Author:
Kai Knudsen
Updated:
12 August, 2024
This chapter describes depth of anesthesia monitoring, with a review of BIS and Entropy, which are standardized methods to determine and monitor anesthesia depth during general anesthesia.
Anesthesia Depth Monitoring
Monitoring of awareness or anesthesia depth has become an integral part of modern anesthesia. Traditionally, and still to a large extent, clinical assessment alone is used to gauge the depth of anesthesia during surgery. Clinicians assess the patient’s pulse, blood pressure, movements, eye movements, pupils, tears, response to surgical stimuli, etc., but overall, this assessment can be challenging, often uncertain, and subject to limitations. There are often significant differences in assessments of anesthesia depth between different individuals. Experienced anesthesia personnel can usually assess anesthesia depth well, but it can still be difficult, especially with complicating factors such as the use of muscle relaxants and various reactions to physiological parameters like pulse and blood pressure. The patient’s reactions to surgical stimuli can vary depending on age, resting pulse, physical condition, presence of heart arrhythmias, beta-blocker medication, bleeding, etc.
BIS Monitor in the Operating Room
The use of available anesthesia depth monitors now allows for better control of anesthesia and helps avoid both over- and under-dosing of anesthetic agents. However, it has not been proven that the use of anesthesia depth monitoring reduces the incidence of awareness during anesthesia.
Anesthesia depth monitoring allows for controlled induction, steerable general anesthesia, more controlled awakening, and often a faster recovery. It becomes easier to manage the awakening process in relation to the surgery, which is useful both with inhalation anesthetics and with intravenous anesthesia or a combination of both. Several scientific studies have shown that overdosing of anesthetics with EEG suppression in the form of “Burst Suppressions” increases the risk of postoperative memory loss (POCD – Post Operative Cognitive Dysfunction), which can be reduced with anesthesia depth monitoring.
Measuring Anesthesia Depth with Simple Skin Electrodes
The methods used clinically include a simplified form of EEG recording via 3-4 flat skin electrodes attached to the patient’s forehead. EEG measures weak electrical activities in the cerebral cortex, about 100 times weaker than the ECG signal. The most commonly available clinical systems are Entropy and BIS (Bispectral Index). There are also other systems for monitoring awareness, such as brainstem audiometry, Auditory Evoked Potentials (AEP), which involves measuring electrical activity from the auditory centers with sound stimulation, and Sensory Evoked Potentials (SEP), used to map the function of nerve pathways that transmit sensory input. SEP involves stimulating a sensory nerve in an arm or leg while monitoring signals from sensory centers in the brain.
The most common methods for measuring anesthesia depth in clinical practice are the EEG-based methods BIS and Entropy. In EEG-based anesthesia depth monitoring, four parameters are analyzed in the EEG measured online. These parameters include activation of high-frequency waves (14-30 Hz), the level of synchronized low-frequency waves, the occurrence of burst suppressions, and the presence of isoelectric EEG.
Medical Illustration of the Human Brain with Electrical Pulses and Activity
The EEG output is displayed in standard anesthesia monitoring as an index from 0 to 100, along with a continuous EEG curve. Entropy shows two values in parallel, while BIS shows an indexed value. In BIS, a numerical value is displayed based on the EEG signal over the last 15 seconds. A fully awake patient normally has a BIS value of 90-100, corresponding to a fast, frequent EEG wave with low amplitude. Between 80-90, the patient is usually asleep but can be awakened. Between 60-80, the amplitude is higher, and the patient is somnolent and may react to pain stimuli but responds sluggishly or not at all to verbal cues. Between 40 and 60, the patient is under general anesthesia, with high amplitudes and low-frequency fluctuations in the EEG. Deep anesthesia is indicated by BIS values around 20, which again show low amplitude with some fluctuations. In very deep anesthesia, burst suppressions increase in number, eventually leading to isoelectric EEG. Burst suppressions are displayed in BIS as an SR value, indicating the percentage of the EEG signal that was completely suppressed during the last minute. Burst suppression shows peaks in EEG activity alternating with isoelectric EEG. BIS is less effective with anesthesia induced by ketamine, nitrous oxide, or dexmedetomidine. Patients with dementia may have a lower baseline BIS value.
- 90-100: Fully awake. The patient is fully conscious, with a fast, frequent EEG wave and low amplitude.
- 80-90: Light sedation. The patient is asleep but easily arousable, often used for procedural sedation where the patient may need to respond to commands.
- 60-80: Moderate sedation. The patient is somnolent (drowsy) and may react to painful stimuli but may not respond to verbal commands. The EEG shows higher amplitude and lower frequency waves.
