Fluid resuscitation remains the foundation for septic treatment.The evaluation of fluid responsiveness has significance in volume resuscitation for septic shock patients. As the sedative which is commonly used in ICU, Midazolam is supposed to change fluid responsiveness for the pharmacological effect of venodilation. However, the hypothesis has not been certified clinically. In this research, the investigators aim to test the hypothesis that Midazolam can increase fluid responsiveness(using passive leg raising test) in septic shock patients.

Patients are included if they meet the following inclusion criteria:

1. Above 18 years old
2. Endotracheal intubated and mechanical ventilated
3. Awake, can communicate with researchers
4. Diagnosed as septic shock. The definition of septic shock was clarified in Year 2001 ACCP/SCCM guidelines
5. Within 24 hours after the EGDT resuscitation goals are achieved
6. Informed consent are signed.

Exclusion Criteria

1. Under 18 years old
2. Intra-abdominal hypertension
3. Central nerves system dysfunction
4. Heart failure and other cardiac dysfunction such as cardiac conduction block,acute coronary syndrome, cardiogenic shock
5. Proof of volume overload or contradiction of fluid infusion
6. Existence of arterial aneurysm, serious valvular disease,extensive peripheral vascular occlusion disease and insertion of artificial pacemaker