In this condition, there's no tricuspid valve. That means no blood can flow from the right atrium to the right ventricle. As a result, the right ventricle is small and not fully developed. The child's survival depends on there being an opening in the wall between the atria called an atrial septal defect (ASD) or patent foramen ovale (PFO). Otherwise, the blood returning from the body to the right atrium gets “stuck” in the right atrium and cannot return to the body. Since blood cannot get to the body, death occurs unless something is done to create an ASD or PFO.
The other problem with this defect is that no blood can get to the pulmonary arteries because the right ventricle is usually undeveloped or nonexistent. There are special cases of tricuspid atresia with a hole between the ventricles (ventricular septal defect, VSD, and/or transposition of the great arteries, TGA). These are complex heart defects with a combination of defects which can change the types of treatment needed although they are overall similar.
Heart with Tricuspid Atresia