The purpose of this study is to analyze raw electroencephalogram in infants younger than 2 years old undergoing general anesthesia using sevoflurane.

After obtaining an informed consent from one of the parents, patients undergo routine anesthetic induction using sodium thiopental. Bispectral index and patient state index are both monitored simultaneously. Raw electroencephalogram data are transmitted and recorded via a dedicated equipment. Anesthesia is maintained using sevoflurane with end-tidal level of less than 2 minimum alveolar concentration (MAC) adjusted to age. Fentanyl of up to 1 mcg/kg can be administered in case of increased heart rate and mean blood pressure with a degree of 30% of baseline.

During anesthesia, patients' status of sedation is defined as 'maintenance of surgical state of anesthesia (MOSSA)' when the end-tidal concentration of sevoflurane is maintained between 0.7 and 1.3 MAC and mean blood pressure and heart rate is between 80-120% of baseline.

'Emergence' is defined as a 3-minute period in the middle of continuous decrease in end-tidal concentration of sevoflurane in the range of less than 0.7 MAC and finally reaching 0.2 MAC.

Raw electroencephalogram waves are analyzed by distribution of power along frequency bands and compared according to the state of anesthesia or bispectral index / patient state index value.

Inclusion Criteria:

• Pediatric patients aged between 4 months and 2 years planned to undergo surgery under general anesthesia
• American Society of Anesthesiologists physical status 1 or 2

Exclusion Criteria:

• History of preterm birth (postconceptual age < 32 wks) or history of mechanical ventilation immediately after birth
• Presence of genetic disease or chromosomal abnormality that can affect brain development
• Presence of disease in central nervous system
• History of head trauma or surgery involving head
• History of hypersensitivity to anesthetic drugs
• Intubated or sedated state before induction of anesthesia
• Difficulty in attaching both the bispectral index and patient state index probes to patient's forehead
• Expected operation time of less than 5 minutes

Pediatric patients aged between 4 months and 2 years planned to undergo surgery under general anesthesia, whose American Society of Anesthesiologists physical status are 1 or 2