Hypoplastic right heart syndrome (HRHS) causes inadequate blood flow to the lungs postnatally and thus leads to cyanosis. HRHS involves pulmonary valve atresia, hypoplasia of the right ventricle, hypoplasia of the tricuspid valve, and a small, hypoplastic pulmonary artery. As the ductus arteriosus closes postnatally, infants with HRHS become critically ill.