Transposition of the great arteries (or the great vessels), is a rare condition in which the arteries that carry blood from the heart’s two lower chambers —the right ventricle and the left ventricle— are each attached to the wrong ventricle. The aorta receives blood from the right ventricle instead of the left, while the pulmonary artery receives blood from the left ventricle instead of the right. To have a better image of what this means, let’s review the circulation of blood both after birth (in children and adults) and before (in the fetus). Risk factors for giving birth to a baby with transposition include alcohol abuse and use of certain drugs in the mother as well as maternal infection with rubella virus.
In normal circulation in children and adults, there really are two circulatory pathways, one pumped by the right side of the heart, the other pumped by the left side. After supplying body tissues with oxygen (O2), glucose (blood sugar), and other vital chemicals, and carrying away carbon dioxide (CO2) and other waste products, blood arrives in the heart’s right atrium (the upper chamber on the right side of the heart). From there, the blood is pumped through a structure called the tricuspid valve into the right ventricle (the lower chamber on the right side of the heart), which sends it through another valve called the pulmonary valve into a large vessel called the pulmonary artery. The pulmonary artery splits into left and right branches, each of which branches into an increasing number of vessels that carry blood through the left and right lungs, respectively. In special capillaries in the lungs’ air sacs, blood releases CO2, which is exhaled, and absorbs O2, then moves into a system of increasingly larger veins that converge into a few (usually four) pulmonary veins that deliver blood to the left atrium (upper chamber in the left side of the heart). When the left atrium contracts, the blood moves through the bicuspid valve, also called the mitral valve, into the left ventricle (the lower chamber on the left side). When the ventricles contract, the blood in the left ventricle moves through the aortic valve into a large vessel called the aorta, from which numerous arteries branch off, supplying body tissues, from which blood returns again to the right atrium, where we began our discussion.
Normally, as blood moves through the heart, there is no mixing between blood on the left and right sides of the heart, because there is a septum (a wall) dividing the right and left atria and the right and left ventricles. But during fetal life, right up to birth, there are some connections between the left and right circulation, so that oxygenated blood can bypass the fetal lungs, which are collapsed. Since the embryo-fetus has no direct access to the outside air, she must depend on the mother’s lungs, and thus oxygenated blood arrives from the mother’s lungs via the placenta and the umbilical vein. The umbilical vein, in turn, delivers the oxygenated blood into a large vein that carries the blood into the fetal right atrium. Although this fresh blood mixes in the right atrium with blood from other parts of the fetal body that is depleted of O2 and loaded with CO2, it is fresh enough to nourish the fetal body tissues. Since the developing fetal lungs are collapsed with no air to inflate them, pressure in the fetal pulmonary vessels is high. Consequently, when the right ventricle contracts, sending blood into the pulmonary artery, that blood is detoured through a fetal blood vessel called the ductus arteriosus, which leads directly into the fetal aorta. Since blood from the right atrium needs to get into the aorta and bypass the collapsed lungs, it is vital that the ductus arteriosus remains open until birth, which it does, as does a connection between the right and left atria, called the foramen ovale.
At birth, the closure of the foramen ovale and the change in pressures between the left and right circulation lead to the normal child/adult circulatory pathway that we described above. In the transposition congenital anomaly, however, the great vessels are reversed. The aorta, whose blood is bound for body tissues, is attached to the right ventricle, whose blood has just arrived from body tissues. The pulmonary artery, whose blood is bound for the lungs to receive O2 and release CO2 already contains oxygenated blood that has been purged of CO2. Basically, transposition means that the right and left blood circulations are not hooked up in series, but run in parallel —the left ventricle pumping blood around and around through body tissues, but not to the lungs, the right ventricle pumping blood through the lungs, only for it to return directly to the right side of the heart.
The only reason why a newborn with transposition can stay alive is the persistence of other connections, between the left and right blood. One such important connection is the ductus arteriosus, the blood vessel that connects the pulmonary artery and aorta during fetal life and which is still open at the moment of birth, but tends to close up by a month or so after birth. Despite the connections between left and right circulation, transposition causes what doctors call cyanosis. This means that blood with insufficient oxygen is circulating from the aorta through body tissues. Transposition is thus called a cyanotic heart disease. In babies with transposition, doctors will want to keep the ductus arteriosus open as long as possible (by giving the child chemicals called prostaglandins), until the child can have surgery. In some babies, there also can be other connections between left and right blood, which can be helpful.