BIS Monitor: How It Measures Depth of Anesthesia

A BIS monitor is a device that reads electrical activity from the brain during anesthesia and converts it into a single number, scaled from 0 to 100, that estimates how deeply unconscious a patient is. The number 100 means fully awake; 0 means virtually no detectable brain activity. In practice, anesthesia providers aim for a reading somewhere in the middle, typically between 40 and 60 for general anesthesia, adjusting drug doses up or down to keep the patient in that window. The device has become one of the most widely used brain monitors in operating rooms worldwide, though its value depends heavily on the clinical context and the drugs being used.

What the BIS Monitor Actually Measures

The monitor uses a disposable sensor strip placed across the forehead, which picks up the brain’s electrical signals, essentially a simplified version of what a full electroencephalogram (EEG) records. But raw EEG traces are too complex for a busy anesthesia provider to interpret in real time, so the BIS system runs those signals through a proprietary algorithm that distills the data into one number. That algorithm blends several components: it looks at how power is distributed across different frequency bands (low-frequency waves dominate during deep anesthesia, for instance), it tracks the ratio of fast to slow brain wave activity, and it monitors for a pattern called burst suppression, where the brain alternates between bursts of activity and stretches of electrical silence.

Researchers have reverse-engineered and reimplemented the algorithm to understand what drives it. One open-source version, called openibis, showed that the system generates separate scores for sedation and general anesthesia, then blends them with different weights depending on the depth range. The sedation score relies on the balance of power between mid-range and high-frequency bands, while the general anesthesia score focuses on how concentrated the brain’s energy is in the highest frequencies compared to the full spectrum.¹ A separate data-driven analysis built a decision tree from BIS sub-parameters and found that the algorithm shifts which inputs matter most depending on the BIS range, with an average error of about 4 BIS points.²

What the Numbers Mean in Practice

The 0-to-100 scale is easy to glance at, but the clinical meaning is not perfectly uniform across the range. A reading of 100 down to roughly 70 corresponds to various degrees of sedation, where a patient might still respond to voice or gentle stimulation. The range from 60 down to 40 is the sweet spot most anesthesia teams target for surgical anesthesia, where the patient is deeply unconscious but the brain is not being excessively suppressed. Below 40, the brain is approaching very deep anesthesia, and below about 20, burst suppression is prominent, meaning the brain is going quiet for stretches at a time.

Traditional ways of judging anesthetic depth, like watching heart rate, blood pressure, sweating, or limb movement, are not reliable indicators of what is happening in the brain itself. A patient’s heart rate can be high because of a drug’s side effects or because the surgeon is working near sensitive tissue, not because the patient is waking up. The BIS monitor was developed to fill that gap by providing a more direct window into brain state.³

Does It Prevent Waking Up During Surgery?

Intraoperative awareness, where a patient becomes conscious during general anesthesia, is rare but deeply distressing. This is the outcome that originally drove interest in BIS monitoring. The evidence here is real but more nuanced than the marketing suggests.

A Cochrane review of 36 trials found that BIS-guided dosing cut the risk of awareness by about 75% compared to relying on clinical signs alone in high-risk patients. But when BIS was compared against a simpler and cheaper alternative, end-tidal anesthetic gas monitoring (which just tracks how much inhaled anesthetic the patient is breathing out), there was no clear difference.⁴ A large trial of nearly 2,000 patients comparing BIS directly to end-tidal gas monitoring found two cases of definite awareness in each group, with no meaningful difference.⁵

Another major trial randomized over 18,000 patients to either BIS monitoring or end-tidal gas concentration protocols and found no statistically significant difference in the rate of definite awareness between the two. However, a secondary analysis within that study did find that the BIS protocol was associated with roughly a fivefold reduction in awareness events compared to a cohort receiving no structured monitoring at all.⁶ A 2024 systematic review also found no statistically significant difference in awareness between BIS and conventional monitoring across roughly 8,600 patients.⁷

The upshot: BIS monitoring clearly beats flying blind, but for cases where inhaled anesthetics are the main agents, simply tracking the exhaled gas concentration works about as well. Where BIS has a stronger argument is during total intravenous anesthesia with drugs like propofol, where there is no exhaled gas to measure and the clinician would otherwise have no objective brain-state feedback at all.

