State entropy and burst suppression ratio can show contradictory information: A retrospective study

Burst suppression is a characteristic electroencephalographic (EEG) pattern that reflects very deep levels of general anaesthesia and may correlate with increased risk of adverse outcomes such as postoperative delirium. EEG-based monitors such as the Entropy Module estimate the level of anaesthesia (state entropy) and provide another index reflecting the occurrence of burst suppression, that is the ratio of burst and suppression (BSR). In the Entropy Module, state entropy and BSR are not interconnected, as they are in the bispectral index (BIS). Hence, state entropy and BSR may provide contradicting information regarding the level of anaesthesia.

We aimed to describe the frequency and characteristics of contradicting state entropy and BSR and to present possible strategies of how to act in these situations.

We based our analyses on state entropy and BSR trend recordings from 2551 patients older than 59 years that showed BSR was > 0 throughout their intervention under general anaesthesia. We determined the maximum state entropy when BSR was > 0, the minimum state entropy with BSR = 0 and the duration of high state entropy with BSR > 0. Further, we selected four exemplar patients to present details of how state entropy and BSR can contradict each other during anaesthesia.

We observed a wide range of state entropy values with BSR > 0. The median [IQR] of the maximum state entropy with BSR > 0 was 53 [45 to 61] and the median of the minimum state entropy without BSR was 21 [15 to 26]. Contradictory BSR and state entropy could persist over several minutes. The presented cases highlight these contradictory BSR and state entropy situations.

Our results illustrate contradictory state entropy and BSR indices that may be relevant for anaesthesia navigation. Longer-lasting episodes may lead to incorrect titration of the depth of the hypnotic component of anaesthesia. Hence, our results demonstrate the necessity to monitor and check the raw EEG or EEG parameters that are less processed than the commercially available indices to safely navigate anaesthesia.