Ligamentum arteriosum also known as the arterial ligament refers to a small ligament that connects the superior surface of the proximal descending aorta with the left pulmonary artery. It is an actually non-functional form of the ductus arteriosus and it forms around the third week after the birth.
Ductus Arteriosus And Ligamentum Arteriosum
During the intrauterine life, the fetus gets oxygen and all nutrients needed for life from a mother’s circulation. The main organ where the exchange of these elements happens is the placenta. Placenta represents a unique organ present only during pregnancy. It is also considered to be the only organ formed by two different bodies since it has fetal and maternal surface. In the complex villi of the placenta, oxygen and nutrients are released from mother’s blood and are transferred to fetal blood enabling the fetus all the elements needed for life. This exchange happens with minimal or without any direct contact of these two circulations. Only small number of cells can pass, but this amount rarely causes problems. One of them can be Rh incompatibility.
Considering there is no breathing during intrauterine life, there is no need for blood to circulate through the lungs. In order for lungs to be bypassed, there has to be a connection between pulmonary and systemic circulation.
This particular connection is formed by ductus arteriosus. It is a small part-time blood vessel that connects aorta to left pulmonary artery, thus leading all the blood from the pulmonary artery to flow, instead to lungs, into the aorta. All this results in the blood to circulate through the fetal body without a problem while bypassing lungs. Lungs are at these 9 months collapsed since there is no breathing.
After the birth of babies, first breathe expands the lungs, causing the blood to start circulating from the right ventricle of the heart through the pulmonary artery and finally into the pulmonary circulation. This will from now on be the only way for the body to get needed amounts of oxygen and nutritions since the connection to the mother’s circulation is disconnected.
This very expansion of lungs causes bradykinins to be released to baby’s circulation while also going to withdrawal from mother’s prostaglandins. These changes cause the ductus arteriosus to obliterate in the manner of hours and sometimes even minutes. By the time, approximately after 3 weeks, this obliterated ductus turns into a connective tissue and changes its name to ligament arteries.
Once it is completely obliterated, ligament doesn’t have lumen anymore, causing the circulation of blood impossible. But, unfortunately, this physiological process doesn’t always come to an end and it does not obliterate. This state causes is called PDA (Persistent Ductus Arteriousus) and has to be treated surgically during infancy.
Location of the ligamentum arteriosum is the same as one of the ductus arteriosus since it is just the postnatal form of the ductus. It is located on the superior surface of the proximal descending aorta connecting it to the left pulmonary artery.
Ligamentum arteriosum lies almost horizontally while the left recurrent laryngeal nerve arches around it. Heart plexus, specifically its shallow sections, is located in front of the ligamentum arteriosum. Heart plexus represents a tangle of nerves responsible for innervating the heart from its basal area. Behind and deeper towards the right lies the left main bronchus.
Cranial nerves, as the name says, represent the pairs of the nerves origin from different centers located inside the brain. These nerves are responsible for motoric and sensitive innervation of various structures in the body. There are 12 paired cranial nerves, a tenth of them is called vagus nerve thus also being called cranial nerve X.
Left vagus nerve branch is located lateral to the ligamentum arteriosum tissue. Left recurrent laryngeal nerve, also known as Gallen’s nerve, springs from the left vagus nerve. It serves for the innervation of the larynx, more specifically of the voice box enabling for the voice to be created, which is how the nerve got its name. On its way towards the pharynx, it hooks around the aortic arch behind the ligamentum arteriosum.
Phrenic nerve, typically originating from the 4th cervical nerve, is also located in the front of the ligament.
Some refer to ligamentum arteriosum as nothing more than an obliterated form of ductus arteriosus. They do not link any function to it once the baby is born. On the other side, there are some referring to ligamentum as one of the factors that contribute to the aorta rupture during times of some major trauma. At the time of rapid deceleration, aorta, the biggest artery in the body, shifts from its normal place and it is assumed that this particular ligamentum keeps aorta in its place when it returns to its normal location. This forced caused by the ligamentum on the wall of the aorta can, in some cases, cause the artery to burst open.
Persistent Ductus Arteriosus
As already mentioned, ductus arteriosus goes through the physiological process of obliteration, enabled by the release of bradykinins from expanded lungs. During this time, since it is detached from placenta and automatically from a mother’s circulation, the body of the baby goes through withdrawal from a mother’s prostaglandins. This chemical changes in baby’s circulation cumulatively start the process of obliteration.
This process may be stopped at some point, due to various disorders, enabling ductus to obliterate. Since the pressure in the air is far greater than one in the pulmonary circulation, the blood flow through the lungs is not increased. This all can lead to pulmonary hypertension and various different disorders.
Treatment of choice for PDA is surgery.