This study is to investigate on the objective relevance between bispectral index (BIS) and Observer's assessment of alertness/sedation (OAA/S) scale in patients sedated with Midazolam, propofol and dexmedetomidine during spinal anesthesia. Also, we will evaluate the reflection of actual sedation levels on BIS monitoring.

Sedation is an important element of regional anesthesia during surgery, which allows to finish surgery smoothly by reducing patients' anxiety and fear who undergo regional and local anesthesia.

If adequate sedation is not maintained during regional anesthesia, surgery may be disturbed by patient's movement. This can cause the patient to be physically and mentally stressed with discomfort.

Commonly used measurement to determine the adequate sedation level relies on subjective physician's assessment such as scoring for response to talk, shake and pain. This measurement is unreliable and not continuous.

Recently, BIS is used to guide sedation during spinal anesthesia as an objective monitoring method in many studies. but previous studies result contrasting in the correlation between BIS and sedation levels with lack in evaluation between sedative drugs.

In this study, the investigators will investigate the usefullness of BIS monitoring during regional anesthesia with sedation and evaluate the suitability of correlation BIS and OAA/S between drugs (midazolam, propofol and dexmedetomidine). The anesthetic and sedation protocol will be standardized.

Inclusion Criteria:

• Patients who scheduled for knee arthroscopic surgery under spinal anesthesia

Exclusion Criteria:

• Patients with a history of hepatic, renal, cardiovascular disease, and those who have allergy to study drugs (dexmedetomidine, propofol, midazolam).
• Hemodynamically unstable patients.
• Patients who is noncommunicative.