Assessment of fluid responsiveness (FR) is to detect whether patient could benefit from fluid therapy. Mini fluid challenge has been widely used in clinical practice to prevent patients from volume overload. In clinical practice, 100 mL or 250 mL have been used most frequently and FR was defined as an increase in CO greater than 10% as much as the changes in CO after infusion of 500 mL. While using a half of volume infusion and assessed by the traditional standard of FR, this might misclassify more patients as nonresponders. In that it is imperative to test the predictive ability of mini fluid and find out the threshold of cutoff value. Meanwhile, in almost mini-FC, cardiac output were measured by echocardiography(VTi) and pulse contour,None of the studies conducted cardiac output (CO)measurement by gold standard method of thermodilution by pulmonary artery catheter (PAC). The correlation between new generated CO measurements and PAC varies in different studies as well. It is imperative to investigate the reliability of mini-FC to predicting fluid responsiveness(FR).The investigator's study is to detect the predictive minimal volume using thermodilution by PAC in septic shock patient.