The ductus arteriosus is a vascular connection that links the arch of the aorta and the pulmonary truct. In this article we will cover the embryology, anatomy, and function of the ductus arteriosus. We will also discuss their clinical relevance, with regards to congenital heart disease.
In order to understand the purpose of the ductus arteriosus, you must have an understanding of fetal circulation. The placenta acts as the source of oxygen, nutrients, and waste disposal for the fetus. The single umbilical vein arises from the placenta and runs in the umbilical cord. It joins the inferior vena cava, via ductus venosus. Hence, the blood going to the fetus is able to bypass the liver. The inferior vena cava then empties into the right atrium. In fetal life the lungs are not functional yet. This makes logical sense, as the fetus is surrounded by amniotic fluid, and not air, which humans require for effective gas exchange.
The blood flows from the pulmonary artery to the aortic arch via the ductus arteriosus. This blood then supplies the body with oxygen and nutrients. The two umbilical arteries (branches of the internal iliac arteries in fetal life), then flow into the umbilical cord, to the placenta. Here the waste and carbon dioxide is removed by the placenta.
The fetal heart is very different from the adult heart. In order for the blood entering the right atrium to bypass the lungs, there is a large atrial septal defect. Blood is then able to shunt into the left atrium. During fetal life, there are different atrial septal defects at varying times.
Initially there is a single interatrial septum (the septum primum) which forms during the fourth week of gestation. At this point there is a small opening in the septum called the ostium primum. The septum primum grows and causes the ostium primum to narrow and it is obliterated. There is also a widely believed theory that the ostium secundum results from predetermined apoptosis. The ostium secundum is able to provide communication between the atria after the septum primum closes off. After the septum primum has closed, the septum secundum, a second wall of tissue grows to cover the ostium secundum (in the right atrium). At this point the blood is able to flow from the right atrium to the left by way of a small opening in the septum secundum, and through the ostium secundum. Both openings together constitute the foramen ovale.
The ostium secundum in the septum primum forms in the right side of the interatrial septum during the sixth week of fetal life, approximately the same time as the fetal heart comes into being. The ostium primum in the septum secundum forms in the left side of the interatrial septum. The blood is then able to flow through the foramen ovale (primum) and through the foramen secundum in order to enter the left atrium. From here the blood flows into the left ventricle, and is pumped around the body.
The arch of the aorta is where we find the ductus arteriosus. It allows for blood to flow from the left side circulation to the right side. When the child is born, the pulmonary pressure decreases, causing the pressure in the left atrium to rise above that of the right atrium. This causes the septum primum to abut the septum secundum, and causes functional closure. Over time the two septa fuse, resulting in an embryological remnant also known as the foramen ovale.
The ductus arteriosus is a connection between the arch of the aorta and the left pulmonary artery that is patent during fetal life. In fetal life, the blood shunts from the right atrium to the left across the foramen ovale and up the ductus arteriosus in order to enter the aortic arch and supply the body with nutrient rich oxygenated blood. Normally it closes at birth, but if it doesn’t, it results in a condition known as patent ductus arteriosus. Blood will now flow from the arch of the aorta to the pulmonary artery. If the shunt remains uncorrected, the defect results in problems with the rhythm of the heart, or pulmonary hypertension, which may lead to failure of the heart and its ability to effectively pump (congestive heart failure).
The ductus arteriosus remains patent due to the prostaglandin E2 series. They cause dilation of the smooth muscle within the ductus to ensure it stays patent during the fetal period. After birth, the levels of these prostaglandins drops sharply, resulting in smooth muscle constriction and subsequent duct closure. This effect, combined with the inflation of the lungs and subsequent closure of the foramen ovale, means that the baby now has a functional adult-like circulation with normal pulmonary and systemic vascular circulation.
In some cases, babies may be born with a defect that severely impairs blood flow out of the right or left ventricles, to the lungs and body respectively, such as severe tricuspid or pulmonary atresia, total anomalous pulmonary venous drainage (where the blood from the lungs is returned to the right atrium, not the left), or tetralogy of Fallot, characterized by pulmonary atresia which varies in severity between babies. Transposition of the great arteries is where two separate circulation loops form i.e. the aorta rises from the right ventricle and the pulmonary trunk arises from the left ventricle. The blood that flows around the body therefore has no way of becoming oxygenated, so these shunts must remain patent in order to allow flow between the right and left circulations. All of these shunts are easily remembered, as they all start with the letter T i.e.: Tetralogy of Fallot, tricuspid atresia, total anomalous pulmonary venous drainage, and transposition of the great arteries.
In the same way, if the child has a ventricular or atrial septal defect, there will initially be a left to right shunt (as the left circulation has higher pressure). Over time this can cause congestion of the pulmonary circulation and this, finally, pulmonary hypertension. Surgery is performed before this point is reached. However, only the cyanotic disorders mentioned in the previous paragraph present with acute cyanosis (blue baby syndrome).
The child will likely be given a prostaglandin infusion in order to maintain the fetal circulation pattern. These shunts are the only mechanism by which blood can pass from the right side of the heart to the left.