Ductus Arteriosus: Lifesaver to Liability

The ductus arteriosus is a vital fetal blood vessel that connects the pulmonary artery to the descending aorta. This shunt is open during fetal life to allow blood to bypass the non-functioning fetal lungs which are filled with fluid and not yet involved in gas exchange. Because the fetus gets oxygenated blood through the placenta, blood is diverted from the lungs through the ductus arteriosus.

Why is the ductus arteriosus open before birth?

In utero, the ductus arteriosus remains open due to several factors:

• The low oxygen environment favors ductal patency.
• Circulating prostaglandins, particularly prostaglandin E2 (PGE2) produced by the placenta and ductal tissue, help keep the ductus arteriosus dilated.
• The fetal lungs are collapsed, resulting in high pulmonary vascular resistance, so blood flow naturally prefers the path of lower resistance through the ductus arteriosus to the aorta.

How does the ductus arteriosus close after birth?

At birth, several physiological changes trigger closure of the ductus arteriosus:

• With the first breaths, the lungs expand and oxygen tension in the blood sharply increases. This rise in oxygen causes the smooth muscle in the wall of the ductus arteriosus to constrict.
• The removal of the placenta abruptly reduces circulating prostaglandin E2 levels, which removes its vasodilatory influence.
• These changes initiate functional closure of the ductus arteriosus within hours to days after birth, typically causing it to constrict and eventually close.
• Anatomical closure with fibrosis follows over weeks, turning the ductus arteriosus into the ligamentum arteriosum.

Delayed Closure and Heart Development

Delayed closure or persistent patency of the ductus arteriosus (Patent Ductus Arteriosus, PDA) can occur especially in preterm infants or in cases of low oxygen levels (hypoxia). The failure to close properly can cause abnormal circulation with blood shunting from the aorta back into the pulmonary artery, leading to volume overload on the lungs and heart.

The PDA is important during fetal development as it helps maintain normal fetal circulation. However, if it remains open too long after birth, it can impair normal postnatal heart development and function by causing increased cardiac workload and pulmonary overcirculation. This can lead to issues such as heart failure or pulmonary hypertension if not managed properly.

In summary, the ductus arteriosus is open before birth due to prostaglandins and low oxygen tension, allowing blood to bypass the lungs. Postnatally, increased oxygen tension and reduced prostaglandins trigger its closure. Delayed closure is linked to immaturity or abnormalities in heart and lung development, particularly in premature infants, potentially leading to complications that affect cardiac function and development.