The ligamentum arteriosum is a small, fibrous remnant located near the heart, connecting the pulmonary artery (which carries blood to the lungs) to the aorta (which distributes oxygenated blood to the body). In adults, this structure is a normal and harmless piece of anatomy, serving no active function. It is the leftover tissue from a vessel that was necessary for life before birth.
The Fetal Predecessor
Before birth, the fetus receives oxygen from the mother’s blood through the placenta because its fluid-filled lungs are not yet used for breathing. The circulatory system must therefore include a temporary detour to bypass the non-functioning lungs, which is a vessel called the Ductus Arteriosus.
The Ductus Arteriosus is a muscular shunt between the pulmonary artery and the aorta. This connection allows most blood pumped by the right side of the heart to skip the high-resistance pulmonary circulation and flow directly into the descending aorta. Without this shunt, the fetal heart would struggle to pump blood into the collapsed lungs, and the body would not receive sufficient blood flow. Only about 5 to 10% of the blood from the right ventricle flows to the lungs, with the rest diverted through this vessel.
The Transformation Process
The transition from the Ductus Arteriosus to the ligamentum arteriosum is one of the most immediate circulatory changes that happens at birth. The moment a newborn takes its first breaths, the lungs inflate and begin to function, causing a dramatic drop in the blood pressure within the pulmonary circulation. At the same time, the severance of the umbilical cord stops the flow of blood from the placenta, which was a major source of blood-vessel-relaxing hormones called prostaglandins.
The primary trigger for closure is the sudden increase in blood oxygen levels after the baby begins breathing air. This oxygen surge causes the muscular walls of the Ductus Arteriosus to contract forcefully, shutting down blood flow through the shunt. This initial muscular contraction is called functional closure, and it typically occurs within the first 12 to 24 hours after birth in full-term infants. Over the next two to three weeks, the tissue undergoes a permanent structural change, scarring into the non-functional, fibrous cord known as the ligamentum arteriosum.
When the Closure Fails
If the Ductus Arteriosus fails to close after birth, the condition is known as Patent Ductus Arteriosus (PDA), where “patent” means open. Since aortic pressure is now much higher than pulmonary artery pressure, the open vessel causes blood to flow from the aorta back into the pulmonary artery. This continuous backflow of oxygenated blood recirculates to the lungs, commonly referred to as a left-to-right shunt.
A significant PDA requires the heart to pump the blood needed for the body plus the extra blood shunted back to the lungs. This volume overload strains the heart muscle and can lead to symptoms of heart failure, such as rapid breathing, difficulty feeding, and poor weight gain in infants. If the defect is small, the only sign may be a distinct, continuous heart murmur caused by turbulent blood flow through the open vessel.
Diagnosis of PDA is confirmed with an echocardiogram, an ultrasound of the heart that allows doctors to visualize the shunt and measure blood flow. Treatment depends on the size of the opening and the baby’s overall health, especially in premature infants who are more susceptible to the condition. Medications like ibuprofen or indomethacin can promote closure by inhibiting the prostaglandins that keep the vessel open. If the PDA is large or fails to respond to medication, intervention may be required through cardiac catheterization or surgery to permanently close the connection.