BIS

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Processed EEG monitor, translating EEG waves into a numeric scale ranging from 0 (complete EEG suppression) to 100 (fully awake). It is said that adequate anesthesia may be present with a BIS value of 40-60. Studies have shown that intraoperative BIS, compared to clinical signs, may have a reduction of intraoperative awareness (OR) 0.36, 95% CI 0.21 to 0.60; I2 = 61%; 27 studies; 9765 participants, time to eye opening, time to orientation, and time to discharge patient from PACU. However, BIS guided anesthesia did not show a clinical difference to anesthesia guided by end tidal anesthetic gas concentration (ETAC). Furthermore, studies show that neuromuscular blockade alone, without other anesthetics, decrease the value of BIS. Therefore, the accuracy and reliability of BIS are debated.

There is potential value in monitoring the waveform of the BIS monitor. Figure 1 describes common EEG frequency bands, which can be seen by observing the processed EEG waveform. In an appropriately anesthetized patient under general anesthesia, the EEG waveform should be dominated by Delta waves (1-4 Hz), Theta Waves (4-8 Hz), and Alpha waves (8-12 Hz). Beta waves (12-30 Hz) are found in wakeful patients.

Figure 1 - EEG frequency bands from slow to fast and how they relate to mental state. Brain wave frequency is measured in Hertz (Hz), which is the number of waves per second.

In order to observe and interpret the unfiltered waveform, users of the BIS must de-select "Filters" from the BIS module menu on their monitor. Then, two things can be observed to suggest that the waveform is consistent with general anesthesia. First, there should be the presence of slow waves / Delta waves, lasting 0.25-1 seconds (1-4 Hz) in duration. Second, there should be an absence of high frequency waves (Beta waves, 12-30 Hz). If both of these criteria are met, the BIS waveform is suggesting that the patient is appropriately anesthetized. In most cases, the BIS numerical reading will match this interpretation, but the number can be skewed by some confounders, such as EMG artifact (especially in patients without neuromuscular blockade) and use of ketamine in the anesthetic, leading to a falsely high numerical reading that can be differentiated from "light depth of anesthesia" by observing the BIS waveform.

StartPearls: BIS

Anaestheasier: Bispectral Index (BIS) and depth of anaesthesia monitoring

Effect of BIS-guided anesthesia on emergence delirium following general anesthesia in children: A prospective randomized controlled trial

Anaesthetic depth and delirium after major surgery: a randomised clinical trial