Authors: Aydin D et al.
Source: Anesthesia & Analgesia. 2026;142(6):1056-1066.
Summary:
This article examines an important limitation of processed EEG monitoring during anesthesia. The authors focused on situations where the bispectral index, or BIS, appears to show an acceptable anesthetic depth, while the burst suppression ratio, or BSR, suggests significant brain suppression.
BIS values between 40 and 60 are commonly used as a target range during general anesthesia. However, burst suppression usually suggests a deeper level of cortical depression. The paradox studied in this article was the combination of BIS values between 40 and 60 with a BSR of 5% or greater.
The authors analyzed intraoperative BIS and BSR recordings from 62 elderly patients, with an average age of approximately 81 years. They found that even when anesthesia was titrated toward a BIS range of 40 to 60, only about 57% of BIS values actually fell within that range. They also found that some BIS values, especially 41 to 42, occurred more often than expected, suggesting that BIS values may not be continuously or evenly scaled.
Most importantly, the study found that paradoxical BIS and BSR combinations can occur for sustained periods of time, sometimes lasting longer than 2 minutes. In other words, a patient may have a BIS value that appears acceptable while also showing meaningful burst suppression.
Clinical importance:
This article is important because anesthesiologists and CRNAs may falsely assume that a BIS between 40 and 60 always reflects an appropriate anesthetic depth. In older or vulnerable patients, burst suppression may indicate excessive anesthetic effect even when the BIS number appears acceptable.
The practical message is that clinicians should not rely on BIS alone. When BIS and BSR appear to conflict, the anesthesia provider should evaluate both values and, when available, look directly at the raw EEG. This is especially important in elderly patients, where excessive anesthetic depth and burst suppression may have clinical consequences.
Bottom line:
An acceptable BIS value does not always mean the EEG is appropriate. BIS values between 40 and 60 can occur together with burst suppression, including sustained periods of BSR 5% or greater. The authors recommend checking BIS, BSR, and the raw EEG when using processed EEG to guide anesthetic management.
Thank you to Anesthesia & Analgesia and IARS for publishing this important article on BIS monitoring, burst suppression, and the limitations of processed EEG interpretation during anesthesia.