Skin conductance or entropy for detection of non-noxious stimulation during different clinical levels of sedation

Background State entropy (SE) measures electroencephalographic signals, whereas response entropy (RE) also includes frontal electromyographic activity. In the presence of electromyographic activity, the RE index is larger than the SE index, the difference being denoted as RE-Delta (RE-Delta = RE - SE). Skin conductance (SC) may be expressed by a slow reacting variable, the mean SC level, the derivate of the mean SC level (D-SC), the number of SC fluctuations (NSCF) or the amplitude of the SC fluctuations (ASCF), which directly shows skin sympathetic nerve activity. The goal of this study was to evaluate whether these SC and entropy variables could differentiate between the responses obtained to load sound stimuli at different sedation levels before the induction of general anaesthesia. Methods Twenty women scheduled for gynaecological laparotomy were studied. The modified observer's assessment of alertness sedation (OAAS) was used to classify the patients' hypnotic levels. White sounds (98 dB) were given at OAAS level 5 without propofol, at OAAS levels 4-3 and 3-2 with propofol and at OAAS levels 3-2 and < 2 with propofol and remifentanil. Results RE and SE showed a steady decline from OAAS level 5 to level < 2 (P < 0.01). RE-Delta did not discriminate between any of the OAAS levels (P = NS). The mean SC level discriminated between OAAS levels 4-3 to < 2 (P < 0.01). D-SC discriminated between all the different OAAS levels (P < 0.01). NSCF discriminated between OAAS levels 5 to 3-2 (P < 0.05), but did not discriminate at OAAS level 3-2 between propofol alone or combined with remifentanil, or between OAAS level 3-2 and < 2. ASCF differentiated between OAAS levels 5 and 4 (P < 0.001) and OAAS levels 3-2 and < 2 (P < 0.05) only. Conclusion RE, SE and D-SC showed a similar discrimination between sound responses at the different sedation levels.