Hypoplastic Left Heart Syndrome (HLHS) is characterized by a small (hypoplastic means smaller than normal) and functionally inadequate left ventricle of the heart.

The hypoplasia (reduction in size) of the left heart affects the formation of the left sided structures, resulting in an absent or nonfunctioning mitral valve (mitral valve atresia -1 in diagram), aortic valve atresia (2), and hypoplasia of the ascending aorta (4).

Diagram At Left:
1. Atretic or Stenotic Mitral Valve
2. Atretic or Stenotic Aortic Valve
3. Hypoplastic Left Ventricle
4. Hypoplastic Ascending Aorta
5. Coarctation of the Aorta
6. Atrial Septal Defect