Partial fenestration of the Fontan circuit so as to leave an atrial septal defect prevents the adverse effects of rapid rise in venous pressure as it relieves the excessive right atrial pressure in the immediate postoperative period. . Very high venous pressures can cause hypoproteinemia due to protein losing enteropathy. Venous pressure is relieved, only at the expense of systemic desaturation as the right to left shunt across the atrial septal defect which relieves the venous pressure, causes a fall in systemic oxygen saturation. Sometimes fenestration of a Fontan circuit can also be made by interventional techniques to relieve excessive venous pressures later on in the course of the natural history of a Fontan circulation. It is also possible to close a fenestration in the circuit which is causing undue desaturation using devices by catheter based intervention in the cardiac catheterization laboratory.