What is TGA?
TGA stands for transposition of great arteries. Great arteries refers to the 2 tubes coming out of the heart. The tube from the right side of the heart carries blue blood without oxygen to the lungs. The tube form the left ventricle carries red blood with oxygen to the whole body.
Transposition refers to the 2 tubes exchanging the position between the 2 tubes i.e. the right sided blue blood carrying tube going to the lungs now in this condition comes out of the left pump but carries red blood and goes to the lungs (it takes red blood to get more oxygen ?-does not work out !)
The aorta which is supposed to carry red blood comes out of the right sided ventricle and carries blue blood to the whole body.
How does the blood flow in utero before the baby is born?
The fetal circulation helps the baby overcome the problem. The problem does not manifest itself because the lungs don’t function in a fetus. The blood (blue) which is supposed to go to the lungs ends up in the lower half of the body normally via the PDA. In transposition, it is the aorta coming out of the right ventricle, and this gets the red blood in its lower half circulation from the pulmonary artery arising from the left ventricle. The mixing happens via the ductus arteriosus.
How does the baby manifest in the newborn period?
The baby presents as a very blue baby because the blue blood keeps going to the body (and gets more blue and comes back to go back to the body again) without getting red via the lungs! The red blood gets more red and does not get its oxygen delivered to the body because it can never reach the body. This is incompatible with life. Some little bit of mixing between red and blue blood happens vis PFO and the PDA. These allow the red blood reach the body and the blue blood reach the lungs. If this little bit of mixing does not happen, the child cannot survive even some short time.
So, what is done to make the child better or survive?
The child is started on some medicine called Prostaglandin to keep the PDA open. Some mixing will happen via the PDA. And the red blood will get to the body while some blue blood will get to the lungs. Beyond this when the baby reaches a tertiary care center, the child undergoes a procedure called septostomy where by the hole between the upper chambers of the heart (PFO) is made larger and the baby becomes much pinker.
When is the final repair done?
The septostomy is a measure to stop the prostaglandin and allow the baby to have good and stable oxygen levels. The final surgery happens a week or 2 into the life of the newborn.
Why is the timing of the surgery so important?
The child is born with high lung pressures. The lung blood pressure is same as blood pressure when they are born (incidentally in us adults, it is 30-35mmHg as compared to a blood pressure of 120 in the rest of the body). The blood pressure of a baby is 80-90 mmHg. So, the right pump of the heart in a baby with TGA is connected to the body (with pressures of 80-90mmHg). The left pump of the heart is connected to the lungs with a pressure of 60-70 mmHG at birth. This pressure is too much for the surgery to be performed. By 1 week of age the lung pressure drops to normal or just above it. So the left ventricle starts pumping against a lot lower pressure and gets used to that. Once the surgery is performed, the right would start pumping into the lungs and the left into the body. The left pump would suddenly need to pump against 60-80 mmHg pressure from 30-40 mmHg. This maybe too much of a change.
So, the surgery cannot happen too early in neonatal period because the lung pressure and resistance is too high. Nor can it happen too late because the left pump would be too “lazy” to pump against a higher pressure of the whole body (compared to before: low pressure).
What is done during surgery?
During surgery the 2 major arteries are cut and re stitched to the correct location. This itself is easy. But, in addition, the coronary arteries are transplanted from the artery coming out of the right pump to the artery coming out of the left pump (like it is in all of us).