The purpose of this clinical trial is to compare analgesic effect of sevoflurane and desflurane at equal minimum alveolar concentrations using surgical pleth index.

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.

Inclusion Criteria:

• patients undergoing balanced general anesthesia using volatile anesthetics and remifentanil
• patients with american society of anesthesiologist physical status I, II
• patients aged 19-65 years
• patients obtaining written informed consent

Exclusion Criteria:

• patients with a history of any psychiatric or neurological disease
• patients who had received any medication affecting the central nervous system
• patients who had received medication affecting the sympathetic or parasympathetic nervous systems
• patients undergoing tracheal intubation for airway management
• pregnant women