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Bispectral Index: Is It Ready for Prime Time in the ICU?

  • Authors: David W Crippen, MD, FCCM
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Target Audience and Goal Statement

This activity is intended for critical care and other clinicians who treat critically ill patients.

The goal of this activity is to disseminate the most current, "state-of-the-art" clinical strategies for the diagnosis and management of diseases and conditions requiring critical care management, to enhance the care of persons with these serious conditions, and to improve the clinical practice methods and tools of healthcare professionals involved in the field of critical care and related specialties.

Upon completion of this activity, participants will be able to:

  1. Discuss new data regarding the use of bispectral index in the ICU.
  2. Detail the latest advances in the diagnosis and treatment of cerebral ischemia.
  3. Review current practice variations in dealing with end-of-life care.
  4. Describe some of the research being used as the basis for treatment in sepsis.


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  • David W Crippen, MD, FCCM

    Associate Professor, University of Pittsburgh, Pittsburgh, Pennsylvania; Staff, Critical Care Physician, Presbyterian Hospital Center, Pittsburgh, Pennsylvania


    Disclosure: Dr. Crippen has no significant financial interests or relationships to disclose. He has reported that he does not discuss any investigational or unlabeled uses of commercial products in this activity.

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Bispectral Index: Is It Ready for Prime Time in the ICU?

Authors: David W Crippen, MD, FCCMFaculty and Disclosures



A great deal of interest in practical electroencephalography is developing in intensive care medicine. Patients are frequently heavily sedated for their comfort and infrequently administered neuromuscular blockade (NMB) to decrease oxygen consumption in the face of compromised oxygen delivery.[1] Following therapeutic obtundation, it is sometimes difficult to accurately assess brain function, especially in NMB where any assessment based on physical exam is useless. It is important to monitor neurologic function at all times during a critical illness, and so it has been suggested that a simplified (for the observer) electroencephalograph (EEG) might fill the void between real cerebral function and the intensive care unit (ICU) physician's ability to assess it through the fog of therapeutic obtundation.[2]

Explanation of EEG

Thomas P. Bleck, MD, FCCM,[3] The Louise Nerancy Eminent Scholar in Neurology and Professor of Neurology, Neurological Surgery, and Internal Medicine; and Chair, Critical Care Committee, Director, Neuroscience Intensive Care Unit, The University of Virginia, Charlottesville, Virginia, explained the electrophysiology. Because of the non-invasive nature of EEG, any brain activity signals generated in the cortex must ultimately be received at the scalp. The compact bone of the skull is relatively resistant to the transmission of weak electrical signals. In addition, weaker signals detected further away from the monitor are frequently masked by signals of higher amplitude originating closer to the monitor. Challenges facing signal receivers are the difficulties resolving small amplitudes and unmasking summated signals. The sensitivity and selectivity of the technology needed to interpret such signals and translate them into comprehensible data must be of a very high order. The modern processed EEG is sufficiently sensitive and does not require as many head electrodes to generate a satisfactory signal that can be utilized for useful clinical data in the ICU. Filters are used to reduce the recognition of frequencies unlike real brain activity. These filters tend to reduce very low and high frequency activity. Advantages of the processed EEG are that the data are more easily interpreted by physicians not specifically trained in electroencephalography and that it accentuates trends in brain wave activity, which may be more obscure in the older analog modes. Modern processed EEG sorts data into recognizable patterns, including:

  • Symmetry -- asymmetrical patterns can indicate diminished perfusion to one hemisphere, cerebral embolism, or thrombosis.
  • Amplitude -- the strength of brain wave vectors. Decreased amplitude suggests commensurate brain activity.
  • Frequency -- the distribution of vectors throughout all frequency bands. Lower frequencies suggest increasing somnolence.
  • Spectral Edge -- the Activity Edge. Significant dips in one hemisphere compared with the other suggest focal brain ischemia.

The principle wave seen in normal wakefulness is the alpha rhythm containing waves of 8 to 12 Hz and is very responsive to changing mental activity, increasing with excitement and decreasing with tranquility. Beta rhythm frequently occurs in the prefrontal regions and is indicative of the initial euphoric, anxiolytic, and amnesic stage of sedation. Both theta and delta waves are frequently seen normally during sleep. The administration of neurotransmitter active medications, such as narcotics and benzodiazepines, produce characteristic changes in the patterns of brain waves that are easily interpretable on the cerebral function monitor.

Recent Advances in Processed EEG Monitoring

A recent advance in the realm of processed EEG is the bispectral index (BIS).[4] Dr. Bleck defined BIS as: "An attempt to derive a weighted parameter that reflects both the fast and slow components of the EEG, and gives special prominence to periodic activity." In his presentation, Dr. Bleck suggested that the BIS may be useful in the assessment of neurologic function for ICU patients because simplicity is its essence. The range of therapeutic obtundation is easy to understand and titrate in real time (see Table).

