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    Technology today: Bispectral index monitoring

    BIS Monitor
    CE Center

    RN/Thomson AHC Home Study Program

    CE credit is no longer available for this article. (Expired September 2007)


    Originally posted September 2005

    Michael Luebbehusen, RN

    Michael Luebbehusen is the manager of clinical operations at the surgical intensive care unit of Indiana University Hospital in Indianapolis. The author has served as an educational speaker for Aspect Medical Systems, manufacturer of the bispectral index monitor described in the article.

    No longer used only in the OR, bispectral index monitoring can now be used to help ensure that a patient is sedated safely and effectively.

    Is your patient adequately sedated?

    If you're relying solely on vital signs and traditional sedation scales, your answer is little more than an educated guess.

    According to one study, approximately 69% of patients in one ICU were inappropriately sedated, 54% were oversedated, and 15% were undersedated.1 Undersedation can increase patient anxiety, agitation, and the risk that the patient will be aware of and able to recall the surgery or procedure. Oversedation can adversely affect a patient's vital signs and impair his ability to breathe. It can also increase the risk of complications, prolong the time on mechanical ventilation, extend the length of stay in the hospital, and raise the cost of care.2

    Sedation monitoring can be challenging because the effects of sedative medications vary from patient to patient and because many analgesic and sedative drugs, when used together, have synergistic effects. Traditionally, clinicians have addressed sedation indirectly by monitoring vital signs and using subjective sedation scales.2 But these methods have shortfalls. Because heart rate and blood pressure aren't consistently sensitive to changing levels of consciousness, vital signs don't always adequately reflect the depth of sedation.3 Sedation scales, such as the Ramsay Sedation Scale and the Sedation Agitation Scale, may not accurately gauge moderate to deep states of sedation because they rely on patient movement and response.3 What's needed in the mix is an objective way to measure a patient's depth of sedation.

    BIS Index

    New technology called bispectral index (BIS) monitoring is one such tool. Derived from data from a patient's electroencephalogram (EEG), the BIS index is a measure of the hypnotic effects of sedatives and anesthetics on the brain.3 When used with other currently monitored parameters and clinical signs, it can help nurses and physicians effectively customize sedation to meet their patient's specific needs.

    Originally used to monitor the effects of general anesthesia in the OR, BIS monitoring is now used in a variety of settings where patients require sedation, including critical care units, endoscopy suites, and radiology departments. Here is what you need to know when using it.

    What BIS monitoring is and how it works

    Developed by Aspect Medical Systems (Newton, MA), the BIS system includes a sensor, a patient interface cable, a digital signal converter, a BIS engine (microprocessor), and a monitor. The technology is also available as a module that can be integrated into other manufacturers' monitoring systems.

    The noninvasive sensor has self-adhesive backing, much like a typical EEG pad. After wiping the patient's forehead and temple with alcohol and drying the skin to ensure that a quality signal will be obtained, you'll place the sensor acros the patient's forehead and over either the left or right temple, as explained in the box. The sensor then sends raw EEG information through the cable and converter to the BIS engine. This engine processes the EEG data according to an algorithm that combines select EEG features to produce a BIS index. This index is a number between 0 and 100 that is displayed on the monitor and reflects the patient's level of sedation.

    The illustration shows how the BIS index correlates with depth of sedation. A patient who is awake and alert will have a BIS reading near 100, while one with no brain activity will have a reading of 0 and a flat EEG. At readings greater than 70, patients are more likely than those with lower readings to be conscious and able to follow commands and recall the procedure they're undergoing.3

    A patient with a BIS reading that's lower than 60 is not likely to be conscious.3 A range of 40 – 60 is typical for a patient under general anesthesia.3 For patients who undergo anesthesia while little or no analgesic or opioid is used, a range of 25 – 35 may be necessary.3

    In addition to a numeric BIS index, the monitor displays other important data. The BIS trend, for example, is a graphic representation of BIS index values over time. It can help you monitor changes in a patient's level of consciousness in response to, for instance, drug administration or stimulation during a procedure.

    Other displayed data include the signal quality index (SQI) bar and the suppression ratio (SR) number. The SQI bar measures the reliability of the signal; higher SQI numbers indicate more reliable BIS values. The SR number is the percentage of time over the last 63 seconds that the EEG was suppressed (flatlined). The monitor also displays a single-channel EEG waveform and an electromyography (EMG) bar graph, which like SQI is used to help determine whether the BIS index values you're seeing are reliable.

    The importance of identifying and managing artifact

    While studies have shown BIS monitoring is reliable in a wide variety of patients and situations, in certain circumstances the BIS index may not accurately reflect a patient's level of sedation. The most frequent source of unreliable BIS readings is signal artifact from muscle activity, typically from the patient's face or forehead muscles.3 This muscle activity, measured as EMG in decibels, can be caused by pain, twitches, seizures, eye movement, or anything that results in increased muscle tone or movement.2,3 This activity can generate high-frequency signals that can contaminate the EEG and make the BIS index unreliable.