Faster Wake-Ups and Less Drug Use

Where BIS monitoring shows more consistent benefits is in drug economy and recovery speed. If you can see exactly how deeply anesthetized someone is, you tend to use less drug to maintain the target window rather than overshooting to be safe. One early trial found that propofol infusion rates dropped by about 13% in BIS-guided patients, who then woke up faster (extubated in about 7 minutes versus 11), were more alert on arrival to the recovery room, and became eligible for discharge sooner.⁸ Similar patterns appeared with inhaled anesthetics: a study using isoflurane found that BIS-guided dosing cut consumption by 12 to 25%.⁹

A Cochrane review pooling data across many trials quantified these recovery benefits. BIS-guided anesthesia shaved about 1.8 minutes off the time to eye opening, about 3 minutes off the time to orientation, and roughly 7 minutes off recovery room stay compared to standard practice.¹⁰ Those numbers sound modest in isolation, but across a high-volume surgical center running dozens of cases a day, shaving several minutes off each recovery translates into meaningful throughput gains and lower cumulative drug costs. A review of the economics found that these benefits come at an incremental cost of roughly five dollars per case.¹¹

Postoperative Cognitive Effects

One of the more compelling arguments for BIS monitoring involves thinking and memory problems after surgery, particularly in older patients. Deep anesthesia has been linked to worse cognitive outcomes, and BIS gives clinicians a tool to avoid going unnecessarily deep. A 2024 meta-analysis found that BIS monitoring was associated with a roughly 15% reduction in the risk of postoperative cognitive dysfunction compared to conventional monitoring.¹²

A randomized trial in older surgical patients reported more dramatic differences: delayed cognitive recovery at one week occurred in 3% of BIS-guided patients versus about 22% of controls, and cognitive disorder at one year was found in about 4% of the BIS group versus 16% of controls.¹³ The picture for postoperative delirium, a distinct condition involving acute confusion and disorientation in the days after surgery, is less clear. That same trial found no significant difference in delirium rates, and a systematic review with meta-analysis of multiple trials in elderly patients also found no statistically significant reduction in delirium with BIS-guided anesthesia.¹⁴ That review did, however, find a significant reduction in the broader category of postoperative cognitive dysfunction.

The distinction matters clinically. Delirium appears to involve factors beyond just anesthetic depth, including inflammation, pain management, and pre-existing vulnerability. Longer-term cognitive problems, on the other hand, may be more directly tied to cumulative brain suppression during surgery, which is exactly what BIS is designed to limit.

Drugs That Fool the Monitor

The BIS algorithm was largely developed and validated using the most common anesthetic agents, particularly propofol and inhaled anesthetics like sevoflurane. When other drugs enter the picture, the number on the screen can become misleading.

Ketamine is the most notorious offender. It produces a dissociative state where the patient is unresponsive and unaware of pain, but the brain’s electrical activity actually increases. Studies have shown that ketamine given during sevoflurane anesthesia causes BIS values to rise, even though the patient is becoming more deeply sedated.¹⁵ An anesthesia provider unaware of this effect might see a rising BIS and respond by giving more of the primary anesthetic, pushing the patient unnecessarily deep.

Dexmedetomidine, a sedative increasingly popular in both operating rooms and ICUs, creates a different kind of interpretive challenge. It produces sedation through a mechanism distinct from propofol’s, and BIS values tend to run lower than expected for a given level of clinical sedation. In volunteers, the BIS cutoff for deep sedation was 67 with propofol but only 46 with dexmedetomidine, meaning a patient on dexmedetomidine who looks comfortable at a BIS of 50 is not as deeply sedated as that number would suggest with propofol.¹⁶ However, the monitor’s ability to predict the moment of loss of consciousness was not degraded by adding dexmedetomidine to a propofol infusion.¹⁷

Even benzodiazepines like midazolam can skew readings. During deep sedation, BIS values with midazolam ran significantly higher than with propofol or dexmedetomidine, meaning the monitor might underestimate how sedated a midazolam patient really is at a given BIS number.¹⁸

Muscle Activity and Other Technical Artifacts

The forehead sensor picks up more than brain waves. Electrical activity from facial muscles, called electromyographic (EMG) activity, shares frequency ranges with some of the EEG components the algorithm uses. If a patient is shivering, clenching their jaw, or experiencing fasciculations from a drug like succinylcholine, the EMG signal can inflate the BIS reading, making the monitor suggest the patient is lighter than they actually are.

The degree of this interference varies by drug. A clinical trial comparing remimazolam and propofol found that BIS had only a weak correlation with EMG during propofol anesthesia, but a moderate positive correlation during remimazolam, meaning EMG contamination contributed meaningfully more to the BIS value under one drug than the other.¹⁹ Most modern BIS monitors display both the BIS value and an EMG bar, which helps clinicians decide when to trust the reading and when muscle activity might be confounding it.

