Dexmedetomidine administration for minor procedure, such as extracorporeal shock wave lithotripsy, provides good quality of sedation and analgesia. Dexmedetomidine is a highly selective α2-adrenergic receptor agonist which causes sedative effects and reduces opioid requirements in the perioperative period. Extracorporeal shock wave lithotripsy may cause pain and result in tachycardia and vasoconstriction-related hypertension. Surgical or procedural stress may induce inflammation and it may cause further microthrombosis. Both vasoconstriction and microthrombosis will alter the microcirculatory status. Dysfunction of microcirculation may impair tissue perfusion and result in organ dysfunction. Dexmedetomidine may induce vasodilation by slow intravenous infusion for sedation. Dexmedetomidine may also reduce inflammation. After reviewing the literature, the investigators found that the effect of dexmedetomidine on microcirculation was not well investigated. By application of the non-invasive technique of sidestream dark field imaging, the investigators can investigate the vessel density, percentage of perfused vessels, and microvascular flow index on sublingual vessels.

The goal of this study is to identify the effect of dexmedetomidine on sublingual microcirculation in patient undergoing extracorporeal shock wave lithotripsy.

Inclusion Criteria:

• Patient who is older than 18 years old and less than 75 years old
• Patient who receive extraproporeal shock wave lithotripsy with ASA class I or II
• 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 s dexmedetomidine
• Patient who has history of cardiovascular, renal or hepatic dysfunction
• Patient who has participated in any other investigational study of other drugs currently
• Female patient who is pregnant or considers breast feeding currently
• Patient who has suspected full stomach