Does spectral entropy reflect the response to intubation or incision during propofol-remifentanil anesthesia?

BACKGROUND: Spectral entropy is an electroencephalogram-based monitoring technique with a frequency band enlarged to include the electromyogram spectrum, which is intended to help to assess analgesia. Although its correlation with hypnosis has been shown, its performance during a noxious stimulation and the influence of neuromuscular blockade have not been described. METHODS: In this prospective, open, multicenter study, 105 patients received propofol then remifentanil target-controlled infusion for induction of anesthesia, with randomized remifentanil targets ranging from 2 to 8 ng/mL. Half of the patients received neuromuscular blockade. Intubation and incision were used as standard noxious stimulations, motor or hemodynamic responses were recorded, and spectral entropy values before and after stimulations were compared between responders and nonresponders. RESULTS: No difference was found in response entropy (RE), state entropy (SE), or (RE - SE) between patients with or without hemodynamic response to stimulations. Patients with motor response to intubation had higher values of RE, SE, and (RE - SE) both before and after the intubation than patients with no response. These results were confirmed by a prediction probability analysis, showing a significant but weak predictive value of entropy for motor response only. CONCLUSIONS: Entropy predicted a motor response to noxious stimulations but not a hemodynamic response, which limits its usefulness for assessing the analgesic component of anesthesia in paralyzed patients. High values (RE >55) before the stimulation should be avoided in order to decrease the risk of motor response, but lower values might not prevent this response when the opioid concentration is insufficient, despite an adequate hypnosis.