Awareness Despite Low Spectral Entropy Values
Arvi Yli‐Hankala · Anesthesia & Analgesia · 2008
To the Editor: Vassiliadis et al.1 presented a case of awareness during propofol–remifentanil anesthesia, despite low-spectral entropy values. Their infusion rate for propofol was less than that recommended by the manufacturer of remifentanil2 (i.e., <100 μg · kg−1 · min−1) to avoid awareness when remifentanil and propofol are infused together. The anesthesia technique described by Vassiliadis et al. resulted in propofol and remifentanil effect–site concentrations (Ce) of 1–1.5 μg/mL and 8–10 ng/mL, respectively. This sedative concentration for propofol was insufficient to produce amnesia for their paralyzed patient. Simultaneously, the very high concentration of remifentanil most probably slowed the frequency of the electroencephalogram (EEG),3 thus decreasing its entropy content. I think that this explains the awareness described by the patient, despite low spectral entropy values. More conventional dosing of remifentanil (Ce up to 6 ng/mL) would have revealed patient's consciousness in entropy monitoring. Large opioid doses change the EEG apart from their unreliable effect on consciousness. When the EEG changes without adequate hypnotic medication, all indices of anesthetic “depth” remain useless. Remember: EEG during anesthesia is never a universal direct measure of consciousness, but an indicator of drug effect! A.Y.-H. has participated in the development of Spectral Entropy Module. A.Y.-H. is serving as a paid, part-time Medical Advisor for GE Healthcare Finland, Helsinki, Finland. Arvi Yli-Hankala, MD Department of Anesthesia Tampere University Hospital, and Medical School University of Tampere Tampere, Finland [email protected]