The results of this study show the course of skin impedances measured by the ESG during induction of cardiosurgical anaesthesia with fentanyl and etomidate. Fentanyl bolus injection, and particularly the additional etomidate bolus injection, produced a noteable change in skin impedance over the whole clinical course. Skin impedances measured with the ESG showed a significant correlation with the bispectral index values at individual measurement time points. Many nonanesthetic factors affecting the EEG . Interpretation of the raw EEG is dependent on the experience of the electroencephalographer. But with an EEG sedative effects of different agents can be monitored [14, 15].
For this study to also assess the effect produced by fentanyl and etomidate, EEGs were administered using BIS monitoring in addition to the skin impedance measurements. Although detailed information regarding the computation algorithms used  are still to be provided by the manufacturer, bispectral EEG analysis is deemed to be the most established EEG procedure for predicting anaesthetic depth [17, 18].
Many studies were able to demonstrate that the BIS offers a reliable measure of sedation during general anaesthesia [19, 20]. The weak points of this method lie in the fundamental difficulty the EEG has in recording the subcortical activities during general anaesthesia. During a sevofluran anaesthesia, Katoh and colleagues found an insufficient prediction reliability of several EEG parameters with respect to patients movements in response to skin incision . Another point is that also the effects of nitrous oxide, ketamine and opiates cannot be computed with the help of the BIS during general anaesthesia . In a multicenter study involving 304 patients, Sebel and colleagues found that the BIS provides an insufficient prediction reliability regarding patients movements in response to skin incision in opiate-supplemented anaesthesias .
Fentanyl reacts on μ1, μ2, δ, as well as on κ receptors. Agonization of the κ-receptor results in slight sedation. The cortex has a great abundance of κ-receptors which may reduce electrical activity in the cortex and thus influence the EEG values measured.
The fact that our study found an impact on BIS values after injection of fentanyl can therefore be explained by a fentanyl agonization of the κ-receptor, but not by an agonization of the receptors μ1, μ2, δ that are responsible for analgesia . μ receptors are located in the substantia gelatinosa of the spinal cord, and supraspinal locations include brainstem and sub-cortex .
The difference between our study and other studies addressing skin conductance measurements [26–29] is that we have evaluated the effects upon the impedance of the skin of a bolus dose of fentanyl, given without additional hypnotics. Furthermore, we studied the effects generated by an additional bolus administration of hypnotics to the same patients. Impedance electrodermal response and skin conductance reflect the activity in the skin sympathetic fibers. However, the experimental settings are different. Skin sympathetic reaction is triggered in subcortical and cortical regions of the CNS with input arising from the hypothalamus, the limbic system and the basal ganglia . Sympathetic efferent pathways descending from the hypothalamus run through the brainstem, synapsing with pre-ganglionic neurons in the Nc. intermediolaterales of the lateral horn. The second synapse is in the sympathetic trunk. The post-ganglionic neurites take their path as sympathetic-sudomotor fibers, together with the remaining portions of the peripheral nerves, up to neuroglandular junctions. Sympathetic activation by cortical and subcortical areas [31, 32] will therefore result in changes in the target organ, the eccrine sweat glands, that are exclusively innervated by the sympathetic nervous system. There is a direct correlation between the activity of the eccrine sweat glands and the changes in skin resistance . Using the constant current technique, Lidberg and Wallin have proved that there is a direct correlation between the discharge rate of skin sympathetic nerve portions and the amplitude of the simultaneously recorded skin impedance responses [34, 35].
The involvement of cortical structures has certainly interfered with skin impedance measurements under the influence of fentanyl. It is a commonly known fact that also emotions have an influence on the values obtained from skin impedance measurements, in addition to manual, visual and acoustic stimuli [36–38]. It can therefore be assumed that, depending on the vigiliance level , emotions, anxieties and acoustic disturbances encountered during induction of anaesthesia for a risk-bearing cardiosurgical intervention counteracted the sedative and vigilance-reducing effect of opiate drugs . The effect of fentanyl upon skin impedance probably would have been even more evident without these cortical influences. An interaction with opiate-induced effects such as nausea, thoracic rigidity, and influence on thermoregulation  cannot be excluded. In an intraoperative study involving 11 patients, Storm and colleagues have postulated that the analgesic effect of a general anaesthesia can be determined through measurements of the skin conductance . Geddes and colleagues examined 45 patients preoperatively and found a decrease in skin conductance only after premedication with diazepam, but not after premedication with intramuscular morphine.
In our study we were able to demonstrate that fentanyl has a significant impact on the skin impedance in the absence of additional hypnotics.
The fact that there are great variances in the baseline values of the individual patients poses a problem for the interpreation of skin impedance measurement data; our own data revealed that there were patients whose skin impedance was up to ten times higher than that of others. Skin resistance is also influenced by factors such as skin condition, the measurement electrodes used, and the recording site. This causes the measurement values to spread across a wide range.