Monarch Disease Ontology entry MONDO_0004933 (hypoplastic left heart syndrome) (HLHS) refers to the abnormal development of the left-sided cardiac structures, resulting in obstruction to blood flow from the left ventricular outflow tract. In addition, the syndrome includes underdevelopment of the left ventricle, aorta, and aortic arch, as well as mitral atresia or stenosis. Also known as: HLHS.