Unusual case of low bispectral index values due to electrocardiographic interferences
Montreal, Canada
* E-mail: thomashemmerling{at}hotmail.com
Editor—The bispectral index (BIS, Aspect Medical Systems, Newton, MA, USA) is widely used as an accurate measure of the hypnotic effect of anaesthetics and sedative drugs.1 Values between 40 and 60 are recommended for maintenance of general anaesthesia.2 We report a case where BIS values were constantly below expected levels, probably caused by interference with ECG monitoring.
A 59-yr-old man (weight=95 kg, ASA I) underwent laparoscopic partial gastrectomy. Anaesthesia monitoring consisted of three-lead ECG, non-invasive arterial pressure monitoring, peripheral pulse oximetry, and BIS monitoring [BIS-VistaTM (version 1.01) monitoring system, BIS-Quatro-sensorTM]. Anaesthesia was induced using propofol 200 mg and fentanyl 250 µg, and endotracheal intubation was facilitated by rocuronium 50 mg. Anaesthesia was maintained by a continuous infusion of propofol titrated to BIS and fentanyl boli of a total of 700 µg throughout surgery. During the first 25 min of anaesthesia, BIS varied between 21 and 27 (Fig. 1); propofol infusion was adjusted according to the BIS value to a minimum value of 90 µg kg–1 min–1. Systolic and diastolic arterial pressure was in the range of 120–140 and 50–75 mm Hg and heart rate was around 60–80 beats min–1 during this period. Since the anaesthetist managing the case (T.H.) had extensive experience with BIS monitoring and interferences with different devices, the BIS sensor was replaced—without improvement of tracing—35 min after beginning of surgery; 90 min after beginning of surgery, another BIS monitor (BIS XP monitoring system) was also applied to verify any monitoring failure. During the surgery, the EMG graph did not indicate any significant activity (<30 dB) and the signal quality index (SQI) showed good to excellent signal quality on any device (>50%, Fig. 1). Approximately 2 h after the beginning of anaesthesia, the patient showed clinical signs of insufficient hypnosis and started to wake up at a BIS of 25 (Fig. 1). Forty milligrams of propofol were immediately injected. A thorough look at the raw EEG trace revealed a pattern that could be most likely attributed to an ECG signal. The three ECG leads were applied on both shoulders and lower left thorax. Propofol dosing continued using solely clinical parameters. The patient was extubated 12 min after the propofol infusion was stopped and did not report awareness when asked after surgery (once immediately, and then 1 month after surgery). EEG signals are known to be vulnerable to noise or artifacts situated in the same frequency range as awake EEG and the detection of those artifacts is tricky owing to the variability of the frequency and amplitude of EEGs.3 High-frequency artifacts may be physiological (coming from sources other than the patient's brain) and non-physiological (generated by external devices). Examples of the latter case have been reported to falsely increase the BIS. They include but are not limited to: cardiac pacing devices,4 5 cardiopulmonary bypass machine,6 warming blankets,7–9 endoscopic shavers,10 electromagnetic systems,11 and EMG tracheal tubes.12 Interference of the patient's own ECG with the BIS reading has been scarcely documented. Myles and Cairo13 and Puri and Nakra14 reported cases where the ECG was detected by the BIS monitoring system and not recognized as artifact in patients with severe brain injury. The BIS values were relatively high (38, 45) whereas they were expected to be extremely low due to low cerebral perfusion and neurological damage. Our patient was on no previous medication known to suppress EEG activity, the awake BIS value was more than 90 and the raw EEG appeared normal. We hypothesize that instead of reading EEG, the BIS monitor read ECG tracings, which reflected long isoelectric periods interrupted rhythmically by the ECG signal with relatively high amplitude, thus mimicking a deep anaesthetic state. We recommend that the raw EEG tracing should be thoroughly investigated whenever BIS values seem clinically dubious.
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References
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