Transposition of the great vessels

Normally, the heart is set up so that the left ventricle pumps oxygenated blood out to the body through the aorta; deoxygenated blood comes back to the right atrium, flows into the right ventricle, and is pumped to the lungs through the pulmonary artery. From the pulmonary artery, it comes back to the left atrium, flows into the left ventricle, and the whole process restarts. The “great arteries,” are the two main arteries taking blood away from the heart: the aorta and pulmonary artery. “Transposing” means that two things switch places with each other. So, transposition of the great arteries, or TGA, is when these two arteries swap locations.

Normally, blood flows through all of these chambers and blood vessels in a big circuit, but if you switch these two main arteries, you switch from one big circuit to two smaller circuits. On the left side, blood is now pumped from the left ventricle, to the pulmonary artery, and to the lungs; it then comes back to the left atrium and left ventricle, and restarts the circuit. On the right side, blood is pumped out of the right ventricle through the aorta, and then goes to the body; blood comes back the the right atrium and right ventricle, and restarts the circuit. Blood on the right side therefore never gets oxygenated, and blood on the left side never gets deoxygenated. This isn’t good. This situation is actually called complete TGA, or sometimes dextro-TGA or d-TGA; dextro means “right,” because, in this case, the aorta is in front of and primarily to the right of the pulmonary artery.

All right, when the fetus in still in the mother’s uterus, babies with d-TGA don’t have any symptoms because they aren’t using their lungs yet. Instead, they rely on blood from the mother and a few shunts for blood flow, including: the foramen ovale, a gap between the atria; the ductus arteriosus, a vessel connecting the aorta and pulmonary artery; and the ductus venosus, a vessel connecting the umbilical cord to the inferior vena cava.

However, after birth, when the baby has to use its lungs for oxygen, these shunts normally go away, the foramen ovale closes, and the vessels become ligaments. This essentially means that d-TGA leads to death, unless there is some way for blood between the pulmonary and systemic circulations to mix. Some possibilities of this happening are if the foramen ovale or ductus arteriosus stay open, or if the baby has a ventricular septal defect, which happens when there’s a shunt between the ventricles — this is actually present in about a third of cases. Any of these possibilities allow the two independent circuits to mix blood and deliver some oxygenated blood to the tissues.

This being said, this system still isn’t very efficient, and a significant amount of deoxygenated blood gets sent to the body’s tissues, which causes cyanosis, a bluish-purple discoloration of the mouth, lips, fingertips, and toes — all areas furthest away from the heart. Sometimes, babies might be given prostaglandin E, which keeps the ductus arteriosus open; however, this is typically only a short-term solution, and ultimately the baby’s going to need surgical repair.