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Bispectral Index

Editor: Ankit Jain Updated: 5/21/2023 6:54:15 PM

Monitoring the depth of anesthesia is very important during any procedure, as anesthesia that's too deep can cause hemodynamic changes. Anesthesia that is too light carries the risk of recall or awareness during anesthesia. Awareness during anesthesia is a very serious complication with potential long-term psychological sequelae such as anxiety and post-traumatic disorder.[1] When the bispectral index (BIS) value is maintained below 60, it decreases the incidence of anesthesia awareness.[1] Several neuro monitors based on the processed electroencephalogram or evoked potentials have been developed to evaluate the anesthetic depth. The BIS monitor is the first method that is FDA approved to assess the hypnotic effects of drugs.[2]


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BIS Monitoring and Principle

The bispectral index monitor processes electroencephalographic signals to obtain a value, which reflects the level of consciousness of the patient. The bispectral index (BIS) monitor collects raw EEG data through its sensors and uses an algorithm to analyze and interpret the data. The data displays as a number on the BIS view monitor. BIS values range from 0 to 100. A value of 0 represents the absence of brain activity, and 100 represents the awake state. BIS values between 40 to 60 represent adequate general anesthesia for a surgery, values less than 40 represent a deep hypnotic state. BIS value is usually maintained between 40 to 60 to prevent awareness under anesthesia.

Technique or Treatment

BIS monitoring involves the application of four electrodes on the forehead. The skin on the forehead is cleaned with an alcohol swab, and 2 to 5 seconds of digital pressure applied over the sensor leads.[3] The sensor is comprised of disposable wet gel electrodes. Electromyographic activity of the frontalis muscle is measured by lead 4, which is the ground electrode as well. The 2-channel monitor includes a user-configurable display. The 4-Channel monitor has enhanced bihemispheric capabilities. BIS extended sensors are available for use in ICU where patients require long-term monitoring. To measure brain activity in the pediatric population BIS pediatric sensor is available, which utilizes 'zipper' technology and can capture low voltage EEG. 

The BIS view monitor displays:

  • BIS number
  • Trend graph of BIS values over time
  • Raw EEG waveforms in real-time
  • Various signal quality indicators as SQI, EMG
  • Alarm indicators and messages

The signal-quality-index (SQI) bar predicts the reliability of the signal; the higher the SQI, the more reliable is the BIS number. The electromyographic (EMG) bar indicates EMG activity, which reflects muscle stimulation caused by an increase in muscle tone or muscle movement.[4]


Limitations/ Factors Affecting BIS Monitoring

  1. Anesthetic Agents: The anesthetic agents used affect BIS values. The patient anesthetized with one anesthetic drug may be more sedated than another patient with the same score anesthetized with a different combination of drugs. BIS monitor is unreliable with certain anesthetics, such as ketamine and nitrous oxide (N2O). In 2017, Mishra et al. studied the effect of nitrous oxide on the bispectral index and found out that the BIS value increases with the addition of nitrous oxide. This effect could be due to the decrease in the suppressant effect of the inhalational anesthetic agents on EEG with the use of NO and the neuro-stimulant properties of NO. NO increases cerebral blood flow velocities along with an increase in cerebral consumption of oxygen (CMRO).[5]
  2. Age: It is difficult to titrate anesthetic agents in infants younger than 6 months of age with BIS; this could be due to a difference in EEG in this population than older children as brain maturation and synapse formation occur during that period. In 2001, Bannister et al. found that there were no significant differences between the standard practice and BIS groups in anesthetic use or recovery measures in the children 6 months to 3 years of age.[6]
  3. Hypothermia: Doi et al. [7] studied 12 patients undergoing hypothermia for a cardiopulmonary bypass pump; they noticed a wide variation in BIS values during the decrease in body temperature. The BIS goes down by 1.12 units for each degree Celsius reduction in body temperature. Reduction in temperature results in decreased cerebral metabolic rate for oxygen which is reflected on the EEG by the progression to an isoelectric/burst suppression pattern dominated by isoelectric periods. Aortic cannulation causes EEG slowing. The onset of cardiopulmonary bypass produces a transient EEG depression most likely due to perfusion of the brain with a crystalloid prime solution. EEG slowing that may persist into the post cardiopulmonary bypass period is seen during decreased cardiopulmonary bypass flow rates with the aortic cross-clamp release. Decreased CPB flow rates requested by the surgeon during aortic cross-clamp release often correlated with EEG slowing that may persist into the post-CPB period.[8]
  4. Neurological impairment: Neurological impairment alters the ability of the BIS to monitor the depth of consciousness; it is not a reliable tool to assess the level of consciousness in this patient population. Generally, BIS values are lower in patients with neurological impairment. BIS values reflect the activity of the cortical structure of the brain but do not reflect the activity of subcortical structures such as a spinal cord. 
  5. Interference with medical devices: Interference from the medical devices causes artifacts and impairs the ability of the BIS monitor to assess the changes in the depth of anesthesia accurately. Chan et al. found that SQI decreases during the use of electrosurgical cautery.[9]

Clinical Significance

Alternative Technologies

Alternative technologies to monitor the depth of anesthesia are as follows:[10]

  • SNAP EEG monitor system
  • Auditory evoked potential monitor
  • 4-channel processed EEG
  • EEG monitor
  • Spectral edge frequency 95 
  • Automated responsiveness test

