Left heart hypoplasia syndrome: A form of congenital heart disease in which the whole left half of the heart (including the aorta, aortic valve, left ventricle and mitral valve) is underdeveloped (hypoplastic). Blood returning from the lungs has to flow through an opening in the wall between the upper chambers of the heart (an atrial septal defect). The right ventricle pumps blood into the pulmonary artery, and blood reaches the aorta through a shunt (the ductus arteriosus). The child may seem normal at birth but have trouble within a few days of birth (when the ductus closes), become ashen, have rapid and difficult breathing and have problems feeding. This heart defect is usually fatal within the first days or months of life without treatment, namely a sequence of surgical operations or a heart transplant.