Absence of any connection between the right atrium and the right ventricle leads to blood being diverted from the right atrium to the left atrium. The right ventricle (RV) is usually small (Hypoplastic) Survival depends on an associated VSD (often quite small), in order for blood to reach the lungs, or on the ductus remaining open in the early days of life. Most babies will need a 'Shunt' operation during infancy, involving insertion of a tiny piece of artificial tube (made from Goretex) between the Aorta, or a branch (usually one of the arm arteries), and one of the branch Pulmonary Arteries. 

At a second operation some blood from veins in the upper part of the body may be connected directly to the lung arteries (BCPC)
Definitive surgery is usually delayed until the age of at least two years. It involves the so called Fontan operation. This involves connecting the veins from the main circulation (SVC & IVC) directly to the pulmonary arteries. Blue blood is thus directed into the lungs rather than to the left atrium. A patch is placed to prevent blood passing from the RA to the LA - though sometimes a small hole (a 'Fenestration') is deliberately left. 

Bidirectional Cavo Pulmonary Connection (BCPC)

A Cavo-Pulmonary shunt involves connecting the main vein from the upper circulation (the SVC) to the right pulmonary artery (RPA), in order to direct the blood from the upper part of the main circulation into the lung circulation. The main vein from the lower circulation (the IVC) still carries blood into the right atrium. This operation will often improve cyanosis (blueness), but does not eliminate it completely. Many children will go on to have a Fontan operation later, when the IVC blood will also be channelled into the pulmonary arteries and the cyanosis will be completely, or almost completely, relieved.

Fontan operation

The Fontan operation involves connecting the veins from the main circulation (SVC & IVC) directly to the pulmonary arteries. Blue blood is thus directed into the lungs rather than to the left atrium. A patch is placed to prevent blood passing from the RA to the LA - though sometimes a small hole (a 'Fenestration') is deliberately left.