Monitoring Depth of Anesthesia

Anesthetic Depth Monitoring

Alertness measurement or anesthetic depth monitoring has become an integral part of modern practice of anesthesia. Previously and still, only  clinical assessment of the patient’s anesthetic depth during anesthesia is still common practice. Pulse, blood pressure, movements, eye movements, pupils, tears, responses to surgical stimuli etc. are assessed but overall, one can say that this assessment is difficult, in many cases insecure and inclusive. There are many times clear differences in the estimates of the anesthetic depth among different doctors and nurses. Experienced anesthetic staff can usually assess the anesthetic depth well but it can still be difficult in many cases and the assessment is complicated by a number of aggravating circumstances, such as the use of muscle relaxants and the response to physiological parameters such as pulse and blood pressure. The patient’s reactions to surgical stimuli vary with regard to age, resting period, fitness, cardiac arrhythmias, beta-blocking medication, bleeding etc.

The use of available anesthetic depth monitors now allows for better control of the anesthesia procedure, and a lower chance of over and underdosing of anesthetics. On the other hand, it has not been shown that there are fewer cases of perception of drug awareness when using anesthetic depth monitors. Anesthetic monitoring allows for controlled induction, a controllable general anesthesia, more controlled awakening and, more often, a faster awakening. It will be easier to control the awakening in relation to the surgery, which is useful both with inhalation anesthetics and in intravenous anesthesia or a combination of both. It has been shown in several scientific studies that overdose of anesthetics with extinction of EEG in the form of Burst Suppressions gives higher risk of postoperative cognitive dysfunction, which can be reduced by anesthetic diaphragm monitoring.

The methods used clinically includes, a more simple version of EEG registration via 3-4 flat skin electrodes attached to the forehead of the patient. EEG measures weak electrical activities in the cerebral cortex, about 100 times weaker than the ECG signal. The most common clinically available systems are Entropy and BIS (Bispektral Index). There are also other systems for alertness monitoring such as brain stem audiometry, Auditory Evoked Potentials (AEP), which means that audio stimulation measures the electrical activity of hearing centers and Sensory Evoked Potentials (SEP) that are used to map the function of nerve sensors that convey sensory input. SEP means that a sensory nerve in an arm or leg is stimulated while monitoring signals from sensory centers in the brain. The most common methods for measuring anesthetic depth in clinical practice are the EEG-based methods BIS and Entropy. In EEG-based anesthesia screen monitoring, there are four parameters that are analyzed in the EEC, which are measured on-line. The four parameters are the activation of high frequency waves (14-30 Hz), the level of synchronized low-frequency waves, the rate of burst suppression and the occurrence of isoelectric EEG.

The response to EEG lead is displayed in the usual anesthesia monitoring as an index from 0 to 100 while a continuous EEG curve is displayed. Entropy displays two values ​​in parallel while BIS displays an indexed value. BIS displays a numeric value based on the EEG signal reading for the last 15 seconds. A fully awoke patient usually has a BIS value of 90-100 which corresponds to a fast-frequency, low EEG wave. Between 80-90 you are usually sleeping but awkward. Between 60 and 80, you have a higher amplitude and a somnolent patient who can respond to pain stimuli but respond sluggishly or not at all. Between 40 and 60, you have general anesthesia with high amplitudes and low-frequency oscillations on the EEG. Deep anesthesia has BIS values ​​around 20, which again yields low amplitude with some fluctuations. In very deep anesthesia, Burst suppression is gained in increasing numbers and finally an isoelectric EEG. Burst suppressions are displayed in BIS as an SR value. It indicates how much (%) of the EEG signal was completely subdued in the last minute. Burst suppression shows tops of EEG activity that alternate with isoelectric EEG. BIS works worse in anesthesia given with ketamine, nitrous oxide or dexmedetomidine. Patients with dementia may have lower baseline on BIS.

Anesthesia depth monitoring provides a good opportunity to awake the patient in a fast and controlled manner. At the values ​​between 80 to 90, the patient is eligible and at lower values, it is hardly meaningful with speech or questions to the patient, but you can wait until the patient rises in vigilance before active wake. When attaching their flat skin electrodes to the forehead of the patient, the skin should first be wiped off with alcohol. You can also clean the skin with a little sandpaper. The skin plates can be applied after the anesthesia induction but are usually fixed before. Anesthesia diets should be standard practice in a modern anesthesia. These also appear to work on young children but have not been evaluated to the same extent as adults.

