<qa>  What is the purpose of this clinical trial?Volatile anesthetics vary in their relative hypnotic potency. Recent studies demonstrated that equi-minimum alveolar concentration of various volatile anesthetic agents may produce different bispectral index values. However, the difference between analgesic effects of volatile anesthetics have not yet been investigated. The purpose of this clinical trial is to compare analgesic effect of sevoflurane and desflurane at equal minimum alveolar concentrations using surgical pleth index-guided remifentanil infusion. 