During general anesthesia, the anesthesiologist must maintain a balance between an adequate hypnotic effect and excessive or insufficient suppression of brain activity. Clinical signs and hemodynamic parameters help assess the patient's condition but do not provide complete information about the depth of anesthesia. BIS Vista supplements standard monitoring with electroencephalography (EEG) data, which help assess the brain's response to anesthetic agents and support decisions regarding dose adjustments. Let us examine how the monitor works and the key aspects of interpreting its readings.

How BIS Vista Works and What the Bispectral Index Measures

A special sensor is placed on the patient's forehead to record EEG activity. The monitor processes the signal and calculates the bispectral index (BIS) on a scale from 0 to 100. A value of 100 corresponds to the awake state, while 0 indicates the absence of detectable electrical brain activity. A range of 40–60 is commonly used as a reference during general anesthesia; however, its interpretation depends on the medications used and the patient's clinical condition.

BIS reflects the hypnotic component of anesthesia but does not directly measure pain intensity or the degree of neuromuscular blockade. Therefore, the index should be assessed alongside other clinical data.

Which BIS Monitoring Parameters Should Be Assessed Together

BIS Value, Trend, and Signal Quality

The current BIS value represents the calculated index, while its trend helps track changes throughout surgery. For example, a sustained increase in the index may prompt the anesthesiologist to reassess the hypnotic effect of anesthetic agents and the reliability of their delivery.

It is also important to assess the Signal Quality Index (SQI) and the sensor's contact with the skin. If the signal is unreliable, the BIS value may not reflect the patient's actual condition. Therefore, a dose adjustment should not be based on a single number.

EEG, Muscle Activity, and Artifacts

The index calculation may be affected by electromyographic activity in the facial muscles, electrosurgical interference, and other EEG artifacts. These factors can distort the signal and make interpretation more difficult.

Additional information is provided by the raw EEG, muscle activity measurements, and the EEG suppression ratio, which reflects the degree of suppression of electrical brain activity.

For a comprehensive BIS assessment, the following parameters should also be considered:

• EMG level. Helps assess muscle activity that may affect the BIS value.
• Raw EEG waveform and the presence of artifacts. Allow clinicians to assess the signal characteristics and identify potential interference that may affect the index calculation.
• Burst Suppression Ratio (BSR). Reflects the proportion of time during which EEG electrical brain activity is suppressed and helps identify pronounced suppression.
• Hemodynamic parameters. Blood pressure and heart rate complement the assessment of the patient's condition during anesthesia but are not independent measures of its depth.
• Type of anesthetic agent used. BIS interpretation may vary depending on the drug class, which should also be considered when evaluating the index.

Thus, BIS should be interpreted alongside other parameters rather than in isolation. The availability and display of individual measurements depend on the monitor configuration.

Intraoperative Awareness: The Role of BIS Vista

Intraoperative awareness refers to the conscious perception of events during general anesthesia, followed by subsequent recall. Particular attention to monitoring the hypnotic effect is required during total intravenous anesthesia (TIVA). Unlike inhalational anesthesia, TIVA does not provide an end-tidal anesthetic concentration that can be used to monitor anesthetic delivery.

BIS Vista helps track the brain's response to anesthetic agents but does not guarantee the prevention of intraoperative awareness. Monitoring should be combined with checks of the infusion system, drug dosages, standard vital signs, and other available methods of anesthesia monitoring.

Optimizing Anesthetic Administration: Individualized Titration

BIS values help clinicians correlate the hypnotic effect with hemodynamic parameters, the stage of surgery, and the doses of administered medications. If the index consistently falls outside the target range and the signal is reliable, the anesthesiologist can assess whether a dose adjustment is needed. This may involve either reducing the anesthetic dose when the hypnotic effect is excessive or increasing it when the effect is insufficient.

The impact of BIS monitoring on drug consumption depends on the type of anesthesia and the protocol used. A 2024 meta-analysis of 27 randomized controlled trials, involving 35,585 patients, found a reduction in sevoflurane consumption but no statistically significant difference in propofol or desflurane consumption. Therefore, applying a universal percentage of drug savings would be inaccurate.

Limitations of BIS Vista and Practical Considerations

BIS readings may be affected by electromyographic activity, electrosurgical interference, and the specific effects of certain medications. In particular, dexmedetomidine may alter the relationship between the index value and the clinical depth of hypnosis. In addition, EEG processing involves a certain delay before changes are displayed. Therefore, it is important to assess signal quality and interpret BIS alongside other clinical parameters. The index also does not directly measure analgesia or the degree of neuromuscular blockade.

Despite these limitations, BIS Vista helps track changes in brain activity throughout surgery and adjust anesthetic doses to individual patient needs. When combined with standard monitoring, it provides the anesthesiologist with additional data to maintain the target hypnotic effect and make informed decisions about the anesthesia strategy.