<qa>  What is the SVV derived from?Perioperative fluid management during thoracic surgery is a significantly important, because it is quite difficult to prevent pulmonary edema due to the fluid overload and compromise perfusion of vital organ. So, it is essential to maintain optimal organ perfusion by appropriate fluid management during thoracic surgery. Stroke volume variation SVV is derived from pulse contour analysis and it is known that SVV 1215 correlate with fluid responsiveness, defined as a significant increase in cardiac output with fluid loading, dung two-lung ventilation. It is a parameter derived from changes in stroke volume SV that is according to the heart-lung interaction during mechanical ventilation. positive pressure ventilation induces cyclic changes in left ventricular SV that are related mainly to the expiratory decrease in right ventricular filling and ejection. This is a reflected by variations in the SV. However both ventilator issues, such as tidal volume, PEEP, chest and lung condition, and the cardiovascular condition, such as heart rate, rhythm, ventricular function, cardiac afterload, arterial compliance may affect SVV. Recently some studies reported that SVV could predict fluid responsiveness in mechanically ventilated patients under various conditions. But it is still unclear whether SVV could predict fluid responsiveness during one lung ventilation with the chest open via a thoracotomy. During one-lung ventilation, the shunted blood flow through the non-ventilated-lung dose not contribute to the generation of SVV. And with the chest opening by thoracotomy, the pressure generated by ventilator would not be transmitted to the pulmonary vessels but rather to the atmosphere. So, the purpose of this study is to evaluate the ability of SVV as an indicator a fluid responsiveness particularly in patients undergoing one-lung ventilation with thoracotomy and to found the optimal threshold value of SVV for fluid management during thoracic surgery. 