Background: Electroencephalogram (EEG) waveforms vary widely among individuals, this decreases the usefulness of BIS™ monitors for assessing the effects of propofol. Practically, anesthesia is only seen as too deep when evidence of burst-suppression is seen. We designed an experiment to help towards better assessment of individual anesthetic needs. First, to mark the Ce (effect-site concentration) of propofol at loss of response to calling name and gently shaking shoulders (LOR), we defined Ce-LOR. To mark the transient power increase in the alpha range (9-14 Hz), common to all patients, when propofol concentration gradually increases, we defined Ce-alpha as the highest recorded alpha power for Ce. We also defined Ce-OBS as the Ce of propofol at initial observation of burst-suppression. Then we tried to predict Ce-LOR and Ce-alpha from Ce-OBS, vice versa, and considered the significance of these parameters.
Methods: We enrolled 26 female patients (age 33-65) who were undergoing scheduled mastectomy. During anesthesia, we recorded all raw EEG packets as well as EEG-derived parameters on a computer from BIS-XP™ monitor. Propofol was infused using a TCI pump. Target concentration was adjusted so that Ce of propofol was gradually increased.
Results: We obtained the following regression equation; Ce-alpha or Ce-OBS = Ce-LOR × 0.87 + 1.06 + dummy × 0.83 (for Ce-alpha dummy = 0, and for Ce-OBS = 1; adjusted r = 0.90, p < 2.2e-16) by ANCOVA. At Ce-alpha, BIS was 50.2 ± 7.7.
Conclusion: Ce-alpha and Ce-OBS could be estimated from Ce-LOR. Based on Ce-LOR it is possible to manage the hypnotic level of individual patients.
Keywords: Electroencephalogram; Inter-individual variation; Pharmacodynamics; Propofol.