We report a case of an erroneously elevated bispectral index (BIS) during robot assisted thyroidectomy using an electromyographic endotracheal tube (EMG tube), which is safe and useful for laryngeal electromyographic monitoring. Ten minutes after start of the operation, a sudden increase of BIS value up to 98 was noticed. The BIS values were not decreased to < 65 with supplemental anesthetics. The anesthetic method was changed from total intravenous anesthesia to balanced anesthesia. The BIS sensor and monitor were changed and other models were used. These interventions did not alter BIS values. BIS levels remained between 60 and 70 throughout the main procedure and intermittently increased to the mid-90s without any trace of poor signal quality. At the end of the surgery, the BIS values returned to normal range. The patient did not complain of intraoperative recall. Knowledge of potential interference from the use of an EMG tube must be considered when interpreting BIS