- 40-60: General anesthesia. This range is typically the target during surgery. The patient is unresponsive to verbal commands and has low-frequency, high-amplitude EEG waves. This is considered a safe range for general anesthesia.
- 20-40: Deep anesthesia. The patient is deeply anesthetized, with further reduction in BIS value indicating deeper levels of unconsciousness.
- 0-20: Burst suppression and isoelectric EEG. This range indicates very deep anesthesia, where burst suppression occurs (brief periods of electrical silence in the brain), or the EEG may become isoelectric (flat), indicating little to no brain activity.
Anesthesia depth monitors provide a good opportunity to awaken the patient quickly and in a controlled manner. At values between 80-90, the patient is responsive to verbal cues, and at lower values, it is usually not meaningful to interact with the patient verbally, as you can calmly wait for the patient to become more awake before active awakening. When attaching the flat skin electrodes to the patient’s forehead, the skin should first be wiped with alcohol. The skin can also be cleaned with a little sandpaper. The electrodes can be applied after anesthesia induction but are usually attached beforehand. Anesthesia depth monitors should be standard practice in modern anesthesia. These also seem to work on small children but have not been evaluated as extensively as in adults.
BIS – bispectral index
BIS (Covidien) is an anesthesia depth monitor that uses a simplified form of EEG recording via four flat skin electrodes attached to the patient’s forehead during anesthesia. EEG measures weak electrical activities in the cerebral cortex, about 100 times weaker than the ECG signal. In BIS, a numerical value between 0 and 100 is displayed based on the EEG signal. BIS also provides an EMG signal reading. BIS allows for the measurement of anesthesia depth during general anesthesia. The term BIS stands for bispectral index, which decreases during anesthesia, allowing for continuous scale registration. BIS primarily shows a numerical value (BIS value) in parallel with a continuous EEG curve.
BIS Interval and Anesthesia Depth
| BIS-value | Anesthesia Depth |
|---|---|
| 100 | Fully awake. The patient is fully conscious, with a fast, frequent EEG wave and low amplitude. |
| 80 | Light medium sedation. May respond to commands spoken in a loud voice or slight tremors |
| 60 | General anesthesia. Low probability of explicit memory experience. Do not respond to verbal stimuli. |
| 40 | Deep hypnotic state |
| 20 | Burst suppression |
| 0 | Isoelectric EEG |
Alongside the BIS value, three numerical values are presented on the anesthesia monitor in the form of EMG, SQI, and SR. EMG is the electromyographic signal (0-100). Ideally, the EMG value should be below 5. At values above 50 in EMG, the patient is likely too awake and needs to be more deeply anesthetized, or there are too many artifacts. At high EMG values, the BIS value should be interpreted with caution. SQI (Signal Quality Index) provides a value of the EEG signal quality (0-100) measured over the last minute. SQI should be as close to 100 as possible; over 90% indicates good quality (it can also be displayed as a bar graph). At values below 80%, the signal may be insufficient to present a reliable value. SR (Suppression Rate) measures the percentage of burst suppressions recorded in the last minute as a percentage of the EEG curve (%). These usually occur during deep anesthesia, often at BIS values below 20. Above 20, burst suppressions are much less common.
The BIS value is presented as continuous values between 0 and 100, calculated from the EEG curve over the last 15 seconds. BIS trends can show BIS values over time. The measurement of the BIS value is based on the principle that the EEG signal is progressively reduced during anesthesia, eventually leading to isoelectric EEG. At a value of 100, the patient is fully awake, and a value of 0 corresponds to isoelectric EEG. Burst suppression shows peaks in EEG activity alternating with isoelectric EEG.
Using BIS allows for the control of anesthesia and avoids over- and under-dosing of anesthetic agents. Anesthesia depth monitoring permits controlled induction, steerable general anesthesia, more controlled awakening, and often a faster recovery. Anesthesia depth monitors should always be used in conjunction with clinical assessment of anesthesia depth. BIS can present 5-minute and 30-minute trends.
The goal is usually to achieve BIS values between 40 and 50 during surgical anesthesia. A fully awake patient normally has a value of 90-100, corresponding to a fast, frequent EEG wave with low amplitude. Between 80-90, the patient is usually asleep but can be awakened. Between 60-80, the amplitude is higher, and the patient is somnolent and may react to pain stimuli but responds sluggishly or not at all to verbal cues. Between 40 and 60, the patient is under general anesthesia, with high amplitudes and low-frequency fluctuations in the EEG. Deep anesthesia is indicated by values around 20, which again show low amplitude with some fluctuations. In very deep anesthesia, burst suppressions increase in number, eventually leading to isoelectric EEG.
