In intensive care unit, patients suffered pain and anxiety from mechanical ventilation, presence of endotracheal tube, central venous catheter, postoperative wound, and invasive procedures. Adequate analgesia and sedation can reduce pain and anxiety. However, traditional sedatives carry the risk of unstable hemodynamic status, respiratory depression, increased mechanical ventilation time, incidence of delirium, and length of ICU stay. Dexmedetomidine is a highly selec¬tive α2-adrenergic receptor agonist which causes sedative effects and reduces opioid requirements in the perioperative period. Memis et al had found that dexmedetomidine may prevent inflammatory effects in sepsis patients during sedation. Oxidative stress status is related to the inflammatory response. Moreover, oxidative stress may result in dysfunction of microcirculation. Dysfunction of microcirculation may cause vasoconstriction or microthrombosis, and it will impair tissue perfusion and result in organ dysfunction. The goal of this study is to compare the effects of dexmedetomidine and midazolam on requirement of analgesics, weaning parameter, hemodynamic status, time of extubation, incidence of delirium, and length of ICU stay, oxidative stress status, and microcirculation.

Inclusion Criteria:

• Patient who is older than 18 years old and less than 80 years old
• Patient who is admitted to intensive care unit with artery line
• Patient's hemodynamic status is stable under adequate management and absence of signs of shock
• Patient who has understood the inform consent and agree to participate this study

Exclusion Criteria:

• Patient's hemodynamic status is unstable in spite of adequate management or presence of signs of shock
• Patient who has a past history of allergy to study drugs
• Patient who has renal failure or requires renal replacement therapy
• Patient who has liver cirrhosis or liver failure
• Patient who has participated in any other investigational study of other drugs currently
• Female patient who is pregnant or considers breast feeding currently