Authors: Hight, Darren et al.
Anesthesiology October 2025 | DOI: 10.1097/ALN.0000000000005762
This letter to the editor exposes a long-standing technical flaw in Bispectral Index (BIS) monitors (Medtronic, Ireland) that directly affects calculation of the spectral edge frequency 95 (SEF95), a standard electroencephalographic (EEG) measure used to gauge anesthetic depth. The authors—building on prior reports of similar processing errors in Entropy and SedLine monitors—describe how the BIS display filter setting unintentionally alters the SEF95 output by removing low-frequency signals, thereby misrepresenting the true depth of anesthesia.
The SEF95 should reflect the frequency below which 95% of total EEG power lies (0.5–30 Hz). However, the BIS monitor’s user-selectable display filter excludes frequencies below 2 Hz from the SEF95 computation. This artificially raises SEF95 values, suggesting patients are more lightly anesthetized than they actually are. Testing across multiple BIS generations—from the A-1050 and A-2000 to the current BIS Advance—confirmed the persistence of this error, with SEF95 increasing by roughly 30% when the display filter was active.
The authors emphasize two critical implications. Clinically, anesthesiologists could unintentionally underdose anesthetics if relying on inflated SEF95 values, especially during deep states such as burst suppression. In research, studies that used BIS-derived SEF95 values without noting filter status may have reported misleading data that subsequently influenced reference literature.
The authors urge Medtronic to correct the SEF95 algorithm and disclose this issue to users. They further call for improved validation standards for EEG-based depth monitors—arguing that devices used to guide anesthetic dosing should be held to the same quality assurance standards as the drugs they inform.
What You Should Know
Activating the display filter on BIS monitors distorts SEF95 readings, making patients appear more awake than they are. This error has persisted for over two decades and may affect both clinical decisions and published research relying on BIS data.
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