What BIS Measures
The Bispectral Index processes raw frontal EEG into a single dimensionless number, 0 (isoelectric/no detectable activity) to 100 (fully awake), intended as a proxy for hypnotic depth of anesthesia.
What it is: a proprietary algorithm combining time-domain, frequency-domain, and higher-order spectral (bispectral) EEG analysis into one trended number.
What it is not: a measure of analgesia, a measure of paralysis, or a direct measure of "consciousness" — it is specifically a hypnotic-depth correlate, and a correlate only.
Value Ranges & Waveform Patterns
| BIS range | State | EEG character |
|---|---|---|
| 90–100 | Awake | High frequency, low amplitude, random — inadequate anesthesia if this appears intraoperatively |
| 70–89 | Light anesthesia / sedation | Beta activity decreasing, alpha activity emerging |
| 40–60 | Adequate general anesthesia (target range) | Alpha activity predominant, <12 Hz |
| 20–39 | Deep anesthesia | Delta activity dominant, 1–4 Hz |
| 0–19 | Very deep / burst suppression | Periods of EEG suppression (flat segments) interspersed with bursts of activity |
| 0 | Isoelectric / flatline | No detectable cortical activity — excessive anesthesia, cardiac arrest, or severe hypothermia |
Target BIS for general anesthesia: 40–60. This range is associated with a low likelihood of explicit recall while avoiding the unnecessary depth (and hemodynamic cost) of burst suppression.
BIS is a trend monitor. A single number in isolation is far less useful than watching where it's heading and how it responds to a dose change — treat it the way you'd treat a single blood pressure reading versus a trend.
The Evidence: Does It Actually Work?
This is the section most BIS reference material skips, and it's the most clinically important one — the trial evidence on whether BIS monitoring actually reduces awareness is genuinely mixed, not settled.
| Trial | Design | Finding |
|---|---|---|
| B-Aware (Myles et al., Lancet 2004) | 2,463 high-risk patients, BIS-guided vs. routine care | BIS-guided anesthesia reduced awareness incidence — the landmark positive trial. Number needed to treat to prevent one case of awareness: 138. |
| B-Unaware (Avidan et al., NEJM 2008) | 1,941 high-risk patients, BIS-guided (target 40-60) vs. end-tidal anesthetic gas (ETAG)-guided (target MAC 0.7-1.3) | No significant difference in awareness between BIS-guided and ETAG-guided protocols — directly contradicted the premise that BIS specifically (rather than any structured protocol) was the active ingredient. |
| BAG-RECALL (Avidan et al., NEJM 2011) | Prevention of intraoperative awareness in a high-risk surgical population | Again found no significant awareness reduction from BIS-guided protocols compared to ETAG-guided protocols. |
Why the trials disagree: a commonly raised critique of B-Aware is that its "routine care" comparison arm lacked a structured alternative protocol — meaning some of BIS's apparent benefit may reflect the effect of following any defined depth-monitoring protocol (increased vigilance) rather than something specific to the EEG-derived number itself. B-Unaware and BAG-RECALL, by giving the comparison arm an equally structured ETAG-based protocol, found the BIS-specific advantage disappeared.
The honest summary: BIS monitoring is widely used and reasonable, especially in high-risk-for-awareness cases or when volatile agent use is being minimized (TIVA, where end-tidal gas monitoring isn't available as a cross-check) — but "BIS prevents awareness better than watching your anesthetic gas concentration" is not a settled claim, it's a disputed one across three major trials.
Artifacts & Limitations
| Source of error | Effect |
|---|---|
| EMG activity (shivering, muscle tone, patient movement) | Falsely elevates BIS — can look like inadequate anesthesia when the patient is actually adequately anesthetized but under-paralyzed |
| Electrocautery interference | Falsely elevates BIS during active cautery use |
| Poor electrode contact / loose sensor | Unreliable or falsely elevated reading — always check signal quality index before trusting the number |
| Ketamine | Can paradoxically increase BIS despite clinically adequate or even deep anesthesia — a genuine paradox, since ketamine's dissociative EEG pattern doesn't map onto the BIS algorithm the way propofol/volatile-driven EEG changes do |
| Hypothermia | Decreases BIS independent of anesthetic depth |
Before trusting any BIS value: confirm electrode contact, check the signal quality index (>50%), rule out cautery/EMG interference, and correlate with hemodynamics and clinical signs — a BIS number that doesn't match the clinical picture is a reason to distrust the monitor, not the patient.
BIS does not detect analgesia and does not detect paralysis — a patient can be fully paralyzed, inadequately anesthetized, and in significant physiologic distress from nociception with a BIS reading that looks reassuring. It is one input among several, not a replacement for clinical judgment.
Quick Reference Card & Primary Sources
| Situation | Target / action |
|---|---|
| Induction | BIS 60–80 transiently acceptable while titrating |
| Maintenance, general anesthesia | Target 40–60 |
| Deep/high-risk cases | 20–40 acceptable if clinically indicated |
| Recovery | >70 expected |
| Burst suppression appearing unexpectedly | Consider hypothermia, excess anesthetic, or reduce dose |
| BIS rising unexpectedly on ketamine | Known paradox — correlate clinically before increasing anesthetic |
| BIS not matching clinical picture | Check electrodes, SQI, cautery/EMG interference before trusting the number |
Primary sources checked for this guide:
- Myles PS, Leslie K, McNeil J, Forbes A, Chan MT. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial. Lancet. 2004;363(9423):1757-1763.
- Avidan MS, Zhang L, Burnside BA, et al. Anesthesia awareness and the bispectral index (B-Unaware trial). N Engl J Med. 2008;358:1097-1108.
- Avidan MS, Jacobsohn E, Glick D, et al. Prevention of intraoperative awareness in a high-risk surgical population (BAG-RECALL). N Engl J Med. 2011;365:591-600.
- To BIS or not to BIS. British Journal of Anaesthesia, editorial reviewing the trial landscape, 2016.
- Protocol for the Michigan Awareness Control Study, contextualizing B-Aware/B-Unaware discrepancy and NNT. BMC Anesthesiology. 2009;9:7.
- Remifentanil alleviates propofol-induced burst suppression without affecting bispectral index. Randomized controlled trial, reviewed via PMC.
- Cochrane Database Syst Rev. 2014;6:CD003843 (depth-of-anesthesia monitoring systematic review).