Frequently Asked Questions: BIS™ Brain Monitoring Technology

*The BIS™ monitoring system should not be used as the sole basis for diagnosis or therapy and is intended only as an adjunct in patient assessment. Reliance on the  BIS™ system alone for intraoperative anesthetic management is not recommended.

The BIS™ system helps you measure brain activity in adult and pediatric patients within a medical facility through data provided by EEG signals. It may be used to help guide anesthetic administration and can help reduce the incidence of awareness with recall in adults during anesthesia and sedation.1

In this blog post, we’ll answer some common questions around your BIS™ brain monitoring system, including:

  • Why does my BIS™ index value suddenly drop?
  • Why does my BIS™ index value suddenly increase?
  • Why doesn’t my sensor stay in place for the entire case?
  • What should my suppression ratio reading be vs. burst suppression?
  • What is the ideal setting for the smoothing rate?
  • Why is there so much EMG?
  • Should I use the BIS™ system in the ICU?
  • And much more

Why does my BIS™ index value suddenly drop?

An acute decrease in BIS™ monitor numbers may be caused by:2

  • Pharmacologic changes such as:
    • Bolus administration of intravenous anesthetic
    • Recent changes in inhalation anesthesia
    • Administration of adjuvant agents (beta blockers, alpha 2 agonists)
  • Decreases in noxious stimulation
  • Administration of neuromuscular blocking agent, especially if excessive EMG was present prior to administration
  • Profound hypotension, hypothermia, hypoglycemia, or anoxia can produce decreases in the brain state activity
  • Paradoxical emergence patterns have been described with transient abrupt decreases in the BIS™ index value prior to awakening during inhalation anesthesia. The clinical significance is unknown.

Related: Learn how brain function monitoring during general anesthesia can help improve patient outcomes.

Why is my BIS™ index value high when I know my patient is asleep?

Observe for the presence of artifacts such as EMG, electrocautery, or high-frequency signals. High-frequency artifacts like those may contaminate the EEG signal and bias the BIS™ monitor towards a higher value. Ensure that anesthetic delivery systems are operating properly so that the intended dose of anesthetic agent is reaching the patient.

A sudden change in the level of anesthetic effect and the resulting BIS™ index value may be caused by changes in:2

  • Vaporizer setting
  • Fresh gas-flow rates
  • Intravenous infusion pump setting
  • Intravenous delivery routes

Ensure that the anesthetic dose is enough. An abrupt change in the BIS™ system value may reflect a new cortical state relative to anesthetic dosing and changes in surgical conditions. Assess the current level of surgical stimulation. The BIS™ monitor may show a transient increase in response to increases in noxious stimulation.

Is the BIS index value an instantaneous, real-time number?

All consciousness monitoring technologies require processing time, and none are technically real-time. The displayed BIS™ index value lags behind the patient’s EEG state by about 10–15 seconds.

It takes two things to generate a BIS™ index value: EEG activity from a patient and considerable signal processing. A patient’s BIS™ index value is calculated using about 15–30 seconds of captured EEG data. You may observe a 10–15 second lag if the clinical situation changes rapidly.

Related: How much does the BIS™ brain monitoring system cost?

Why doesn’t my sensor stay in place for the entire case?

Prior to application of the sensor, make sure to wipe the patient’s skin with an alcohol pad and dry thoroughly. After applying, press the edges of the sensor to ensure adhesion. Circle all four elements/circles to seal in the gel. Press each element/circle for five seconds to ensure proper contact.

What should my suppression ratio reading be vs. burst suppression?

The percentage of epochs in the past 63 seconds in which the EEG signal is considered suppressed (zero–100 percent). Burst count is the number of EEG bursts per minute, where a burst is defined as a short period of EEG activity preceded and followed by periods of inactivity/suppression (zero–20).

What is the ideal setting for the smoothing rate?

The BIS™ monitoring system offers three smoothing rate options over which the BIS™ index value is averaged. There are three default settings based on monitor mode. For Monitor Mode I and IV, the default setting is 15 seconds. For Monitor Mode II, it’s 30 seconds, and 10 seconds for Monitor Mode III.

Related: Is a video laryngoscope right for my OR?

Why is there so much EMG?

This frequency range contains power from muscle activity — electromyography or EMG — as well as power from other high-frequency artifacts. When the BIS™ monitor indicator bar is low, it indicates that EMG activity is low. BIS™ system monitoring conditions are optimal when the bar is empty. One bar represents power in the 30–38 range; two bars represent power in the 39–47 range; three bars represent power in the 48–55 range; and 4 bars represent power greater than 55.

I push hard on the sensor to get the gel pack to break, but it still doesn’t, why?

If you push too hard and the zip prep breaks, the sensor may not work. Follow the steps below to ensure the sensor is applied properly:

  1. Prep the skin by wiping with an alcohol pad and allow to dry thoroughly.
  2. Apply the sensor to the forehead.
  3. Starting with circle one, center the sensor on the forehead, approximately two inches (5 cm) above the nose or 1.5 inches (4 cm) for a pediatric sensor.
  4. Place circle four directly above and adjacent to the eyebrow.
  5. Place circle three on either temple area between the corner of the eye and the hairline.
  6. Press the edges of the sensor to ensure adhesion.
  7. Circle all four elements/circles to seal the gel.
  8. Press each circle for five seconds to ensure proper contact.
  9. Use a fingertip to press each circle one at a time.
  10. Press firmly to adhere the sensor to skin.

What does the signal quality mean?

The Signal Quality Indicator (SQI) measures the signal quality for the EEG channel source and is calculated based on impedance data, artifact, and other variables. It’s displayed in the upper left corner of the screen, to the right of the “BIS™” label.

Signal quality is optimal when all five bars of the SQI icon are green. When signal quality is too low, the BIS™ index value and other trend variables that are adversely affected by artifact won’t be displayed on the screen.

What value does the BIS™ system bring to the ICU for moderate/deep sedation during mechanical ventilation?

BIS™ monitoring technology provides additional information to assess patient response and sedation level — and improves the application of anesthesia through:

  • Improved drug titration
  • Better controlled sedation to help avoid use of neuromuscular blockers (NMB)
  • Reduced adverse events associated with over- and under-sedation that can lead to self-extubation or patient harm

Related: What are the advantages of multi-parameter monitors versus a stand-alone box?

Medtronic has offered an awareness-prevention guarantee in the past. Does that still exist?

Yes, indemnification will be provided in accordance with the terms agreed upon between Medtronic and the hospital or purchaser if your patient experiences anesthesia awareness while using either standalone or integrated OEM platform bispectral index technology, and the electronic record shows that the BIS™ index value was below 60 at the time of anesthesia awareness.

1. Lewis SR, Pritchard MW, Fawcett LJ, Punjasawadwong Y. BIS™ pectral index for improving intraoperative awareness and early postoperative recovery in adults. Cochrane Database Syst Rev. 2019; 26;9:CD003843.
2. Dahaba AA. Different conditions that could result in the bispectral index indicating an incorrect hypnotic state. Anesth Analg. 2005;101(3):765–773.

About the Author

Stephanie Aranowitz is operating room specialist for the Medtronic Respiratory and Monitoring Solutions portfolio. She provides consultation on the McGRATH™ MAC video laryngoscope, BIS™ brain monitoring system, and INVOS™ regional oximeter to healthcare facilities in New York and New Jersey.

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