Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
Normally, the heart’s set up so that the left ventricle pumps oxygenated blood out to the body through the aorta, then deoxygenated blood comes back to the right atrium, flows into the right ventricle, and is pumped to the lungs through the pulmonary artery, where it comes back to the left atrium, flows into the left ventricle, and this whole process restarts. The “great arteries”, are these two main arteries taking blood away from the heart—the aorta and pulmonary artery. “Transposing” is when two things switch places with each other. So transposition of the great arteries, or TGA—you might be able to guess—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’s now pumped from the left ventricle out to the pulmonary artery to the lungs, and then comes back to the left atrium and left ventricle and restarts. On the right side, blood’s pumped out the right ventricle through the aorta to the body, and blood comes back the the right atrium and right ventricle and then restarts. Blood on the right side therefore never gets oxygenated, and blood on the left side never gets deoxygenated. This isn’t good, and this situation is actually called complete TGA, or sometimes dextro-TGA or d-TGA, dextro meaning “right”, since in this case the aorta’s in front of and primarily to the right of the pulmonary artery.
Alright 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, and rely on blood from the mother as well as a few shunts for blood flow like the foramen ovale, a gap between the atria, the ductus arteriosus—a vessel connecting the aorta and pulmonary artery, and the ductus venosus, which is a vessel connecting the umbilical cord to the inferior vena cava.
After birth, though, when the baby has to use its lungs for oxygen, normally these shunts go away, the foramen ovale closes up 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 are having the foramen ovale or ductus arteriosus stay open or, for the baby to have what’s called a ventricular septal defect, where there’s a shunt between the ventricles—which is actually present in about a third of cases. Any of these allow the two independent circuits to mix blood and deliver some oxygenated blood to the tissues.
That being said, this system still isn’t very efficient, and a significant amount of deoxygenated blood is sent to the body’s tissues, which causes cyanosis, this bluish-purple discoloration of the mouth and lips, fingertips, and toes—areas furthest away from the heart. Sometimes babies might be given prostaglandin E, which keeps the ductus arteriosus open, even though this is typically only a short-term solution and ultimately the baby’s going to need surgical repair.
If the shunt is large enough such that the initial symptoms aren’t noticed and the TGA isn’t repaired, the heart can progress to congestive heart failure. And, this is because the roles of the ventricles have been switched, right? Meaning the right ventricle now pumps out to the higher-pressure systemic circuit, even though it’s built for low-pressure systems, and the left ventricle pumps out to the lower pressure pulmonary circuit even though it’s built for high pressure systems. So in response, the right ventricle can hypertrophy, or get larger, and the left might atrophy, or get smaller, and these massive changes in heart structure can ultimately cause the heart to fail.
Less commonly, babies can have levo-transposition of the great arteries, or l-TGA, with levo meaning that the aorta’s primarily to the “left” of the pulmonary artery. In this form, the great arteries are still connected to the wrong ventricle, but it’s just that the ventricles are the one’s that switch places along with their atrioventricular valves. So in this case, unlike d-TGA, circulation is preserved and therefore sometimes this is called congenitally corrected TGA. This type is acyanotic, meaning “not-blue”. For that reason, babies with l-TGA usually won’t have any obvious symptoms at birth. Now if you take a step back and look at this heart, it doesn’t look that different from a normal heart, but remember that the left ventricle and mitral valve are built to withstand higher pressures, and the right ventricle and tricuspid valve are built for lower pressures. So if these guys switch roles, and now the right ventricle and tricuspid valve see higher pressures than they’re used to, after years pumping against higher pressures, the right ventricle can hypertrophy, grow in size, and stretch out the tricuspid valve, and so adults are at greater risk of heart failure.
Ultimately the cause of TGA is unknown, but there are diseases and behaviors that are associated with higher risk of babies having TGA. Some known risk factors are related to the pregnancy, like the expecting mother having diabetes, rubella, poor nutrition, consuming alcohol, and being over 40 years old.
Sources edit
http://www.cdc.gov/ncbddd/heartdefects/tga.html
http://emedicine.medscape.com/article/900574-overview#a4
https://en.wikipedia.org/wiki/Levo-Transposition_of_the_great_arteries
https://en.wikipedia.org/wiki/Dextro-Transposition_of_the_great_arteries