What Are Its Effects?

In Single Ventricle, there is complete mixing of the blood returning from the body with blood returning from the lungs. Therefore, blood leaving the heart through the pulmonary artery will have basically the same oxygen content as that leaving the heart through the aorta on its way to the body.

In the example defect shown (Tricuspid Atresia), pulmonary blood flow for blood oxygenation is dependent on the presence of a Patent Ductus Arteriosus (PDA) as there are multiple levels of right heart obstruction that limit pulmonary blood flow (small ASD, small VSD, subvalve and valvular pulmonary stenosis).  When the PDA closes, the infant can become cyanotic ("blue", decreased blood oxygenation), with insufficient blood flow to the lungs and inadequate oxygen supply to the body.