BISPECTRAL INDEX

💆The EEG bispectrum is a high-order statistical computation derived from the analog EEG.

💆The BIS is a combination of three weighted parameters: (i) the burst suppression ratio (the proportion of isoelectric EEG signal in an epoch); (ii) the beta ratio (a measure of the proportion of signal power in the high vs medium frequency range); and (iii) the SynchFastSlow (relative synchrony of fast and slow waves)

💆Changes in frequency and power alone ( as done with conventional power spectral analysis) have been shown to be inconsistent when attempting to measure anesthetic depth.

💆Bispectral analysis incorporates information on power and frequency with the phase coupling information that is more indicative of anesthetic depth but not present in other clinical applications of EEG.

💆The BIS uses a combination of EEG subparameters that were selected after analysis of a large database of EEGs to demonstrate specific ranges for varying phases of anesthetic effect

💆These parameters were then combined to form the optimum configuration for monitoring of the hypnotic state.

💆The BIS is then displayed as a dimensionless number between 0 and 100 with the lower numbers corresponding to deeper levels of hypnosis.

💆There are normal, genetically determined low-voltage EEG variants among the population that can result in abnormally low BIS values in awake patients; therefore, it is important to obtain baseline values before the induction of anesthesia

💆BIS is not able to predict movement in response to surgical stimulation because the generation of reflexes is likely to be at spinal cord rather than cortical level

💆BIS does not fully reflect the synergistic effect of opioids with hypnotic agents

💆The presence of electromyographic artefacts, poor signal quality, and electrical artefacts such as those from electro-cautery and forced air warming units can cause spurious values to be displayed by the BIS monitor.

💆With the administration of ketamine, the BIS may remain high, possibly due to the excitatory actions of ketamine, and, therefore, the BIS monitor is not reliable when used to monitor hypnosis with ketamine.

💆There have been studies in which the BIS monitor has not been shown to reflect the hypnotic contribution to the anesthetic by nitrous oxide.

💆Potential benefits from the routine use of the BIS monitor include

➖decreased risk of awareness

➖improved titration of anesthetic agents and

➖decreased recovery room time

💆The BIS also gives the anesthetist additional information to consider when selecting drugs for interventions, for example, when making the decision whether to deepen anesthesia with a volatile agent, add more analgesia with an opioid, or use a vasoactive drug.

💆Also note:

➖The BIS may drop after giving a neuromuscular blocking agent if excessive EMG was present prior to giving it.

➖Ischemia attenuates the amplitude and frequency of the EEG signal, which may result in a decrease in BIS

➖Hypothermia decreases brain activity, and may decrease BIS

➖Muscle shivering, tightening, twitching etc may increase EMG and increase BIS

➖Artifacts in the higher frequency ranges [e.g. use of any mechanical device that could generate high frequency activity like patient warmer]can artificially increase the BIS value

➖Is the BIS decreasing when you think it should be increasing? Think of Paradoxical Delta pattern (characterized by a pronounced slowing of the EEG) which occurs over a short period of time (2-3 minutes).

➖If the sensor is placed over the temporal artery, pulse artifacts can cause the BIS value to be inappropriately low. Check EEG waveform for presence of pulse artifacts and move sensor if necessary.

➖Blinking or rolling his/her head by the patient, may cause artifacts that mimic slow frequency EEG patterns.

Reference: The BIS monitor: A review and technology assessment, James W. Bard, AANA Journal/December 2001/Vol. 69, No. 6

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