Relationships between Bispectral Index, Implicit Memory, Dream Recall and Minimum Alveolar Concentration in Blended Anaesthesia

Objective: The aim of this study was to estimate relationships between Bispectral Index and awareness and the possibility to reduce halogenate administration during blended anaesthesia. Methods: An epidural catheter was placed and an bolus of 7-10 mg of levobupivacaine 0,5% and 30 mcg of sufentanil injected. Anaesthesia was induced with propofol 2 mg/kg and cis-atracurium 0.15 mg/kg and fentanil 2 mcg/kg. Analgesia was carried out with levobupivacaine (0.125%) and sufentanil (1 mcg/ml) in epidural infusion. During anaesthesia, patients listened two stories at 1 and 0.5 MAC of sevoflurane. Cardiovascular parameters and BIS were continuously recorded. Patients were interviewed to detect awareness 24 h after awakening. To identify differences between patients with and without implicit memory or/and dreaming recall, the Fisher exact test was applied for independent quantitative variables and the Mann-Whitney test for quantitative variables (p=0.05) and also Cohen’s d. Results: 32 patients were enrolled. There were not events of explicit memory. Seven patients (21.9%) showed dream recall; two (6.3%), presented implicit memory for the story played a 0,5 MAC. Conclusion: Maintaining Bispectral Index values within the range recommended for general anaesthesia, did not permit to avoid unconscious memory formation. Therefore, it should be suppose that, the end-tidal concentration of volatile anesthetics, to maintain the loss of consciousness during blended anaesthesia, is higher than 50% of MAC for sevoflurane.