Table. Bispectral Index Scale

100 Awake
80 Sedated
60 General anesthesia
40 Deep hypnosis
20 Burst suppression
1 Flat line

Controversies Surrounding BIS Monitoring

There are some reports in the literature that suggest the BIS is reasonably accurate in predicting awareness or correlating with sedation scores during therapeutic obtundation[5-7] and decreasing dosages of agents needed to yield adequate anesthesia.[8] The usual artifacts, eye blink (low frequency), and muscle electrical activity (high frequency) seem to be adequately sorted out for usage in assessing gross changes in awareness. In the end, Dr. Bleck suggested that (processed) EEG "could help to prevent over- or under-sedation of ICU patients, especially those receiving NMB." He suggested that use of the BIS might also speed ventilator weaning (by analogy to the daily wake-up), decrease complications of immobility, decrease unplanned extubations, and decrease ICU staff workload. In conclusion, Dr. Bleck suggested that the BIS may be a more reliable measure of cortical function than the clinical rating scales

Alternatively, Stanley A. Nasraway, MD,[9] Chief, Surgical Critical Care and Associate Professor, Tufts/New England Medical Center, Boston, Massachusetts, presented a less optimistic picture of BIS. Dr. Nasraway suggested that there are no convincing data to evaluate the use of the BIS in the ICU. Virtually all of the current data are from short-term use in operating theaters. Dr. Nasraway suggested that the current literature does not necessarily support the proposition that BIS accurately measures awareness under therapeutic obtundation,[10-12] and its use is not necessarily cost effective.[13] Dr. Nasraway suggested that the ICU is a very different environment from operating theaters in terms of monitoring brain function. Studies demonstrating efficacious cerebral function monitoring in the ICU suffer from weak statistical correlation, and the aggressive marketing practices of the Aspect Company are not borne out in evidence-based medicine. He concluded that BIS may get better in time, but it is not yet ready for ICU practice.


In conclusion, there does not yet appear to be a meaningful consensus on monitoring brain function in the ICU and that default has prompted skepticism among critical care providers.[14]


  1. Crippen D. Life-threatening brain failure and agitation in the intensive care unit. Crit Care. 2000;4:81-90. Abstract
  2. Crippen D. Role of bedside electroencephalography in the adult intensive care unit during therapeutic neuromuscular blockade. Crit Care. 1997;1:15-24. Abstract
  3. Bleck TP. Pro/con: BIS-valuable or not? Pro viewpoint. Program and abstracts of the 33rd Annual Congress of the Society of Critical Care Medicine; February 20-24, 2004; Orlando, Florida.
  4. Ball J. How useful is the bispectral index in the management of ICU patients. Minerva Anestesiol. 2002;68:248-251. Abstract
  5. Ekman A, Lindholm ML, Lennmarken C, Sandin R. Reduction in the incidence of awareness using BIS monitoring. Acta Anaesthesiol Scand. 2004;48:20-26. Abstract
  6. Riker RR, Fraser GL, Simmons LE, et al. Validating the sedation-agitation scale with the bispectral index and visual analog scale in adult ICU patients after cardiac surgery. Intensive Care Med. 2001;27:853-858. Abstract
  7. Iselin-Chaves IA, El Moalem HE, Gan TJ, Ginsberg B, Glass PS. Changes in the auditory evoked potentials and the bispectral index following propofol or propofol and alfentanil. Anesthesiology. 2000;92:1300-1310. Abstract
  8. Luginbuhl M, Wuthrich S, Petersen-Felix S, Zbinden AM, Schnider TW. Different benefit of bispectral index (BIS) in desflurane and propofol anesthesia. Acta Anaesthesiol Scand. 2003;47:165-173. Abstract
  9. Nasraway SA. Pro/con: BIS-valuable or not? Con viewpoint. Program and abstracts of the 33rd Annual Congress of the Society of Critical Care Medicine; February 20-24, 2004; Orlando, Florida.
  10. Nasraway SA, Wu EC, Kelleher RM, Yasuda CM, Donnelly AM. How reliable is the bispectral index in critically ill patients? A prospective, comparative, single blinded observer study. Crit Care Med. 2002;30:1483-1487. Abstract
  11. Schneider G, Gelb AW, Schmeller B, Tschakert R, Kochs E. Detection of awareness in surgical patients with EEG-based indices--bispectral index and patient state index. Br J Anaesth. 2003;91:329-335. Abstract
  12. Messner M, Beese U, Romstock J, Dinkel M, Tschaikowsky K. The bispectral index declines during neuromuscular block in fully awake persons. Anesth Analg. 2003;97:488-491 Abstract
  13. Lehmann A, Karzau J, Boldt J, Thaler E, Lang J, Isgro F. Bispectral index-guided anesthesia in patients undergoing aortocoronary bypass grafting. Anesth Analg. 2003;96:336-343. Abstract
  14. Crippen D. High-tech assessment of patient comfort in the intensive care unit: time for a new look. Crit Care Med. 2002;30:1919-1920. Abstract