Hypoplastic left heart syndrome (HLHS) is one of the most complex cardiac defects seen in the newborn and remains probably the most challenging to manage of all congenital heart defects. It is one of a group of cardiac anomalies that can be grouped together under the description single ventricle defects.
In a child with hypoplastic left heart syndrome, all of the structures on the left side of the heart (the side which receives oxygen-rich blood from the lungs and pumps it out to the body) are severely underdeveloped.
The mitral and aortic valves are either completely "atretic" (closed), or they are very small. The left ventricle itself is tiny, and the first part of the aorta is very small, often only a few millimeters in diameter.
This results in a situation where the left side of the heart is completely unable to support the circulation needed by the body's organs, though the right side of the heart (the side that delivers blood to the lungs) is typically normally developed.
Blood returning from the lungs to the left atrium must pass through an atrial septal defect (ASD) to the right side of the heart.
The right ventricle must then do a "double duty" of pumping blood both to the lungs (via the pulmonary artery) and out to the body (via a patent ductus arteriosus). The patent ductus arteriosus, a normal structure in the fetus, is often the only pathway through which blood can reach the body from the heart. When the ductus arteriosus begins to close, as it typically does in the first days of life, the blood flow to the body will severely diminish, resulting in dangerously low blood flow to vital organs and leading to shock.
Without treatment, hypoplastic left heart syndrome is uniformly fatal, often within the first hours or days of life.