In Tetralogy of Fallot there is right outflow obstruction due to pulmonary stenosis with supravalvular pulmonary artery obstruction.In case of severe obstruction, the right ventricular pressure becomes greater than the left ventricular presence and the deoxygenated blood starts moving to the left ventricle resulting in severe cyanosis and erythrocytosis. In this situation the aim is to increase blood flow towards the right heart. The methods adopted are compression of Aorta (knee chest position) and use of vasopressor drugs like methoxamine (Phenylepinephrine is also a vasopressor). Any condition or drug which causes peripheral pooling of blood should be avoided because this will reduce return of blood to heart. Therefore isoprenaline which has marked 13, agonistic action should be avoided as it will cause vasodilatation which leads to peripheral pooling of blood. This will be harmful in a case of Tetralogy of Fallot.