For the patient who passed the screen of weaning from ventilation, the traditional sedation drug such as propofol and midazolam should be decreased or stopped, then it would induce the stress response and agitation, such as hypertension,tachycardia etc. In order to resolve that problem, the sedation should be given again. but if renewed to sedation, it would cause prolonged sedation and ventilation duration.

So as to resolve above issues, after the patient passed the screen of weaning, sedation is changed to Dexmedetomidine, a new sedation drug, that could wake up at any time. So our study to compare the efficacy and safety of sequential sedation of propofol or midazolam and dexmedetomidine in mechanical ventilated ICU patients.

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 -