Falsely Increased Bispectral Index During Endoscopic Shoulder Surgery Attributed to Interferences with the Endoscopic Shaver Device

The A-2000™ Bispectral Index™ (BIS™) monitoring system (Aspect Medical Systems, Natick, MA) is the only commercial system available for monitoring depth of anesthesia. The important new features of this system consist of a bar reading indicating signal quality and a bar reading indicating electromyelogram (EMG) activity. Poor signal quality combined with increased EMG activity should alert the anesthesiologist to interpret BIS values with care because of possible artifact signal pollution. We present a case of a patient undergoing endoscopic shoulder surgery whose depth of anesthesia was monitored using BIS. The BIS reading was falsely increased during activity of the endoscopic shaver despite optimal signal quality indication and no indication of EMG activity. Case History A 54-year-old patient (ASA physical status I) underwent endoscopic arthroplasty of the right shoulder using general anesthesia. Because the patient was scheduled for day surgery, fast tracking of anesthesia was facilitated by using a BIS monitoring system (A-2000 Bispectral Index). Anesthesia was induced by using a continuous infusion of remifentanil 0.5 μg · kg−1 · min−1 for 2 min, followed by propofol 3 mg/kg; tracheal intubation was facilitated by rocuronium 0.6 mg/kg. After induction, the BIS sensor was applied to the forehead, the first BIS reading showing a value of 40. Anesthesia was maintained by using remifentanil 0.15 μg · kg−1 · min−1 and sevoflurane administered according to the BIS; the BIS target was 40–45, and positive pressure ventilation was set to maintain an end-tidal Petco2 of 30–40 mm Hg. The systolic/diastolic blood pressure was maintained at approximately 90–120 mm Hg/45–65 mm Hg, and heart rate at 45–55/min throughout surgery. The first endoscopic shaver oscillations (Apex® Universal Drive System; Linvatec Company, Largo, FL) occurred 15 min after the start of surgery. The anesthesiologist noted a sudden increase of BIS from 40 to 62 with no change in heart rate or blood pressure. There was no sign on the BIS screen indicating poor signal quality—the signal quality column indicated optimal signal quality, the EMG column showed no sign of EMG activity, and there were no artifacts on the raw electroencephalogram (EEG) tracing. Alerted by prior experience with device interferences, we decided not to change the infusion rate of remifentanil or the dose of sevoflurane but to closely monitor the BIS. Throughout the 2-h surgery, the BIS increased suddenly (within 30 s) after the start of endoscopic shaver oscillations, remained at values around 60 and decreased equally abruptly within 30 s after the end of the use of the shaving device, thus creating a wave form pattern of BIS, during shaver activity levels around 60, without shaver levels around 40 reflecting the clinically estimated depth of anesthesia (Fig. 1). At the end of surgery, the trachea was immediately extubated at BIS levels of 90. There was no postoperative recall.Figure 1: Bispectral index (BIS) levels throughout endoscopic shoulder surgery. BIS values during periods of endoscopic shaver oscillations are encircled.Discussion Interferences with electrical devices in the operating room are a common problem during the everyday use of the BIS monitor. Although interferences with electrocautery are very common and usually easily recognizable as artifacts—because of the A-2000 system indicating an artifact in the signal quality bar and raw EEG tracing—there are more subtle interferences with forced air warming therapy (1,2), which are not recognized as artifacts and cause falsely increased BIS values. We speculate that shaver-caused oscillations (up to 1500 rpm) at the shoulder joint are transmitted via bone contact to the forehead and temporal area bones. These oscillations could then cause minimal vibrations of the BIS sensor itself and cause falsely increased BIS values. It is obvious that the increasing use of BIS monitoring will lead to more reports of interferences with other medical devices. BIS values that do not correlate with clinical judgment of depth of anesthesia might well be caused by such interferences, even though the A-2000 monitoring system indicates optimal signal quality, no EMG activity, and no artifacts on the raw EEG trace.

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