This is a single-blind randomized clinical trial. Patients undergoing abdominal surgery will be enrolled and randomly divided into two groups: dexmedetomidine group and propofol group. In the dexmedetomidine group, patients will receive continuous intravenous infusion of dexmedetomidine (infusion dosage range: 0.1 ~ 0.7 mcg/kg/h). In the propofol group, patients will receive continuous intravenous infusion of propofol (infusion dosage range: 0.3 ~ 1.6 mg/kg/h). Hemodynamics will be continuously monitored using a non-invasive monitoring with chest bioreactance technique at preset time points (0, 2, 4, 6, 12h and 24h). Clinical data such as vital signs, hemodynamic parameters, laboratory results and fluid balance will be recorded. The microcirculation will be examined by Cytocam. The following microcirculatory parameters will be investigated: total small vessel density, perfused small vessel density, microvascular flow index, and heterogeneity index. The enzyme-linked immunosorbent assays will be used to measure serum level of endocan, diamine oxidase, and neutrophil gelatinase-associated lipocalin at preset time points. The primary goal of this study is to compare the effect on microcirculation between dexmedetomidine and propofol in intensive care unit patients after abdominal surgery.

Inclusion Criteria:

• non-emergent major abdominal surgery with ICU admission
• the need of sedation during ICU stay

Exclusion Criteria:

• age younger than 20 years and older than 89 years
• refractory bradycardia (HR < 60 bpm after treatment)
• severe AV block (2nd or 3rd degree)
• refractory shock (MAP < 60 mm Hg after treatment)
• severe heart failure or NYHA 4
• new onset of myocardial infarction within 4 weeks
• receive CPR within 4 weeks
• APACHE score > 30 before enrollment
• severe liver cirrhosis or CHILD B or C
• pregnancy
• allergic history to dexmedetomidine or propofol
• be enrolled in other clinical trials related to dexmedetomidine or propofol within 4 weeks
• non-native speakers