 After induction of anesthesia, pediatric SomaSensor for INVOS 5100C is attached at calf muscle and non-invasive blood pressure cuff is applied to ipsilateral thigh. VOT is performed as: after baseline oxygen saturation value is recorded, cuff pressure is rapidly increased to 30 mmHg over than systolic blood pressure. After 3 minutes of ischemic time, cuff pressure is rapidly decreased. It is performed at 3 time periods : before start of operation, during cardiopulmonary bypass, and after weaning from cardiopulmonary bypass. Postoperative outcome variables are recorded after surgery : major adverse event, use of inotropic and vasoactive drugs, duration of mechanical ventilation and ICU stay. The relationship between the results of VOT and postoperative outcome will be investigated. 