The alpha2 agonist dexmedetomidine is a new sedative agent combined with the analgesic qualities and lack of respiratory depression. Patients sedated with dexmedetomidine could be easily roused, these advances shows dexmedetomidine may be a effective and safe sedative agent. But some studies showed some adversely effects of dexmedetomidine on haemodynamics (such as bradycardia, hypotension), the investigators want to further research the effects of dexmedetomidine on haemodynamics, such as Cardiac Output (CO), Systemic Venous Resistance Index(SVRI), and so on. Propofol is widely used sedative agent in ICU, it also has adversely effects like bradycardia and hypotension, so the investigators want to compare the effect of dexmedetomidine with propofol on haemodynamics after major abdominal surgery. Expect to further research the mechanism of haemodynamics of dexmedetomidine.

When the patients can be roused, they will first receive analgesia with bolus of fentanyl 0.03mg, followed by a continuous infusion at a fixed dose of 0.3μg/kg/h during study period. Then patients will be randomly divided into two groups (Dexmedetomidine and Propofol). All of them will receive hemodynamic monitoring through "Vigileo"(Edwards Lifesciences) and cardiogram monitor. The study will continue for 6-24 hours.

Inclusion Criteria:

1. Postoperative patients of major abdominal surgery
2. Age ≧ 18

Exclusion Criteria:

1. heart rate ≤ 50 bpm
2. allergy with dexmedetomidine or propofol
3. pregnancy

Postoperative patients of major abdominal surgery