Record the renal resistive index and hemodynamic parameters ( record the cardiac output and stroke volume if the patient's next to kin agree to undertake a PiCCO monitoring ) before and after resuscitation for severe sepsis or septic shock patients, to determine whether the changes of resistive index or hemodynamic parameters, especially the cardiac output can be a better parameter to predict AKI

• Purpose: To examine whether the resuscitation-induced changes of renal resistive index or hemodynamic parameters has the superiority as a valid tool to evaluate the renal perfusion improvement and whether the changes can better predict AKI occurrence in severe sepsis or septic shock patients.
• Methods: Measure the renal resistive index and take record of hemodynamic parameters (use a PiCCO monitoring to measure cardiac output if the patient's next to kin agree to undertake such a catheter ) before and after successful resuscitation for severe sepsis or septic shock patients, followup to determine the occurrence of AKI and mortality.
• Hypothesis: The changes of resistive index or cardiac output during shock resuscitation may be a better parameter to evaluate renal perfusion and predict AKI, and may have a better value to guide renal protective therapy in septic shock.

Inclusion Criteria:

• ≥18 years, Sex unlimited
• Arriving ICU in 6 hours and diagnosed with severe sepsis or septic shock
• Physician in charge decide that an aggressive fluid resuscitation was in order

Exclusion Criteria:

• Pregnancy, Age <18 years
• Ongoing recovery from AKI
• Known end-stage renal disease or chronic kidney disease(glomerular filtration rate <30mL/min/1.73m2)
• Known other causes of shock morbidly
• conditions known to modify RRI, such as renal-artery stenosis, intra-abdominal hypertension(>35cmH2O)

Critical ill patients diagnosed with septic shock who required a fluid resuscitation.