Blalock - Taussig shunt is an operation done to improve the oxygen levels in blood of children with certain birth defects of the heart. Typical example is a condition known as tetralogy of Fallot with four defects. Components of tetralogy of Fallot are a defect in the wall between the lower chambers known as ventricular septal defect, narrowing of the opening to the pulmonary artery known as pulmonary stenosis, thickening of the wall of the right ventricle, and overriding of the ventricular septal defect by the aorta.