Hypoplastic right heart syndrome (HRHS) is a range of congenital defects in which the right-sided structures of the heart are underdeveloped or not formed.
When these structures (triscupid valve, right ventricle and/or pulmonary valve) are too small or do not function properly, the right side of the heart cannot send enough blood to the lungs.
Untreated HRHS causes a baby to have abnormally low oxygen levels.