Hypoplastic left heart syndrome (HLHS) is a congenital heart defect that occurs when the left side of the heart does not develop normally. The left ventricle, which normally pumps oxygen rich blood to the body, and the opening of the aorta, the vessel that carries that blood, are too small. The aortic and mitral valves that control the flow of blood into the left ventricle and out to the aorta are narrowed or completely closed.