Background: It is generally thought that opioids have little effect on EEG, especially when noxious stimuli were not added.. However, we felt that EEG amplitude (Amp) seemed to become smaller after starting the infusion of remifentanil during propofol anesthesia. To examine this, we compared the Amp observed without remifentanil and that with remifentanil during propofol anesthesia. 
Materials and Methods: We analyzed the EEG data obtained in the previous study. After approval of the ethical committee of our institute and obtained written informed consent from the participants, we enrolled 26 female patients (aged 33-65) who were scheduled mammectomy for breast cancer. Besides the standard monitors, we used BIS monitor (BIS-XP) and all raw EEG packet as well as EEG derived parameters were recorded on a computer using our original software “BSA for BIS”. We defined the amplitude of EEG as the half of the voltage difference between the adjacent two local peaks, and Amp was defined as the average of EEG amplitude in the latest 20 seconds of EEG signals. Propofol was infused using TCI pump (TE-371; TERUMO, TOKYO, JAPAN). We determined Amp without remifentanil and Amp with remifentanil infused at 0.2 μg/kg/min at the same effect-site concentration of propofol before incision. We also compared the powerspectrum of EEG, with or without remifentanil. The propofol concentration was adjusted to surgical level of anesthesia in each patient by EEG monitoring. 
Results and Discussion: In all cases, amplitudes of EEG were decreased after infusion of remifentanil. Amp without remifentanil was 13.1±2.6 (μV) and Amp with remifentanil was 10.7±2.3 (μV) (p<0.001) (Mean±SD). And power of alpha frequency (8-13 Hz) was also significantly decreased after infusion of remifentanil, 20.7±14.2 (μV2) vs. 10.2±8.1 (μV2) (p<0.001), which suggested that spindle activity was attenuated by remifentanil. 
Conclusion: We found that EEG amplitude became smaller when remifentanil was co-administered with propofol. Remifentanil seemed to attenuate the spindle activity at this anesthetic level.

Remifentanil attenuates the spindle activity and decrease EEG amplitude during propofol anesthesia