The goal of this clinical trial is to learn about the effects of binaural beats on maintenance of general anesthesia in patients undergoing thyroidectomy without intraoperative neuromonitoring.

The main question it aims to answer is:

• 1) Does applying binaural beats during surgery reduce the gas anesthetics (especially sevoflurane) requirement to maintaining adequate anesthetic depth during general anesthesia?
• 2) Does applying binaural beats during surgery affect intraoperative hemodynamic stability or post operative nausea and vomiting?

Participants will wear headsets with a sound generator which contains music files (binaural beat file in the intervention group (BB) ; silent file in control group (C)) according to the randomization. Researchers will compare the BB and C group to see if intraoperative binaural beats reduce the requirements of sevoflurane for maintaining adequate anesthetic depth.

This study will look at thyroidectomy patients without neuromonitoring. Before entering the operating room, patients will be randomized according to the randomization table, with the test group receiving a sound generating device with a binaural sound file and the control group receiving a sound generating device with a silent file. After entering the operating room, electrocardiogram, pulse oxymetry, non-invasive blood pressure monitor, and sensor for depth of anesthesia will be attached. At the beginning of anesthesia induction, the headset will be placed on the patient and a sound generator will be connected to the headset to play the file. The induction of anesthesia will be done with administration of fentanyl and propofol in both groups, and after confirming that the patient is unresponsive to voice, rocuronium and sevoflurane will be administrated to the patient. During the operation, the inhaled anesthetic concentration will be adjusted to maintain a patient state index (PSI) between 25 and 50. Fentanyl can be titrated up to 100 mcg to account for the hemodynamic response to intraoperative pain, and neuromuscular blocking agents are titrated to maintain a train of four (TOF) count of 1-3. The headset is continuously applied to the patient during surgery, and blood pressure, pulse oximetry, PSI, end tidal sevoflurane and end tidal minimal alveolar concentration will be monitored during the operation. At the time of the final suture of the skin, the sound generator will be removed from the headset. Save the raw EEG data from the Sedline® sensor for further analytical evaluation.

Inclusion Criteria:

• Patients who are scheduled for thyroidectomy without neuromonitoring
• Patients aged 20-60 who require general anesthesia over 2 hours
• Patients who are able to provide written consent to participate in the clinical trial, to understand the procedure of this clinical trial, and to fill out the questionnaire appropriately
• Patients with ASA physical status classification 1-2

Exclusion Criteria:

• Patients with hearing loss or using hearing aids
• Patients who received narcotic analgesics or sedative drugs within 1 week
• Patients with alcohol or drug dependence
• Patients with drug hypersensitivity to sevoflurane
• Patients with family history or past history of malignant hyperthermia
• Patients with neuromuscular disease of myasthenia gravis
• Patients with arrhythmia, cardiovascular disease, and decreased heart function
• Patients with kidney failure
• Patients who are judged to be inappropriate for this clinical trial according to the opinions of investigators