Enhancing Healthcare Team Outcomes

Current Evidence of BIS Monitoring

BIS is in current use in the operating rooms, ICUs, and some emergency departments. In 2007, Carlos et al. found that time to extubation and discharge from the operating room and PACU is reduced with the use of BIS monitoring. The risk of postoperative nausea and vomiting was decreased by 12% in patients monitored with BIS.[11] Tong et al., in 2017, demonstrated that the addition of BIS monitoring could result in economic benefit by reducing the use of propofol and faster recovery compared to standard practice.[12]

So far, there is no gold standard to span the entire spectrum of anesthetic effect on the central nervous system. BIS is one of the most studied monitors derived from EEG used nowadays. BIS has been shown to have a positive cost-benefit ratio and lower morbidity than more invasive monitoring methods.[13] The use of this technology requires an interprofessional team approach that includes the surgeon, anesthesiology, nurse anesthetist, surgical nurses, and other support staff to ensure the best possible patient outcomes.[13] [Level 5]


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Bispectral index
Bispectral index
Image courtesy S Bhimji MD



Avidan MS, Zhang L, Burnside BA, Finkel KJ, Searleman AC, Selvidge JA, Saager L, Turner MS, Rao S, Bottros M, Hantler C, Jacobsohn E, Evers AS. Anesthesia awareness and the bispectral index. The New England journal of medicine. 2008 Mar 13:358(11):1097-108. doi: 10.1056/NEJMoa0707361. Epub     [PubMed PMID: 18337600]


Ge SJ, Zhuang XL, Wang YT, Wang ZD, Li HT. Changes in the rapidly extracted auditory evoked potentials index and the bispectral index during sedation induced by propofol or midazolam under epidural block. British journal of anaesthesia. 2002 Aug:89(2):260-4     [PubMed PMID: 12378664]


Johansen JW. Update on bispectral index monitoring. Best practice & research. Clinical anaesthesiology. 2006 Mar:20(1):81-99     [PubMed PMID: 16634416]


Olson DM, Chioffi SM, Macy GE, Meek LG, Cook HA. Potential benefits of bispectral index monitoring in critical care. A case study. Critical care nurse. 2003 Aug:23(4):45-52     [PubMed PMID: 12961782]

Level 3 (low-level) evidence


Mishra RK, Mahajan C, Prabhakar H, Kapoor I, Bithal PK. Effect of nitrous oxide on bispectral index values at equi-minimum alveolar concentrations of sevoflurane and desflurane. Indian journal of anaesthesia. 2017 Jun:61(6):482-485. doi: 10.4103/ija.IJA_363_16. Epub     [PubMed PMID: 28655953]


Bannister CF, Brosius KK, Sigl JC, Meyer BJ, Sebel PS. The effect of bispectral index monitoring on anesthetic use and recovery in children anesthetized with sevoflurane in nitrous oxide. Anesthesia and analgesia. 2001 Apr:92(4):877-81     [PubMed PMID: 11273918]


Doi M, Gajraj RJ, Mantzaridis H, Kenny GN. Effects of cardiopulmonary bypass and hypothermia on electroencephalographic variables. Anaesthesia. 1997 Nov:52(11):1048-55     [PubMed PMID: 9404165]


Mathew JP, Weatherwax KJ, East CJ, White WD, Reves JG. Bispectral analysis during cardiopulmonary bypass: the effect of hypothermia on the hypnotic state. Journal of clinical anesthesia. 2001 Jun:13(4):301-5     [PubMed PMID: 11435056]


Chan MT, Ho SS, Gin T. Performance of the bispectral index during electrocautery. Journal of neurosurgical anesthesiology. 2012 Jan:24(1):9-13. doi: 10.1097/ANA.0b013e31823058bf. Epub     [PubMed PMID: 21946766]


Medical Advisory Secretariat. Bispectral index monitor: an evidence-based analysis. Ontario health technology assessment series. 2004:4(9):1-70     [PubMed PMID: 23074459]


Oliveira CR, Bernardo WM, Nunes VM. Benefit of general anesthesia monitored by bispectral index compared with monitoring guided only by clinical parameters. Systematic review and meta-analysis. Brazilian journal of anesthesiology (Elsevier). 2017 Jan-Feb:67(1):72-84. doi: 10.1016/j.bjane.2015.09.001. Epub 2016 Apr 14     [PubMed PMID: 28017174]

Level 1 (high-level) evidence


Gan TJ, Glass PS, Windsor A, Payne F, Rosow C, Sebel P, Manberg P. Bispectral index monitoring allows faster emergence and improved recovery from propofol, alfentanil, and nitrous oxide anesthesia. BIS Utility Study Group. Anesthesiology. 1997 Oct:87(4):808-15     [PubMed PMID: 9357882]


Bard JW. The BIS monitor: a review and technology assessment. AANA journal. 2001 Dec:69(6):477-83     [PubMed PMID: 11837151]


Ferreira AL, Mendes JG, Nunes CS, Amorim P. [Evaluation of Bispectral Index time delay in response to anesthesia induction: an observational study]. Brazilian journal of anesthesiology (Elsevier). 2019 Jul-Aug:69(4):377-382. doi: 10.1016/j.bjan.2019.03.008. Epub 2019 Jul 29     [PubMed PMID: 31371175]

Level 2 (mid-level) evidence