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.

Fluid therapy is the cornerstone of septic resuscitation. In recent decade, a restrictive fluid strategy has been applied to shock resuscitation for fewer complications and shorter hospital stays compared with a liberal fluid strategy. Therefore, it is a reasonable step to assess the patient's response to fluid infusion to avoid excessive fluid administration.Mini-fluid challenge allows the selection of fluid responders by inducible changes of hemodynamic parameters after changing the preload gaining popularity. The mini fluid challenge was firstly described by Muller et al. in 2011. They found an infusion of 100 mL colloid over 1 minute and the assessment by cardiac output using velocity time integral (VTi) at the aortic outflow tract could predict fluid responsiveness (FR). Since then, a total of seven investigations have been published. Although a mini-fluid challenge may help the decision-making process of fluid management, the investigation results differed from each other, especially in minimal volume and cut off value of hemodynamic parameters change. Meanwhile, we noticed that when measuring CO, the method of thermodilution by pulmonary artery catheter (PAC), which is the gold standard of CO measurement, was not used in these studies. Furthermore, the majority participants of the studies were perioperative patients in a stable hemodynamic state.Therefore, there are two question needed to be answered:what is the minimal infusion volume in effectively predicting fluid responsiveness, and whether mini-volume can perform a real change in septic shock patients. Thus, it is necessary to explore the minimal volume in fluid challenge by PAC and test its reliability in detecting responders (R) and nonresponders (NR).

Inclusion Criteria:

• Patients diagnosis of septic shock and is required fluid challenge in the presence of invasive hemodynamic monitoring.
• Patients with hypotension (SBP <90 mmHg or MAP <65 mmHg)
• Patients with evidences of tissue hypoperfusion (including but not limited to oliguria, skin mottling, altered mental status, cool peripheries, hyperlactatemia, et al).

Exclusion Criteria:

• Age less than 18yrs or greater than 80yrs
• Shock is diagnosed with other types of shock
• Known allergy to colloid fluids
• Pregnancy
• Recent participation in another biomedical study.

In our department, we has done a serial clinical studies of fluid challenge, we will do retrospective study first.