Bolus administration of anesthetic agents can cause sudden drops in BIS values, as can the administration of inhalation anesthetics, beta-blockers, clonidine, or other drugs that reduce alertness. The decrease may also be due to the administration of muscle relaxants, severe blood pressure drops, hypothermia, increased intracranial pressure, hypoglycemia, or anoxia. A paradoxical brief decrease in BIS values may occur just before awakening during excitation, but this change is usually short-lived. A sudden increase in BIS values may indicate that the patient has become more awake but may also be due to high-frequency artifacts, such as from diathermy or NMT; check the EMG value. The increase may also naturally be due to too shallow anesthesia or a sudden increase in stimuli from the surgeon.
Anesthesia depth monitors provide a good opportunity to awaken the patient quickly and in a controlled manner. At values between 80-90, the patient is responsive to verbal cues, and at lower values, it is usually not meaningful to interact with the patient verbally, as you can calmly wait for the patient to become more awake before active awakening. When attaching the flat skin electrodes to the patient’s forehead, the skin should first be wiped with alcohol. The skin can also be cleaned with a little sandpaper. The electrodes can be applied after anesthesia induction but are usually attached beforehand. Anesthesia depth monitors should be standard practice in modern anesthesia. However, BIS is less effective with anesthesia induced by ketamine, nitrous oxide, or dexmedetomidine. Patients with dementia may have a lower baseline BIS value.
BIS (Bispectral Index) monitoring is a method used to measure the depth of anesthesia by analyzing EEG (electroencephalogram) signals. The BIS monitor provides a numerical value between 0 and 100 that helps anesthesiologists assess the level of consciousness and anesthesia in a patient. The BIS value corresponds to the following intervals and levels of anesthesia depth:
Entropy
Entropy (Datex-Ohmeda) is an anesthesia depth monitor that uses a simplified form of EEG recording via three flat skin electrodes attached to the patient’s forehead. EEG measures and reads weak electrical activities in the cerebral cortex, about 100 times weaker than the ECG signal. In Entropy, two numerical values are displayed based on the EEG signal reading over the last 15 seconds, plus the reading of the EMG signal in the forehead. Entropy allows for the measurement of anesthesia depth during general anesthesia. The word entropy means noise or irregularity, which decreases during anesthesia, allowing entropy values to be continuously recorded during anesthesia. Entropy displays three numerical values in parallel with a continuous EEG curve. These are presented as varying values between 0 and 100, similar to monitoring with BIS. The monitoring is based on the principle that the EEG signal progressively reduces during anesthesia, eventually leading to isoelectric EEG. At a value of 100, the patient is fully awake, and a value of 0 corresponds to isoelectric EEG.
Entropy presents RE (Response Entropy), SE (State Entropy), and BSR (Burst Suppression Rate). RE bases its value on high frequencies in the EEG, including signals from the facial muscles (EMG signals in the forehead), and provides a value between 0 and 100. RE signals more easily capture rapid changes in alertness, including signals from the facial muscles (Forehead EMG). SE bases its value on stable lower frequencies corresponding to the anesthesia depth, which reacts slightly slower than RE. SE provides a value between 0-91 and primarily follows the RE value. BSR (Burst Suppression Rate) shows the number of Burst Suppressions recorded within a minute (0-100%). Burst suppression shows peaks in EEG activity alternating with isoelectric EEG.
Using Entropy allows for better control of anesthesia and avoids over- and under-dosing of anesthetic agents. Anesthesia depth monitoring permits controlled induction, steerable general anesthesia, more controlled awakening, and often a faster recovery. Anesthesia depth monitors should always be used in conjunction with clinical assessment of anesthesia depth. It is not uncommon for anesthesia personnel to detect changes in alertness with clinical signs before observing changes in Entropy. Entropy can present 5-minute and 30-minute trends.
The goal is usually to achieve values between 40 and 50 in SE and RE during surgical anesthesia. A fully awake patient normally has a value of 90-100, corresponding to a fast, frequent EEG wave with low amplitude. Between 80-90, the patient is usually asleep but can be awakened. Between 60-80, the amplitude is higher, and the patient is somnolent and may react to pain stimuli but responds sluggishly or not at all to verbal cues. Between 40 and 60, the patient is under general anesthesia, with high amplitudes and low-frequency fluctuations in the EEG. Deep anesthesia is indicated by values around 20, which again show low amplitude with some fluctuations. In very deep anesthesia, burst suppressions increase in number, eventually leading to isoelectric EEG. During surgical stimulation, it is more likely that the RE and SE values will differ.
Anesthesia depth monitors provide a good opportunity to awaken the patient quickly and in a controlled manner. At values between 80-90, the patient is responsive to verbal cues, and at lower values, it is usually not meaningful to interact with the patient verbally, as you can calmly wait for the patient to become more awake before active awakening.
Brain Cells with Electrical Activity
Sedline
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