Sequential Use of Propofol/Midazolam and Dexmedetomidine for Sedation in Mechenical Ventialtion Patients in ICU.

Inclusion Criteria: 1. Written Informed Consent 2. Anticipated Ventilation and sedation duration of at least 48 hours 3. Age≥18 years old 4. Weight is in the range of ±20% of standard weight.\[Standard weight= (Height(cm)-80)×70﹪ for male and (Height(cm)-70)×60﹪for female\] - Exclusion Criteria: 1. Extremely unstable of circulatory system, such as systolic blood pressure less than 90 mm Hg despite continuous infusions of vasopressors before the start of study drug infusion or shock； 2. Heart rate less than 50 bpm 3. Second or third degree heart block 4. Serious central nervous system pathology(acute stroke, uncontrolled seizures,severe dementia) 5. Acute hepatitis or severe liver disease (Child-Pugh class C) 6. Dialysis of all types 7）History of neuromuscular disease 8\) Unstable mental status and metal illness 9) Sure or suspected abuse of narcotics and alcohol independence 10) Allergy to experimental drug and other contraindication 11) Advanced tumor patients 12) Neuromuscular blockade other than for intubation 13) Pregnancy or lactation 14) Patient that participate other trial at past 30 days -