Hypoplastic left heart syndrome is a heart defect that is present at birth (congenital) in which the left side of the heart and aorta are very small or underdeveloped. The small left ventricle makes the right ventricle responsible for pumping blood to the lungs and body.

Over time, too little blood may be pumped to the body, leading to heart failure. Without a series of major heart operations, this defect almost always causes death.