Dynamic Change of Doppler-based Renal Resistive Index in Predicting Renal Recovery

The goal of this observational study is to observe the correlation between dynamic changes in the Doppler-based renal resistive index (RI) and the occurrence of persistent acute kidney injury (AKI) in critically ill patients. The main questions it aims to answer are: * Are dynamic changes of RI within 24 or 48 hours associated with the prognosis of AKI (NO AKI, transient AKI, persistent AKI)? * what are the influencing factors of changes in RI? Participants will measure values of RI in patients at admission (0 hour), after 24 hours and after 48 hours, respectively, and collect characters such as age, sex, vital signs, Apache II, diagnosis, renal function, applied drugs, fluid, 72-hour renal function and renal replacement, followed renal replacement and the mortality rate at 28 days.