Analgesics，Opioid

1. Sufentanil as an strong analgesic is used widespreadly during laparoscopic cholecystectomy. 2. Lower dosages of sufentanil facilitated with dexmedetomidine can offer enough analgesia effect. 3. The investigator are planning use the most classical method for determining the ED50 or half effective concentration of sufentanil facilitated with dexmedetomidine using sequential method, which is also called" up and down method" and" ladder method".