The Effects of Propofol and Dexmedetomidine Infusion on the Fluid Responsiveness in Critically Ill Patients

In this clinical trial,the investigators hypothesized dexmedetomidine or propofol infusion in patients with circulatory failure decrease cardiac preload and enhanced preload-dependency and fluid responsiveness,and PLR induced pulse pressure variation (PLR-ΔPP) could predict the hypotension during dexmedetomidine induction in critically ill patients.Before dexmedetomidine or propofol infusion, the investigators will conduct passive leg raising test. At first, the patient's trunk was elevated 45 degrees for the first set of measurements. Then, the lower limbs were raised to a 45° angle while the patient's trunk was lowered to a supine position to measure peak BP (usually within 30-90 s). Hemodynamic profiles planned to be measured are SBP, DBP, CVP and HR.Sedation was induced with a dexmedetomidine 0.5μg/kg over 10 min loading, followed by a maintenance infusion of 0.2-0.7 μg/kg/h for one hour. The dose of propofol was titrated targeting the Richmond agitation sedation scale (RASS) ranged from -2 to -1 (bispectral index: 60-75).Hypotension was defined as a SBP less than 80 mmHg or DBP was less than 50 mmHg, or greater than 30% decrease from baseline for longer than 60 s. Bradycardia was defined a HR 50 /min or greater than 30% change from baseline for longer than 60 s. Receiver operator characteristic curve analysis was performed to evaluate sensitivity and specificity of the value PLR-ΔPP to detect patients at high risk of hypotension.