Monitoring of intra-operative nociception: skin conductance and surgical stress index versus stress hormone plasma levels.

'Surgical Stress Index' and the 'Number of Fluctuations in Skin Conductance.s⁻¹, use different methods to analyse sympathetic tone and so provide an estimate of peri-operative analgesia. The aim of our study was to investigate the relationship between these methods and stress hormone plasma levels. In 20 patients scheduled for elective surgery, values of the two methods, mean arterial blood pressure, heart rate and blood samples (to measure plasma levels of adrenaline, noradrenaline, adrenocorticotrophic hormone and cortisol) were obtained at five time points. Changes in Surgical Stress Index and the Number of Fluctuations in Skin Conductance.s⁻¹ only partially reflected changes in plasma noradrenaline levels. Surgical Stress Index, heart rate and blood pressure, but not the 'Number of Fluctuations in Skin Conductance.s⁻¹ changed in response to changes in depth of analgesia by showing significant differences between before and after a bolus of fentanyl. However, the overall predictive ability of both methods was poor.