Temperature and Physiological Confounders

Body temperature has a direct, linear effect on BIS that has nothing to do with anesthetic depth. During cardiac surgery requiring deep hypothermic circulatory arrest, where the body is cooled dramatically, BIS drops by roughly 1.1 to 1.8 points per degree Celsius of cooling, independent of the anesthetic drugs being given.²⁰ At the extreme end, patients cooled to the point of circulatory arrest reached BIS values near zero, and recovery of BIS correlated with how long the arrest had lasted.²¹ In these situations, clinicians cannot use BIS to make decisions about anesthetic dosing in any straightforward way.

BIS has also been studied as a prognostic tool after cardiac arrest, where patients are often treated with therapeutic hypothermia. In that setting, patients who ultimately had poor neurological outcomes showed BIS scores roughly 31 points lower than those with good outcomes by seven hours after ICU admission, with corresponding differences in how much sedation they required.²² Here BIS is functioning less as a sedation monitor and more as a rough gauge of overall brain function.

The Burst Suppression Problem

When anesthesia goes very deep, the brain enters burst suppression, a pattern of intermittent electrical silence. The BIS algorithm handles this by incorporating a suppression ratio (the percentage of time the EEG is flat). But the relationship between the suppression ratio and the BIS number is not linear across the whole range. Research has shown that up to a suppression ratio of 40%, BIS values stayed roughly constant and did not track suppression well. Only above 40% suppression did BIS and the suppression ratio become tightly correlated in a linear fashion.²³

This creates a blind spot. A patient could have a BIS of 45, which looks like an appropriate surgical depth, while simultaneously showing a suppression ratio of 5% or more, which is a red flag for excessive brain suppression. A 2025 study confirmed that these paradoxical combinations do occur in clinical practice and can confuse care providers.²⁴ The practical takeaway is that clinicians should look at the suppression ratio displayed on the screen alongside the BIS number, not just the BIS alone.

Pediatric Challenges

The BIS algorithm was developed using adult EEG data, and children’s brains, especially those of infants, produce different electrical patterns. A study of infants and older children during sevoflurane anesthesia found that BIS correlated with clinical depth in both groups, but the concentration of sevoflurane needed to reach a BIS of 50 was higher in infants than in older children, consistent with the known difference in anesthetic requirements by age.²⁵

A broader study of sedated children found additional complications. BIS was reasonably good at distinguishing light from deep sedation, but it struggled to differentiate moderate from deep sedation in any age group. In infants six months and younger, the mean BIS values and cutoff thresholds for each sedation level were significantly lower than those in older children. And the drug being used mattered: BIS tracked sedation depth moderately well during propofol, pentobarbital, chloral hydrate, and midazolam, but poorly during ketamine or opioid use.²⁶ The conclusion was that BIS in pediatric patients demands extra caution and cannot be interpreted with the same thresholds used for adults.

Alternatives and Competing Monitors

BIS is not the only brain monitor available. The GE Entropy module tracks EEG-based indices called Response Entropy and State Entropy, and the SedLine monitor produces a Patient State Index (PSI). In routine clinical use, these devices tend to track anesthetic depth in parallel with BIS but are not numerically interchangeable. A study using remimazolam found a strong correlation between BIS and PSI (r = 0.89), but PSI consistently read about 11 points higher than BIS on average, with the gap widening during periods like intubation and emergence from anesthesia.²⁷ One study of sevoflurane anesthesia suggested that entropy values might adjust more reliably to anesthetic dose changes than BIS.²⁸

The key point for patients and clinicians is that switching between monitor brands mid-case, or comparing numbers from different devices across different patients, is not straightforward. Each system has its own algorithm and its own quirks.

ICU Sedation Monitoring

Outside the operating room, BIS has found a role in the intensive care unit, particularly for patients receiving neuromuscular blocking agents (paralytics). When a patient is paralyzed, clinical sedation scales that rely on observing the patient’s responses become useless. A systematic review found a moderate to strong correlation between BIS and validated sedation scales in non-paralyzed ICU patients, suggesting that BIS provides clinically relevant information about consciousness level. The authors noted, however, that substantial variability across studies limits how precisely BIS can be mapped to any particular sedation target in the critically ill.²⁹ The concern is that without some form of brain monitoring during paralysis, patients risk being oversedated for hours or days, which itself contributes to worse ICU outcomes.

BIS in Veterinary Anesthesia

Veterinary anesthetists have explored BIS as a depth-of-anesthesia tool in dogs and other animals, since these patients also cannot report their experiences. A study in dogs found that while average BIS values did differ between light and surgical levels of anesthesia, the individual values overlapped considerably across those levels. Sensitivities and specificities for distinguishing light from surgical depth ranged widely, and the researchers concluded that using BIS as the sole method to judge anesthetic depth in dogs would be unwise.³⁰ The algorithm was, after all, trained on human brain data, and different species produce different EEG signatures. BIS in veterinary settings remains an adjunct at best, not a standalone guide.