Objectives: Spectral entropy quantifies variations in cortical electrical activity measured by electroencephalography and frontal activity measured by electromyography. The aim of this study, in the context of general anesthesia, was to compare bispectral index values with the two components of spectral entropy: state entropy and response entropy.
Material and methods: Sixteen women (ASA I-II) undergoing gynecological surgery were enrolled. The bispectral index was maintained between 40 and 50 for all patients. Both sensors, for monitoring the bispectral index and spectral entropy, were placed on each patient. Simultaneous readings were recorded at the following moments: operating room arrival, induction, relaxation, intubation, switching on the vaporizer, start of surgery, traction of the intestinal mesenteries and maneuvering of the electric scalpel, switching off the vaporizer, end of surgery, during cough, extubation, and eye opening. The data sets were subjected to analysis of variance, and the intraclass correlation coefficient (ICC) was used to analyze agreement.
Results: Differences between mean values of the bispectral index, state entropy and response entropy were observed at operating room arrival, intubation, and induction. Differences when the patient awakened from anesthesia were observed only between the bispectral index and response entropy, on the one hand, and state entropy on the other. There was no agreement (ICC<0.7) upon operating room arrival, maneuvering the electric scalpel, or awakening from anesthesia.
Conclusion: The bispectral index, state entropy, and response entropy show good agreement during recordings that reflect the effects of anesthesia. The differences observed at baseline and upon awakening can be attributed to frontal electromyographic activity. Distinguishing cortical electrical activity from frontal electromyographic activity may provide additional information in these situations.