Falsely increased bispectral index values caused by the use of a forced-air-warming device

Abstract

EDITOR: The bispectral index (BIS) is an electroencephalogram (EEG)-derived index which has been proposed to monitor the hypnotic component of anaesthesia. It is a dimensionless number between 0 and 100. In non-anaesthetized patients, BIS ranges between 90 and 100. Total suppression of cortical activity results in a BIS of 0. BIS values below 60 are associated with a low probability of consciousness and reflect sufficient anaesthesia. However, BIS might be inaccurate in some instances, e.g. during hypothermia, nitrous oxide administration or the use of electrocautery (for review see [1]). Forced-air-warming therapy has also been shown to produce falsely high BIS values when the monitors Aspect A-1000 (v. 3.12) [2] or the newer A-2000 (both monitors Aspect Medical Systems, Natick, MA, USA) [3] are used. Here we report a similar case using the latest available version of the Aspect BIS XP monitor (Aspect Medical Systems, Natick, MA, USA). Case Report: A 65-yr-old male patient with squamous cell carcinoma was scheduled for abdomino-thoracic resection of the oesophagus. After placement of an epidural catheter at the T5 level, anaesthesia was induced using 2 mg kg−1 propofol and 10 μg sufentanil; intubation was facilitated with 0.6 mg kg−1 rocuronium. Anaesthesia was maintained with 0.7 minimum alveolar concentration (MAC) sevoflurane in 40% oxygen and air. For analgesia, 10 mL of ropivacaine (7.5 mg mL−1) were administered approximately every 2 h via the epidural catheter. Neuromuscular blockade was provided by repeated injections of 0.2 mg kg−1 rocuronium. After induction of anaesthesia, catheters were placed in the left radial artery and the right internal jugular vein. A new original BIS XP sensor was placed on the patient's forehead according to the manufacturer's instructions to monitor the hypnotic component of anaesthesia. For warming, the patient's upper body, arms and head were surrounded by a tent of blankets and a WarmTouch air warming system (Mallinckrodt, Hazelwood, MO, USA) was inserted above the left arm. From the first reading – after induction of general anaesthesia – BIS ranged around 40, no change was noticed after surgical incision. When the WarmTouch was turned on, the BIS value climbed to ˜80 within 10 s. The signal quality index ranged between 80% and 100% during electrocautery and the electromyogram (EMG) bar increased slightly. No clinical sign of insufficient hypnosis was observed. The high BIS value dropped back to ˜40 when the WarmTouch was turned off again. This phenomenon could be repeated several times (Fig. 1). During usage of the WarmTouch system, no interference with other electrical devices, i.e. electrocardiogram, were noted. This is consistent with the results of the testing of the WarmTouch devices by our medical physics group, which is routinely performed every 2 yr. After extubation and on postoperative day 1, the patient did not recall any intraoperative events.Figure 1.: Recording of BIS of an anaesthetized patient warmed using a forced-air-warming device. When the device was turned on, BIS values quickly increased to ˜80. When the device was turned off, BIS values rapidly decreased to ˜40. The recording was started during the operation when the device was on. Values of signal quality index (SQI) and EMG are also shown. An EMG level of 30 corresponds to an empty EMG bar on the BIS display.Since its introduction in 1996, the BIS system has been improved to generate stable signals that are more robust against artefacts to make the interpretation of BIS values more reliable. In the XP system, a new electrode position was added to further enhance artefact detection and reduce the influence of high frequencies on BIS values. According to the manufacturer, this results in better stability against artefacts and more reliable BIS values. Here we report a case where the latest version of the BIS XP system produced falsely high values due to the use of a forced-air-warming system. This problem has been described for older versions of the BIS [2,3]. However, in contrast to these reports, the XP system displayed an increase on the EMG bar.