Comparison of skin conductance with entropy during intubation, tetanic stimulation and emergence from general anaesthesia

Background The number of skin conductance fluctuations (NSCF) expresses sympathetic skin nerve activity. The response entropy (RE) measures electromyographic and electroencephalographic activity in the forehead. The state entropy (SE) measures mainly electroencephalographic activity. When the suppression of frontal muscular activity is complete, RE is equal to SE. RE-Delta is defined as SE minus RE. The purposes of this study were to examine whether NSCF and RE-Delta correlate with signs of clinical stress during intubation and tetanic noxious stimulation and to elucidate how rapidly and accurately entropy and NSCF react during emergence from general anaesthesia. Methods Twenty women scheduled for gynaecological laparotomy were studied. During intubation in remifentanil and propofol general anaesthesia, NSCF and RE-Delta were correlated with the clinical stress score. After a wash-out period, two series of tetanic stimuli were given, the first with (R+) and the second without (R-) remifentanil infusion. The tetanic pre-stimuli periods were compared with the tetanic post-stimuli periods, and R+ was compared with R-. During emergence, the responses of entropy and skin conductance were related to the time of extubation. Results NSCF correlated well with the clinical stress score during intubation (r(2) = 0.73, P < 0.0005). RE-Delta showed a weaker correlation (r(2) = 0.33, P = 0.007). During tetanic stimuli, the NSCF pre-stimuli level was lower than the post-stimuli level (P < 0.001), and the NSCF R+ response was lower than the NSCF R- response (P = 0.002). RE-Delta did not show similar differences. During emergence, RE reacted before NSCF and SE (P = 0.003). Conclusions NSCF was better than RE-Delta for the measurement of clinical stress during intubation, and was sensitive to tetanic stimuli at different opioid analgesic levels, by contrast with RE-Delta. Both modalities were able to predict emergence at the end of anaesthesia, but RE was more rapid.