a healthy 33-year - old female patient ( asa class i ) underwent a robot assisted total thyroidectomy and lymph node dissection . after routine monitoring , general anesthesia was initiated with continuous infusion of propofol and remifentanil using a targeted controlled infusion system ( orchestra , fresinius vial , france ) , and endotracheal intubation was facilitated by rocuronium ( 50 mg ) . the trachea was easily intubated using a medtronic xomed nerve integrity monitor ( nim ) emg endotracheal tube ( jacksonville , fl , usa ) . ( aspect medical systems , newton , ma , usa ) was mounted on the forehead according to the manufacturer 's guidelines and connected to the bisvista monitor ( aspect medical systems , newton , ma , usa ) . anesthesia was maintained with medical air , oxygen , and intravenous infusion of propofol and remifentanil . propofol infusion was titrated to maintain a bis index of 40 - 60 , and remifentanil infusion was titrated to maintain heart rate and arterial blood pressure within 20% compared with baseline values . additional muscle relaxant was not used to monitor integrity of nerves throughout the entire period of surgery . ten minutes after incision , a sudden increase of bis a value of 94 was noticed . the target concentrations of propofol and remifentanil were rapidly increased following intravenous injection of midazolam ( 2 mg ) . at the time , the bis value was maintained between 65 and 75 for 20 minutes and then suddenly increased to 93 . a midazolam supplement ( 2 mg ) all infusions were found to be performed satisfactorily , and there were no signs of patient movement or arousal . the propofol and remifentanil infusions were sequentially increased to 7.0 g / ml and 6.0 ng / ml ( effect site concentrations ) . after 20 minutes a 5 mg bolus of iv ephedrine was given because arterial blood pressure was 74/58 mmhg and heart rate was 56/min . considering a small possibility of intravenous line dysfunction , propofol infusion was stopped and sevoflurane was started . though sevoflurane was gradually increased to an end - expiratory concentration of 3.5 vol% , the bis value did not decrease to < 65 . twenty - five minutes later , sevoflurane was discontinued and total intravenous anesthesia ( tiva ) was restarted to infuse via another intravenous route . the bis quatro sensor replacement could not significantly change the bis values . a similar model of the bis - vista monitor then , the bis values increased to the mid-90s , and an additional 2 mg dose of midazolam was administered . the settings of propofol and remifentanil were adapted according to clinical signs of the patient during the remaining operating time . at the end of the main procedure the remainder of the operation proceeded uneventfully and bis value was maintained within an ideal range at the appropriate level of anesthesia . during the surgery , the emg graph did not indicate any significant activity ( ~40 db ) and the signal quality index ( sqi ) showed good to excellent signal quality on any device ( > 50% , fig . both propofol and remifentanil were discontinued at the end of surgery , and the patient was allowed to recover over a 20-minute span . no clear recall or elicit memory during operation was observed when asked once immediately , and then 3 days after anesthesia . there have been many methods used for intraoperative neuromonitoring to identify recurrent laryngeal nerve . in recent years , wire - reinforced emg tubes equipped with surface electrodes have gained popularity due to their essential advantages including easy placement , noninvasiveness , and ability to sample larger areas of evoked muscle action potentials . the nim monitor tracks emg activity continuously , and enables direct stimulation of an exposed nerve and subsequent recording of muscle activity . bis monitoring has been proven to be a useful procedure for measuring depth of sedation . however , several cases of bis values not coinciding with a clinically judged sedative - hypnotic state have been reported . such reports are as follows : cardiac pacing device , warming devices , endoscopic shaver device , and electromagnetic system . our case differs from previous one in that we changed the anesthetics from intravenous agents to a volatile agent , though the manipulation could not alter the bis values . when measuring eeg in the operating room , it is important to assess the quality of the eeg signal . there are many problems with signal bias due to electrical interference from other equipment used in the operating room . elevated emg activity and neuromuscular blocking drugs ( nmbd ) could significantly influence bis monitoring . emg activity represents artifacts superimposed on the frequency band of eeg signals used by the algorithm of the monitor to determine bis values . the administration of nmbd would decrease the bis value by alleviating the emg contamination and revealing the true calculated bis . however , dahaba et al .   and grief et al . whether or not nmbd affects the value given by sedation depth monitors continues . in our case , the absence of muscle relaxant might cause inaccurate bis values due to emg activity . we could not attempt to administer a nmbd due to intraoperative neuromonitoring . however , judging from the phenomenon that the bis values decreased after cessation of the main procedure , presumed to coincide with discontinuance of emg stimulation , the emg interference might be the cause of the erroneous bis finding . in our center , emg stimulation generally ends with electrophysiologic confirmation of objective preservation of the recurrent laryngeal nerve after completion of thyroid dissection . although the emg scores demonstrated similar levels throughout the duration of the surgery , the appearance of high bis scores were limited to the period of the main procedures . subtle artifact signal pollution arising from electric device interference could not be identified by the signal quality bar nor displayed as artifacts . in this case , the bis reading was falsely increased despite optimal signal quality indication , and no indication of emg activity . recurrent laryngeal nerve stimulation might cause signal interference , simulating eeg waves found in superficial anesthesia or during alertness , which were not identified by the monitor as interferences . other factors causing inaccurate bis values such as nitrous oxide administration , use of electrocautery , and forced air warming device could be excluded . electrocautery was used throughout the operation , even in the period indicating appropriate bis levels before and after the main procedure . although the bis algorithm has been modified several times to improve its performance and decrease the interference of artifacts , bis values can still be influenced by different clinical situations that were not included during its development . the bis quatro sensor has a new electrode which offers enhanced artifact detection and improved resistance to interference from noise sources , such as high frequency / electromyography conditions . the latest model of the bis system produced inaccurate high values due to interference arising from an extraneous source , as previously described in the prior models [ 5 - 9 ] . in conclusion , we present a case where electrical interference with the emg tube was misinterpreted as eeg activity by a bisvista monitor . this misinterpretation was displayed as a high bis value , with optimal signal quality and no significant emg activity .
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 .