BIS Intervall and Anesthetic Depth

BIS-valueDegree of Unconsciousness
100Awake. Responds in a normal manner when spoken to.
80Light to moderate sedation. May respond to commandos out spoken with a high voice or light shakes.
60Generall anesthesia. Low likelihood of explicit memory experience. Does not respond to verbal stimuli
40Deep hypnotic state.
20Burst suppression
0Isoelectric EEG


BIS – Bispectral Index

BIS (Covidien) is an anesthesia depth monitor that uses a simplified form of EEG registration via four flat skin electrodes attached to the patient’s forehead under anesthesia. EEG measures weak electrical activities in the cortex, about 100 times weaker than the ECG signal. BIS displays a numeric value between 0 and 100 based on the reading of the EEG signal. BIS also provides a reading of the EMG signal. BIS provides an opportunity to measure the depth of general anesthesia. The word BIS means bispectral index which is reduced during anesthesia and thus a scale can be recorded continuously. BIS displays primarily a numeric value (BIS value) in parallel with a continuous EEG curve.

In addition to 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 five. At the values ​​above 50 in EMG, the patient is likely to be awake and need to be forged deeper or there are too many artifacts. At high EMG values, the BIS value should be interpreted with caution. The Signal Quality Index (SQI) gives a value of the EEG signal quality (0-100) measured during the last minute. SQI should be as close to 100 as possible, over 90% provide good quality signal (can also be shown as a bar graph). At values ​​below 80%, the signal may be insufficient to present a reliable value. SR (Suppression Rate “Suppression Ratio”) is a measure of the proportion of burst suppression measured in the last minute measured as a percentage of the EEG curve (%). These usually occur in the case of deep anesthesia, usually at BIS values ​​below 20. Over 20 burst suppression occurs significantly less frequently.

 The BIS value is presented as continuous values ​​between 0 and 100 calculated from the EEG curve in the last 15 seconds. The BIS trend can show BIS values ​​over time. The measurement of the BIS value is based on the principle that the EEG signal is gradually reduced during anesthesia to ultimately transform into isoelectric EEG. At the value 100, the patient is completely awake and the value 0 corresponds to isoelectric EEG. Burst suppression shows tops of EEG activity that alternate with isoelectric EEG.

Use of BIS provides the ability to control anesthesia and avoid over- and underdosing of anesthetic agents. Anesthetic monitoring allows for controlled induction, a controllable general anesthesia, more controlled awakening and, more often, a faster awakening. Anesthesia depth monitors should always be used in conjunction with clinical assessment of the anesthetic depth. BIS can present 5-minute and 30-minute trends.

Usually, the optimal values are between 40 and 50 in BIS during surgical anesthesia. A fully awoken patient usually has a value of 90-100 which corresponds to a fast-frequency, low EEG wave. Between 80-90 you are usually sleeping but possible to awake. Between 60 and 80, you have a higher amplitude and a somnolent patient who can respond to pain stimuli but respond sluggishly or not at all. Between 40 and 60, you have general anesthesia with high amplitudes and low-frequency oscillations on the EEG. Deep anesthesia has values ​​around 20, which again yield low amplitude with some fluctuations. In very deep anesthesia, Burst suppression is gained in increasing numbers and finally an isoelectric EEG.

Bolus administration of anesthetics may cause sudden cases in BIS as well as administration of inhalation anesthetics, beta blockers, clonidine or other drugs that lower the alertness. The decrease may also be due to the administration of muscle relaxants, high blood pressure drop, hypothermia, intracranial pressure increase, hypoglycemia or anoxia. Paradoxically short-lived reduction of BIS value may have occurred just prior to awakening during excitation, this change usually appears to be rapidly transient. A sudden increase in BIS may be due to the patient waking up but may also be due to high frequency artifacts such as diathermy or NMT, check the EMG value. The increase can of course also be due to superficial anesthesia or a sudden increased stimulus from the surgeon.

