Issue #1 is an ASD. ASD stands for atrial septal defect and is more commonly known as a heart murmur. This just means there is a hole in the wall that divides the two atria in Scarlette's heart. Sometimes, these holes close on their own, as is the case with the other ASD and VSD (ventricular septal defect) that Scarlette was born with. When an ASD (or VSD) is very large, they do not close on their own, and need to be surgically repaired. Often, this can actually be done via a catheter inserted in the large femoral artery in the leg (amazing, huh?). But in Scarlette's case, they need to do it via open heart surgery BECAUSE of Issue #2, her cor triatriatum. To learn more about ASD you can visit this website: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001210/
Issue #2 is the cor triatriatum. This is much rarer and much more difficult to explain. Basically, in Scarlette's left atria, there is an extra membrane dividing it in two. This membrane is not supposed to be there at all, and it inhibits blood flow. If the membrane were intact, certain important arteries would be starved of blood and MAJOR issues could occur, but Scarlette was born with a hole IN the membrane, allowing for somewhat normal blood flow (though moderately slower, meaning her heart has to pump harder in order to maximize flow). Over time, this membrane could actually "heal" and the hole could close, which could be very bad for Scarlette. This is why they need to take it out now. To learn more about cor triatriatum (one of the rarest of all heart defects) you can go here: http://emedicine.medscape.com/article/895968-overview
Because of both these issues, Scarlette's heart is pumping harder and faster than a normal heart should just to maintain normal blood flow. This means that her heart, a muscle, is working harder and therefore has become enlarged. Some of the benefits we should see after the surgery are: decreased heart rate, decreased breathing rate, increase in weight gain, increase in energy (she will be using less calories for just blood flow/breathing b/c her heart will have to work less) and hopefully a developmental spike as she will be able to utilize those calories into energy to do more! I hope those who were curious about Scarlette's heart issues found this interesting. I know it's somewhat confusing but I tried to translate the best that I know how!