Lillian has a condition known as Hypoplastic Left Heart Syndrome, or HLHS. This is a rare congenital heart defect, occurring at a rate of about 0.16-0.36 per 1000 live births, comprising of about 1.5% of all heart defects. It is also one of the most severe types of congenital heart defects, with a 25 to 35% mortality rate even today, which can be higher (up to 55%) if the baby is premature or has other issues.
The term hypoplastic is derived from the Greek words hypo, meaning low, and plasis, which refers to molding or forming (Wikipedia). It functionally means that something is underdeveloped. Combined with the rest of the syndrome name, HLHS means that the left side of the heart is underdeveloped. In Lillian's heart, this under development is severe, to the point that her left side is so small that it's almost not even there. She has, in essence, half a heart. This may be a little easier to understand if you see it, versus having me describe it, so I've attached a picture below that shows a high level overview of the differences between a normal heart and a hypoplastic one. This picture isn't entirely accurate for Lillian, as far as I'm aware, she also has a hypoplastic left aorta (which the hypoplastic picture shows as normal), and if I recall correctly, she does not have an atrial septal defect.
It should go without saying that having a whole heart is kind of a big deal. I should take one more step back here and explain some basic heart functionality for those who are not aware of how it works. If you know how a normal heart works, go ahead and skip this paragraph. A normal human heart has 4 chambers, divided into the left and right side and also between the top and bottom parts. The top and bottom parts serve different functions (blood collection with the atria and blood pumping with the ventricles), and the two functions are paired on the sides, with one atria and one ventricle working together. As I said, the purpose of the ventricles is pumping the blood, with the right ventricle pumping blood without oxygen from the body to lungs, and the left ventricle pumping oxygen rich blood from the lungs out to the rest of the body.
Now Lillian, being a HLHS baby, has a hypoplastic left ventricle. Therefore her body has no way to pump blood with oxygen to the rest of her body. Thus the normal basic cause of mortality in HLHS babies is functionally suffocation, as the heart can no longer deliver blood with oxygen to the body. A baby with HLHS can survive in the womb and for a couple days after surgery due to the ductus arteriosus. The ductus arteriosus is a shunt that connects the pulmonary artery to the aortic arch. In normal babies, this allows blood to bypass the lungs in utero due to the lungs being filled with fluid. In HLHS children, this allows oxygenated blood to mix with the unoxygenated blood in the right ventricle, where this mixed blood is then pumped to the entire body. However, as soon as a baby takes their first breath after being born, this shunt starts to close, taking anywhere from 4 to 11 days to close normally. This closure is good and normal for a healthy baby, but fatal for a HLHS baby. HLHS children must be given a drug called prostaglandin, which helps to prevent this shunt from closing before the first surgery can be completed.
In Lillian's case, her hypoplasia is severe enough to the point that her left ventricle is almost absent entirely (note having no ventricle at all is a separate, but similarly treated condition), and the heart muscle she does have is extremely thick and tough. That last part might sound like a good thing, but it means that it is more difficult for her heart itself to get blood. Lillian has other complications on top of having a left ventricle the size of a small slit. Her aorta, the main artery of the body which takes oxygen rich blood to all parts of the body has severe atresia. According to one of her cardiac surgeons, her aorta was about half a millimeter, the smallest he's seen in the 20 years of being a cardiac surgeon. On top of that, Lillian's coronary arteries (the vessels that supply blood to the heart itself) are malformed as well, with no apparent blood flow through the left coronary artery, and the right one appearing to extend beyond where it normally would down the heart, and up again the left side. All in all, not exactly what we were hoping for in terms of heart development.
HLHS must be treated surgically or it is uniformly fatal. For most children, this means a 3 surgery regiment, typically consisting of the Norwood procedure at birth, the Glenn procedure at around 4 to 8 months, and the Fontan procedure at around 3 to 5 months. It is also possible that at any stage during surgeries and beyond, a HLHS child will experience general heart failure and have to have a heart transplant. The transplant route is only taken with children who absolutely need it due to an extreme shortage of suitable donors. While this is an aside, I urge any one reading this to register as an organ donor. It only takes a few minutes the next time you renew your license, and you could give many families the gift of life as a final act.
I'll cover the specifics of Lillian's heart that haven't been covered here and surgeries in detailed posts later, this post is intended as a general overview of what specifically is wrong with kids who have HLHS, and how Lillian's version of the condition varies from the standard version.