Anesthesia depth monitors provides a good opportunity to wake up the patient in a fast and controlled manner. At the values ​​between 80-90 the patient is eligible and at lower values, it is hardly meaningful with speech or questions to the patient, but you can wait until the patient rises in vigilance before active wake up.

When attaching their flat skin electrodes to the forehead of the patient, the skin should first be wiped off with alcohol. You can also clean the skin with a little sandpaper. The skin plates can be applied after the anesthesia induction but are usually fixed before. Anesthesia depth monitors should be standard practice in modern anesthetic care. However, BIS works worse in anesthesia given with ketamine, nitrous oxide or dexmedetomidine. Patients with dementia may have lower baseline on BIS.

BIS Intervall and Anesthetic Depth

BIS-valueDegree of Unconsciousness
100Awake. Responds in a normal manner when spoken to.
80Light to moderate sedation. May respond to commandos out spoken with a high voice or light shakes.
60Generall anesthesia. Low likelihood of explicit memory experience. Does not respond to verbal stimuli
40Deep hypnotic state.
20Burst suppression
0Isoelectric EEG

Entropy

Entropy (Datex-Ohmeda) is an anesthesia depth monitor that uses a simplified form of EEG registration via three flat skin electrodes glued to the forehead of the patient. EEG measures and detects weak electrical activities in the cortex, about 100 times weaker than the ECG signal. In Entropy, two numeric values ​​are displayed based on the EEG signal reading during the last 15 seconds plus reading the EMG signal on the forehead. Entropy provides the ability to measure the anesthetic depth under general anesthesia. The word entropy means noise or irregularity which is reduced during anesthesia and therefor the entropy values ​​can be continuously recorded during anesthesia. Entropy displays three numeric values ​​in parallel with a continuous EEG curve. These are presented as varying values ​​between 0 and 100 in the same way as in monitoring with BIS. Monitoring is based on the principle that the EEG signal is gradually reduced during anesthesia to ultimately pass into isoelectric EEG. At the value 100, the patient is completely awake and the value 0 corresponds to isoelectric EEG.

Entropy presents RE (Response Entropy), SE (State Entropy) and Burst Suppression Rate (BSR). RE bases its value at high frequencies in EEG including face musculature signals (EMG signals in the forehead), giving a value between 0 and 100. RE signals capture easier quicker changes in alertness including signals from the facial muscles (Forehead EMG). SE bases its value at stable lower frequencies corresponding to anesthetic depth which reacts slightly slower than RE. SE gives a value between 0-91 and follows essentially the RE value. Burst Suppression Rate (BSR) shows the number of burst suppressions measured in one minute (0-100%). Burst suppression shows tops of EEG activity that alternate with isoelectric EEG.

Use of Entropy provides the ability to control anesthesia and avoid over and underdose of anesthetic agents. Anesthetic monitoring allows for controlled induction, a controllable general anesthesia, more controlled awakening and, more often, a faster awakening. Anesthesia depth monitors should always be used in conjunction with clinical assessment of the anesthetic depth. Anesthetic staff frequently detects clinical changes in the state of unconsciousness before observing changes in Entropy. Entropy can present 5-minute and 30-minute trends.

The ideal values ​​are usually between 40 and 50 in SE and RE during surgical anesthesia. A fully awoken patient usually has a value of 90-100 which corresponds to a fast-frequency, low-EEG EEG wave. Between values of 80-90, you are usually sleeping but awkward. Between 60 and 80, you have higher amplitude and a somnolent patient who can respond to pain stimuli but respond sluggishly or not at all. Between 40 and 60, you have general anesthesia with high amplitudes and low-frequency oscillations on the EEG. Deep anesthesia has values ​​around 20, which again yield low amplitude with some fluctuations. In very deep anesthesia, Burst suppression is gained in increasing numbers and finally an isoelectric EEG. In surgical stimulation, the RE value and SE value are more likely to differ.

Anesthesia meter provides a good opportunity to wake the patient in a fast and controlled manner. At the values ​​in between 80-90 the patient is eligible and at lower values, it is hardly meaningful with speech or questions to the patient, but you can wait until the patient rises in vigilance before active wake.