is getting oxygen from the lungs, the ductus arteriosus is no longer necessary and blood flow through it drops. Additionally, the increased presence of oxygenated blood and other signals encourage the ductus arteriosus to close. With some premature babies, this process of closure either is delayed. The ductus arteriosus might not close sufficiently, leaving a small opening, or it might not close at all. As a result, normal blood flow is affected. This condition is called patent ductus arteriosus (PDA). “Patent,” in this context, means “open.”
If the opening is small, blood flow may not be disturbed at all. However, if the opening is large, blood flow to the lungs increases and the heart must pump harder to compensate. This can lead to congestive heart failure. Increased blood flow to the lungs can cause respiratory complications such as fluid in the lungs, or pulmonary edema. PDA can also cause a decrease in blood flow to the intestinal tract, which may increase the potential for necrotizing enterocolitis (NEC), a condition in which the cells of the intestines do not receive enough oxygen. NEC is described in the “Feeding and Nutrition” section of this site. A PDA can also reduce blood supply to the kidneys and the brain and can lead to or further complicate heart failure.
The staff of the NICU can be alerted to a PDA in several ways. Because blood flow through the heart is altered, the sounds of the heart beating will be different from those of a normal heart, although a PDA is sometimes silent. A baby whose blood pressure drops or becomes irregular or whose breathing becomes laboured will be checked for PDA.
PDA can resolve by itself; it may just be that an immature baby needs a little extra time for the ductus arteriosus to close. If a PDA requires treatment, or is contributing to other complications, a drug called indomethacin may be prescribed. In some situations, surgery may be needed to close the PDA.