The maintenance of an adequate depth of anesthesia (DoA) is important for patients undergoing general anesthesia. Quantitative electroencephalography (EEG) - based monitors, such as the bispectral (BIS) index and entropy, have been established to assess DoA. However, these measures can be affected by various factors, and the processed numerical output can often be misleading; therefore, quantitative EEG-based indices should be interpreted with caution. We report a case of a patient with abnormally high entropy values during ophthalmic surgery with no evidence of intraoperative awareness. Written informed consent was obtained.
A 61-year-old woman was scheduled for retinal detachment repair. Monitoring systems included electrocardiography, noninvasive blood pressure testing, pulse oximetry, end-tidal carbon dioxide, entropy (Entropy™ monitor; GE Healthcare, Finland), and neuromuscular transmission (E-NMT-01; GE Healthcare, Finland). General anesthesia was induced with intravenous injections of fentanyl (75 µg), lidocaine (100 mg), propofol using a target-controlled infusion system (Fresenius Orchestra Primea; Fresenius Kabi AG, Germany) to an effect-site concentration (Ce) of 4.0 µg/ml, and rocuronium (40 mg). General anesthesia was initially maintained with a propofol Ce of 3.0 µg/ml and rocuronium (20 mg/h). However, during the procedure, the entropy values were extremely high (response entropy [RE] > 90, state entropy [SE] between 80 and 90). The train-of-four count of the neuromuscular transmission monitor was 0. Since we assumed there was inadequate DoA, we titrated the propofol Ce to 3.5 µg/ml and administered sevoflurane at a minimal alveolar concentration of 0.5. However, even after titrating the anesthetic doses to deepen the DoA, the entropy values remained abnormally high even though there were no clinical signs of inadequate DoA, such as tachycardia or hypertension. Additionally, the raw EEG waveform changed from ‘fuzzy’ high frequency beta and gamma waves before anesthetic induction to the slow frequency waves of sleep spindles (
This case study demonstrates the importance of examining raw EEG waves in real time rather than relying solely on quantitative EEG-derived indices. Despite the common use of quantitative EEG-based monitors for assessing DoA, the values shown on the monitors can potentially be misleading. For instance, in circumstances with elevated electrode impedance caused by erroneous placement or reduced adherence, the cause of the low-frequency electromyography signals can be misinterpreted as high-frequency EEG signals, iatrogenic movement of the limbs, interference from strong vibration-producing instruments or electrical equipment (such as electric scalpels, electrocautery, or thermal blankets), or a pathological EEG [
Common causes of erroneously high entropy values during ophthalmic surgery include electrocautery, electro-oculography, and electromyographic activity. Alternatively, the surgeon’s hand on the patient’s frontal region, where the entropy sensing lead is placed, may affect electromyography readings and indirectly affect entropy values. However, throughout the reported operation, both RE and SE values remained > 80 despite temporarily ceasing electrocautery. Additionally, the difference between the RE and SE values remained < 8 and the train-of-four count of the neuromuscular transmission monitor was 0, indicating that neither electro-oculographic nor electromyographic activity was the cause of elevated entropy values in this case. All these measurements suggest that none of the common causes of falsely elevated entropy values during ophthalmic surgery were the culprit in this case. Moreover, the particular entropy monitor that was used was presumed to function normally, as no other patients had erroneous results with the model.
The algorithm used to calculate entropy is the mathematical normalization of the overall frequency range of values between 1 (maximum irregularity) and 0 (complete regularity). The theoretical assumption is that irregularities in the EEG signal decrease under anesthesia [
Since BIS and entropy values are calculated using different algorithms, BIS and entropy values that are concomitantly recorded may occasionally show discordant trends during general anesthesia. Aho et al. [
While we are still uncertain of the factors that caused the erroneously high entropy values in this case, we cannot overemphasize the importance of interpreting the raw EEG waveform and considering the patient’s clinical condition rather than relying solely on quantitative EEG-based indices for assuming an inadequate DoA. We suggest that two EEG-based monitors with different algorithms be employed, since this may provide a more accurate assessment when numerical data are inconsistent with the clinical condition. Moreover, we recommend that, as a practical alternative, the EEG sensing electrode be placed at the posterior auricular position during ophthalmic surgery.
None.
No potential conflict of interest relevant to this article was reported.
Yuh-Shyan Wu (Writing – original draft)
Po-Nien Chen (Supervision)
Gwo-Ching Sun (Visualization)
Kuang-I Cheng (Conceptualization)
Zhi-Fu Wu (Writing – review & editing)
High RE and SE value. This case of a 61-year-old female patient undergoing detached retinal repair shows RE and SE values of 100 and 92, respectively; EEG waveforms showing sleep spindles throughout the trace with a consistent and repetitive slow wave background; and stable vital signs. RE: response entropy, SE: state entropy, EEG: electroencephalography.