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A second operation, a Glenn shunt, often performed at about 4 to 12 months of age, replaces the Blalock-Taussig shunt with another connection to the pulmonary artery. In this operation, the Blalock-Taussig shunt is removed, and the superior vena cava (the large vein that brings oxygen-poor blood from the head and arms back to the heart) is connected to the right pulmonary artery. Blood from the head and arms passively flows into the pulmonary artery and proceeds to the lungs to receive oxygen. However, oxygen-poor (blue) blood returning to the heart from the lower body through the inferior vena cava will still mix with oxygen-rich (red) blood in the left heart and travel to the body, so the child will remain mildly cyanotic. This operation helps create some of the connections necessary for the final operation, the Fontan procedure.