Sudden and sustained elevation of bispectral index due to electromyographic interference during closed circuit inhalational anesthesia

Sir,

The placement of the bispectral index (BIS) sensor on the forehead makes it vulnerable to contamination by the frontalis electromyogram (EMG) activity since electroencephalographic (EEG) and EMG signals overlap in the 30-50 Hz range. Here, we highlight such a biasing influence of EMG activity on BIS during breast reconstruction with transverse rectus abdominis myocutaneous flap (TRAM flap).

A 53-year-old woman with essential hypertension underwent excision of a breast mass and reconstruction with TRAM flap. Her physical examination was normal and blood tests were within the normal limits. She had no drug allergies. Standard cardiorespiratory monitors including three lead electrocardiogram, pulse oximetry and noninvasive blood pressure were attached and a depth of anesthesia sensor, the BIS-XP^® four electrode sensor (Covidien Ilc, Mansfield, MA, USA) was placed on the patient's forehead. The baseline BIS index was above 90. General anesthesia was induced with midazolam 2 mg intravenous (i.v.), propofol 150 mg i.v. and fentanyl 75 μg i.v. and endotracheal intubation was facilitated with rocuronium 40 mg i.v. Anesthesia was maintained with sevoflurane at 0.7-1.2 age — adjusted minimum alveolar concentration (MAC) in 30-40% oxygen in air mixture with the Drager Zeus^® auto control closed circuit system (Drager Medical, Lubeck, Germany) to maintain a BIS index between 40 and 60. However, during mobilization of the TRAM flap, the BIS index suddenly increased from 51 to 63. The signal quality index (SQI) was 100%, the EMG bar read 34 and spectral edge frequency 95 (SEF 95) was 14. The end tidal sevoflurane concentration (ETAC) was 1.7% (MAC of 1.0). The BIS then increased to over 70 and it was noted that both the EMG signal and SEF had also increased significantly (63 and 28, respectively), but ETAC of sevoflurane was 1.8% (MAC of 1.1). The sustained elevation of BIS and SEF were not associated with any signs of light anesthesia or awareness. A bolus of propofol 40 mg i.v. reduced the BIS to <50, the EMG reduced to 32 and SEF to 15 while the SQI stayed at 100%. The remaining intraoperative course was uneventful.

The reported incidence of unintended awareness under general anesthesia with subsequent recall (AWR) varies widely with an incidence in the general surgical population of 0.1-0.2% reported in the USA.[1] Currently, it is recommended that depth of anesthesia monitors including BIS monitor should be used at the discretion of the anesthetist on a case-by-case basis for selected patients and not used routinely.[2] The BIS^® monitor calculates the BIS from the weighted sum of three parameters: Beta ratio (frequency analysis), Synch Fast Slow (bispectral analysis), and the burst suppression ratio (time domain analysis).[3] The resulting BIS values are scaled from 100 (awake) to zero (isoelectric EEG) with BIS values between 40 and 60 reported to indicate an adequate depth of anesthesia. Conventionally, EEG and EMG signals are situated in the 0.5-30 band Hz and the 0-300 Hz band respectively with both signals overlapping in the 30-47 Hz range.[4] Thus, the most likely cause of the high BIS index and SEF readings in our patient is EMG activity of either the anterior abdominal muscles or the frontalis muscle, which was subsequently quantified by the EMG variable of BIS-XP^®. This case further demonstrates that a sudden increase in BIS values above 60 may reflect lightening of anesthesia/AWR or the corrupting effect of EMG activity on EEG signals and steps should be taken to differentiate the two.