This study was designed to observe the effects of dexmedetomidine on delirium and haemodynamic in mechanical ventilated elderly patients,and to evaluate the safety of dexmedetomidine.

Inclusion Criteria:

• Written informed consent
• Intensive care unit patients who need mechanical ventilation
• Anticipate duration of mechanical ventilation longer than 24 hours,and shorter than 96 hours prior to giving investigational products
• Acute physiology and chronic health evaluation II score more than 10
• Aged between 60 to 80 years old
• Weight between plus or minus 20% of the standard weight. Male: (height cm - 80) × 70% = standard weight Female: (height cm - 70) × 60% = standard weight

Exclusion Criteria:

• Trauma and burn patients
• Any kind of dialysis
• Suspected or confirmed difficult airway
• Use of neuromuscular blocking drugs(Except using for intubation )
• Neuromuscular disease
• Epidural or subarachnoid anesthesia
• Plan to give general anesthesia before 24 hours drugs intervention or after giving investigational drugs
• Serious central nervous system disease
• Mental disability or mental disease
• Acute hepatitis and serious liver disease(Child-Pugh C)
• Unstable angina and acute myocardium infarction
• Left ventricular ejection volume less than 30%,heart rate less than 50 beats per minute
• II and III degree atrioventricular block
• Systolic pressure less than 90 mmHg after giving vasopressors twice before beginning of this study
• Consciousness disturbance patients whose blood pressure less than 70mmHg and blood lactate more than 4.0mmol/L
• Suspected or confirmed long term use of narcotic analgesics
• Subjects who are breastfeeding or pregnant
• Allergic to investigational products or with other contraindication
• Participated in other study within 30 days