    For this reason, BIS monitors include an EMG bar graph. The higher the EMG, the greater the muscle activity, and therefore the less reliable the BIS index.2

    Because the BIS index tends to increase as EMG increases, a patient with a high EMG level (greater than 50 decibels) may be more deeply sedated than his BIS value indicates.3 With this in mind, pay close attention to your patient's EMG value; if you suspect it's affecting your patient's BIS index, request an order for a muscle relaxant, which can significantly decrease the EMG activity without changing the patient's level of sedation.3 Some clinicians have found that patients in pain often have high EMG levels, but evidence to support this is anecdotal.2

    Other sources of high-frequency activity that can lead to inaccurately high BIS values include medical devices, such as warming blankets, circulatory assist systems, high-frequency ventilators, suction devices, surgical instruments, and pacemakers and defibrillators.3 The closer the device is to the BIS sensor or equipment, the more likely it is to generate artifact.3

    Other factors that may affect BIS values

    Ketamine (Ketalar) and etomidate (Amidate) are two commonly used IV anesthetics whose pharmacologic profiles can affect how you should interpret a patient's BIS index. Ketamine, which is used predominantly in children, causes an increase in high-frequency EEG activity; therefore, BIS values may remain high when ketamine is administered, despite the onset of sedation.3

    Falsely high BIS values can also occur with etomidate, an IV agent commonly used to induce anesthesia. During induction, etomidate frequently produces skeletal muscle excitation, which can cause high-frequency EMG activity and, hence, a high BIS reading.3 In most cases, once induction occurs or the patient receives a neuromuscular-blocking (paralytic) drug, the BIS index will accurately reflect the hypnotic state.3

    Some clinicians have questioned the accuracy and reliability of the BIS index in patients with abnormal brain structure or function—from stroke or encephalopathy, for example. Until there is more research on the use of BIS technology in patients with neurologic disorders, you'll need to interpret their BIS values with caution.3 The same holds true for interpreting the BIS index of infants without a neurologic disorder.4

    While BIS monitoring is a valuable tool, you should not make clinical decisions based solely on a BIS index. To make sure your patient's level of sedation is safe and effective, use BIS monitoring in conjunction with your clinical judgment and other methods of assessment, such as vital signs and sedation scales.

    BIS usage now extends beyond the OR

    A substantial amount of clinical research has shown that BIS monitoring improves patient outcomes. Numerous studies have shown that patients monitored with BIS receive fewer drugs and emerge from anesthesia faster and with fewer problems than those who don't receive such monitoring.5,6,7 Recent studies have demonstrated that patients who are monitored using BIS are less likely to experience awareness and recall of their surgery or procedure, as the box on explains. And other studies have shown that using BIS monitoring allows facilities to lower costs by reducing the amount they spend on sedative drugs while maintaining adequate sedation; in one study, the savings was $185 per patient.2,3

    While BIS monitoring has been used most frequently in the OR for patients undergoing general anesthesia, its use is being expanded to a variety of clinical settings where patients undergo sedation. In critical care units, for example, it's being used to monitor patients during bedside procedures that require procedural sedation or monitored anesthesia care. It has also been used to monitor the level of sedation in patients in neuromuscular blockade or barbiturate coma.

    In some critical care units, BIS monitoring is also used as an adjunct to the neurological exam. And in the perioperative setting, clinicians have used it to confirm brain activity during critical events like cardiac arrest and resuscitation.

    In our facility, we use BIS to monitor patients who are sedated for mechanical ventilation, neuromuscular blockade, bedside diagnostic or therapeutic procedures, and drug-induced coma. We also use it for patients in whom sedation monitoring can be challenging: obese patients and those with a neurological disease or substance abuse problem. In addition, we use BIS monitoring as part of end-of-life care, to ensure the patient's comfort and reduce the risk of awareness when we withdraw support.

    As BIS monitoring moves beyond the OR, more and more nurses will find themselves working with it. Understanding how the technology works and how to best interpret the information it reveals will help you provide your patients with optimal sedation, regardless of the clinical setting.


    1. Kaplan, L. J., & Bailey, H. (2000). Bispectral index (BIS) monitoring of ICU patients on continuous infusion of sedatives and paralytics reduces sedative drug utilization and cost. Crit Care Med, 4(Suppl. 1), S110.

    2. Olson, D. M., Chioffi, S. M., et al. (2003). Potential benefits of bispectral index monitoring in critical care: A case study. Crit Care Nurse, 23(4), 45.

    3. Kelley, S. D. "Monitoring level of consciousness during anesthesia & sedation: A clinician's guide to the Bispectral Index." 2003. www.aspectmedical.com/resources/handbook/default.mspx (8 June 2005).

