The Children's Hospital of Philadelphia
Hypoplastic Left Heart Syndrome — What is Hypoplastic Left Heart Syndrome (HLHS)?
Jack Rychik, MD: Hypoplastic left heart syndrome, or HLHS, is a collection of diseases-- collection of anomalies of the heart in which the left side of the heart, for whatever reason, is inadequate to do the job it's supposed to do in delivering blood flow to the body.
J. William Gaynor, MD: And it's grouped in a group of defects called single ventricle where there's really only one pumping chamber to the heart.
Jack Rychik, MD: In order to understand and to comprehend any of the different forms of the heart disease that exist, it's very important to make sure we understand what the healthy or the normal heart looks like. The normal heart has two sides--a right side and the left side and four chambers--the top receiving chambers or atria and the lower chambers, which are thick-walled pumping chambers called ventricles. Red blood cell will come from either the superior vena cava or inferior vena cava and enter into the right atrium. The blood then flows across the tricuspid valve to the right ventricle. The right ventricle then squeezes and ejects that blood cell into a vessel called the pulmonary artery. Pulmonary artery splits into two vessels, each going to the lungs. As that red blood cell makes its way through the lung it returns through the pulmonary veins to the left atrium. That blood is now oxygenated. It's picked up oxygen then goes across the mitral valve into the left ventricle, which does most of the work in terms of delivery of blood flow to the body. That blood cell is now ejected into the aorta to some organ or muscle or skin in the human body. Now, there are some significant differences between the heart in the newborn and the heart in the fetus.
Elizabeth Goldmuntz, MD: The heart actually is-- assumes its almost complete anatomy often before a woman would even know she was pregnant.
J. William Gaynor, MD: The heart is actually functioning and pumping blood to the baby throughout most of fetal life.
Jack Rychik, MD: Because the lungs are collapsed in the fetus and it's really the placenta through which much of the oxygenation takes place through the mother, there are various bypass pathways within the fetal heart that direct blood away from the lung. The first is a structure called the foramen ovale. That's a communication between the two top chambers of the heart that allows for blood to go from the right atrium to the left atrium. In fact, because there is very little blood that's returning from the lung, which would normally go to the left side, the majority of blood that fills the left atrium and left ventricle is coming across the foramen ovale from the right side. Because the lungs are collapsed, there's high pressure, high resistance in the lungs as blood is ejected out the right ventricle and enters into the main pulmonary artery. Very little goes down into the lungs themselves, the majority goes into a structure called the ductus arteriosus, which is the second point in communication between the pulmonary artery and the descending aorta. The third structure that's important that connects the umbilical vein to the fetal circulation is a site, a junction, called the ductus venosus. That acts as somewhat of a resistor, if you will, in terms of controlling the return of blood from the placenta to the fetal circulation. Birth is a wonderful process and an amazing process, and there's a dramatic change that takes place in what we call the fetal transition-- this transition of the circulation from a fetal life to neonatal life. As soon as the cord is clamped, the ductus venosus ceases to carry blood to the heart, and it begins to constrict within the first few hours or days of life. The very first thing that happens when the fetus is born is it takes its first breath, the lungs expand and so the resistance or pressure in the lungs drop and that promotes blood flow into the lung itself. The ductus arteriosus begins to constrict and is typically fully closed within 24 to 48 hours of life, and blood is now then fully directed into the lung. As the blood returns to the left side of the heart after traversing the pulmonary circulation and picking up oxygen, pressure in the left atrium rises just a bit and the trapdoor of the foramen ovale, which was open before birth, now begins to close, usually within the first few days of life.
So one can imagine that if you are solely dependent on your right ventricle to do the job of delivering blood to the body, once that ductus arteriosus begins to constrict, or close, then there's no way for blood to get to the body, and unfortunately these babies die.
Peter Gruber, MD: But, if we know a complex lesion is coming up, a child that's suffering from one of these, we can prepare for that ahead of time.
J. William Gaynor, MD: We can avoid having a sudden cardiac arrest at home or even death at--sudden death at home. By knowing that the baby has the heart defect, if necessary we can arrange to have the baby delivered at CHOP--
J. William Gaynor, MD: --in a much more controlled treatment strategy. So that's a huge advantage in terms of prenatal diagnosis.