Ligamentum arteriosum and ductus arteriosus
Ligamentum arteriosum (also known as Ligament of Botallo or Harvey's ligament) is a fibrous remnant of the fetal ductus arteriosus (ductus Botalli, Botallo's duct). The ductus arteriosus is a vessel connecting the pulmonary trunk and the aortic arch in the fetus. While this is a vestigial structure in an adult, during fetal development, the ductus arteriosus’ function is to bypass the lungs.
Normally, the ductus arteriosus closes soon after birth; however, occasionally it may remain open, resulting in a condition known as patent ductus arteriosus.
This article will cover the anatomy and function of the ductus arteriosus and the ligamentum arteriosum.
Ductus arteriosus: pulmonary trunk at its division to right and left pulmonary arteries; inferior aspect of aortic arch
Ligamentum arteriosum: Left pulmonary artery near its root; inferior aspect of aortic arch
Functional: up to 48 hours after birth
Anatomic: 1-3 months after birth
|Function||Bypass pulmonary circulation of a fetus|
The fetal ductus arteriosus arises from the distal left portion of the sixth aortic arch. This short artery (8-12mm) forms a connection between the pulmonary trunk, at the point of its division into the right and left pulmonary arteries, and the inferior part of the aortic arch, in line with the origin of the left subclavian artery. The ductus arteriosus location in fetal circulation allows to bypass the pulmonary circuit, which is not needed as oxygenated blood is delivered via the umbilical vein. That means that ductus arteriosus is essentially the connection between the right ventricle and the descending aorta.
It is suggested that the receptors in the walls of the ductus arteriosus react to oxygen causing the walls to contract. However, in an unborn fetus, these receptors are inhibited by prostaglandins E2, I2 and F2a, thereby keeping it open. After birth, this inhibition ceases while a substance called bradykinin is released from the lungs during the first inspiration. Both of these factors result in the contraction of the circular muscle in the tunica media of the vessel’s wall. This process is known as functional closure; it closes off the blood flow which happens up to 48 hours after birth. It occurs alongside other processes, such as closure of the foramen ovale, umbilical vessels and the ductus venosus, all related to changes in the circulation of a newborn. Anatomic closure occurs gradually in the following 1-3 months by the means of thickening of the tunica intima, the innermost layer of the vessel. In short, this means that no more blood is physically able to enter the ductus arteriosus and it begins to gradually break down.
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Although some degree of reversed blood flow may be present in the ductus arteriosus for up to a week, eventually, complete closure and breakdown occurs, leaving the fibrous structure we observe in adults, known as ligamentum arteriosum.
The attachments of the ligamentum arteriosum are very similar to those of the ductus arteriosus - the left pulmonary artery near its root and the inferior part of the aortic arch. Closely related is the recurrent laryngeal branch of the right vagus nerve. Initially, the recurrent laryngeal branches on both sides loop under the 6th aortic arch, the precursor of the ductus arteriosus. The 6th aortic arch past the right pulmonary artery atrophies, causing the right recurrent laryngeal branch to ascend to the right subclavian artery, while on the left, the ductus arteriosus anchors it inferior to the aortic arch.
This relationship may give support to the ductus arteriosus, which results in tunica media containing smooth muscle required at birth to contract the vessel wall and lacking elastin, normally found in other arteries of similar size.
Patent ductus arteriosus (PDA)
The closure of the ductus arteriosus is controlled by a number of factors and if one or more of them become disrupted, the closure may not occur, resulting in a condition known as patent ductus arteriosus. The direction of blood flow in the ductus arteriosus is reversed after birth due to the change in pressure in various parts of the circulatory system. Thus in patent ductus arteriosus, oxygenated blood from the aortic arch is carried into the pulmonary circulation. If left untreated for a long period of time, this anomaly can result in pulmonary hypertension.
Diagnosing this defect can be difficult, as it may be clinically silent. However, it may present as a continuous machinery murmur in the second left intercostal space in the midclavicular line, as well as bounding peripheral pulses.
Treatment includes intravenous non-steroidal anti-inflammatory drugs such as indomethacin, which reduce the prostaglandins in the blood, and hence close the ductus over time. If medical treatment is ineffective, surgical ligation may be necessary. It may also be associated with other congenital defects of the heart, such as aortic or pulmonary atresia, in which case it is the only way for oxygenated and deoxygenated blood to mix and is essential for the survival of the affected individual.