    4. Aspect Medical Systems. "Overview: Bispectral index monitoring in children." 2000. www.aspectmedical.com/assets/documents/pdf/whitepapers/pedi_wp_6231.pdf (13 June 2005).

    5. Zaugg, M., Tagliente, T., et al. (1999). Beneficial effects from beta-adrenergic blockade in elderly patients undergoing noncardiac surgery. Anesthesiology, 91(6), 1674.

    6. Nelskyla, K. A., Yli-Hankala, A. M., et al. (2001). Sevoflurane titration using bispectral index decreases postoperative vomiting in phase II recovery after ambulatory surgery. Anesth Analg, 93(5), 1165.

    7. Juvin, P., Vadam, C., et al. (2000). Postoperative recovery after desflurane, propofol, or isoflurane anesthesia among morbidly obese patients: A prospective, randomized study. Anesth Analg, 91(3), 714.

    Applying a BIS sensor

    Aspect Medical Systems' BIS monitor—available as a stand-alone monitor or a module for use in other manufacturers' monitoring systems—works with four types of sensors: the Standard, Extend, Quatro, and Pediatric. Regardless of which one you use, to obtain accurate readings, you'll need to position the sensor's circular areas properly.

    To apply a Standard sensor on an adult, decide if you're going to use the patient's right or left temple. Hold the sensor so that circle 3 is positioned over that temple, but don't apply it yet. First, remove the backing and apply circle 1 to the center of the forehead, approximately 1½" above the bridge of the nose. Then, apply circle 3 vertically to the patient's temple, between the corner of the eye and the hairline. Press the edges of the sensor to make sure it adheres properly, and press firmly on the circles for five seconds to ensure proper contact.

    Finally, insert the sensor tab into the patient interface cable. Hold it with the blank side facing up, and move it into the cable until you hear a click.

    Source: Kelley, S. D. "Monitoring level of consciousness during anesthesia & sedation: A clinician's guide to the Bispectral Index." 2003. www.aspectmedical.com/resources/handbook/default.mspx (8 June 2005).

    Minimizing awareness during surgery

    Awareness with recall is a rare but potentially devastating consequence of inadequate anesthesia. Also called intraoperative awareness or anesthesia awareness, it occurs when a patient undergoing surgery is paralyzed with muscle relaxants but not given enough general anesthetic to prevent consciousness or recall of the surgical events. One study found the rate of awareness with recall to be one to two cases per 1,000 patients who receive general anesthesia, which translates to an estimated 26,000 patients each year.1 More than just upsetting patients, awareness with recall can lead to symptoms of post-traumatic stress disorder, including severe anxiety, nightmares, flashbacks, and avoidance of medical personnel.2

    The FDA determined that BIS monitoring might lead to a reduction in the incidence of awareness with recall in adults during procedures that require general anesthesia or sedation.3 That's because the technology allows clinicians to continually track a patient's level of consciousness and make the necessary adjustments to ensure that the level of sedation is deep enough. One study of about 2,500 patients at high risk for awareness during general anesthesia found that BIS monitoring reduced the rate of awareness by 82%.4 In a second study of nearly 5,000 patients undergoing general anesthesia, the rate of awareness was reduced by 77% compared to a control group from a previous study.5


    1. Vidt, D. "Hypertensive crises: Emergencies and urgencies." 2003. www.clevelandclinicmeded.com/diseasemanagement/nephrology/crises/crises.htm (4 Mar. 2005).

    2. Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. (2003). The seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. Bethesda, MD: National Institutes of Health.

    3. Mayo Foundation for Medical Education and Research. "Hypertensive crisis." 2004. www.mayoclinic.com/invoke.cfm?objectid=6C95FDF4-3201-4DA8-865B9C4E129DE8D6&MOTT=DS00100 (7 Mar. 2005).

    4. Kajs-Wyllie, M. (1999). Antihypertensive treatment for the neurological patient: A nursing challenge. J Neurosci Nurse, 31(3), 142.

    5. Vaughan, J. C., & Delanty, N. (2000). Hypertensive emergencies. Lancet, 356 (9227), 411.

    6. Castillo, J., Leira, R., et al. (2004). BP decrease during the acute phase of ischemic stroke is associated with brain injury and poor stroke outcome. Stroke, 35(2), 520.

    7. Blumenfeld, D. J., & Laragh, H. J. (2001). Management of hypertensive crises: The scientific basis for treatment decisions. Am J Hypertens, 14(11), 1154.

    8. Varon, J., & Marik, P. (2000). The diagnosis and management of hypertensive crises. Chest, 118(1), 214.

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    Michael Luebbehusen, RN
    Michael Luebbehusen is the manager of clinical operations at the surgical intensive care unit of Indiana University Hospital in ...
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