Sedation in the ICUs is very common. There is no ideal sedative yet, so research has been conducted to replace propofol and midazolam, which are the most commonly used sedatives in ICUs, by inhalation anesthetics. The investigators will sedate the patients who undergo head & neck surgery with tracheostomy for several days using sevoflurane, a kind of inhalation agent, via anesthetic conserving device.

The objective of this study is to confirm the end-tidal sevoflurane concentration for inducing moderate sedation (RASS -2~-3). In addition, the investigators compare the volatile sedation with the IV(intravenous) sedation to see if the volatile sedation could reduce the amount of post-operative opioid consumption.

• RASS: Richmond Agitation-Sedation Scale
• RASS: Richmond Agitation-Sedation Scale

When patients, who need mechanical ventilation for several days after head & neck surgery, enters the ICU, the investigators induce moderate sedation (RASS -2~-3). End-tidal sevoflurane (etSEVO) concentration is adjusted to 0.5Vol% within 30 minutes. Sedation level is evaluated every 5 minutes. After 30 minutes, the investigators check that the sedation goal is achieved.

Following the up & down method, if the moderate sedation (RASS -2~-3) or the deep sedation (RASS -4~-5) is achieved at the etSEVO concentration of 0.5vol% after 30 minutes of sedation, the next patient's target etSEVO concentration is reduced to 0.4vol%. On the contrary to this, if only light sedation (RASS above -1) is achieved, the next patient's target etSEVO is increased to 0.6vol%. This process is repeated to find the appropriate etSEVO concentration at the beginning of sedation, which induces sedation of RASS -2~-3.

In retrospective analysis, the investigators compared the prospective study group with propofol intravenous sedation group.

* RASS: Richmond Agitation-Sedation Scale

Inclusion Criteria:

• Patients who needs sedation in the ICU for several days after head & neck surgery
• ASA class I~III
• Adult patients over 20 years old
• Patients who can read and understand the informed consent

Exclusion Criteria:

• Patients who do not agree to participate in the study
• Past history or Family history of malignant hyperthermia
• End stage renal disease (eGFR<30 or dialysis)
• Moderate to severe liver disease (AST, ALT > 200IU/L)
• Pregnant women
• Patients who cannot read and understand the informed consent