Fluid challenge is often carried out in septic shock patients. Its responsiveness usually requires invasive monitoring. The pulmonary artery catheter(PAC) is the most effective means of monitoring.To use non-invasive methods is very tempting. Investigators hypothesize that venous-to-arterial carbon dioxide difference,venous-to-arterial carbon oxygen difference, central venous-arterial carbon dioxide to arterial-venous oxygen content ratio and Central Venous SO2 variations provides feasible estimation on fluid responsiveness in septic shock patients.

Inclusion Criteria:

• more than 18 years old; less than 75 years old ICU patients
• Septic shock
• Monitored with pulmonary artery catheter (Swan-Ganz catheter)
• The decision of fluid challenge made by the treating physician

Exclusion Criteria:

• Evidence of fluid overload
• Pregnancy
• Recently participated in other studies
• Severe heart failure