Bispectral Index and Detection of Noxious Stimuli

We congratulate Funcke et al.¹  for their work on the validation of various techniques for monitoring nociception. The authors reported that the Analgesia Nociception Index, the Surgical Pleth Index, and pupil diameter are efficient for the detection of noxious stimuli, while the Bispectral Index (BIS) is not a marker of the analgesic level. However, in this study, deep sedation with propofol was used to allow laryngeal mask insertion without opioids or neuromuscular blocking agents, and consequently, all patients required norepinephrine infusion. As reported in table 4,¹  the BIS values varied between (95% CI, 24 to 28) and 31 (95% CI, 28 to 34) before tetanic stimulation or 27 (95% CI, 25 to 29) and 33 (95% CI, 30 to 37) before intracutaneous stimulation. We would like to point out that the BIS level sought by the authors (24 to 37) is lower than the usual recommendations (40 to 60 or 45 to 60). It is therefore possible that tetanic stimulation was not sufficient to provoke electrocortical activation because of the too deep sedation. Let us mention three studies showing a quite different result compared to that of Funcke et al. Laryngoscopy induces an increase in BIS value when the patients have a BIS value around 50 before stimulation, BIS variation being inversely proportional to the administered concentration of remifentanil.²  Similarly, it has been reported that BIS increases in moderately sedated patients who have received a painful stimulus and that this response was blocked by the analgesic or increasing propofol concentrations.³  The last study showed that BIS increases during moderate and severe noxious stimuli, but the variation was moderate when patients are deeply sedated (BIS < 40).⁴  Finally, regarding BIS variation after noxious stimuli, Funcke et al. demonstrated that the electrocortical activation after noxious stimuli is abolished during too deep sedation.

Support was provided by Hôpital Foch, Suresnes, France.

Dr. Liu and Dr. Chazot are the cofounders of MedSteer (Groslay, France), a company dedicated to creating closed-loop systems for the delivery of anesthetic drugs. Dr. Fischler is the president of the scientific committee of MedSteer.