In this double-blinded randomized controlled study, serum diamine oxidase activity, which is a sensitive and specific marker for the detection of intestinal injury, is taken as the primary endpoint. Other parameters reflecting the functions of liver (AST/ALT), lung (lung injury score and CC-16), kidney (BUN/Cre), and heart (CK-MB/Troponin T), the biomarker of endothelial injury (endocan) will also be determined. Besides, microcirculation parameters measured with Cytocam® and near-infrared spectroscopy (NIRS) will be used to estimate the protective effect of dexmedetomidine on microcirculation. The variables will be collected perioperatively and will be followed up for 3 days after the surgery. Clinical outcome parameters will be followed up for 3 months after the surgery.