Normalized skin conductance level could differentiate physical pain stimuli from other sympathetic stimuli

Abstract

Skin conductance monitoring is one of the promising methods for objectively evaluating pain. However, skin conductance might possibly increase in response to sympathetic stimulation other than pain. In this study, we aimed to test whether skin conductance monitoring can distinguish physical pain stimulation (heat, mechanical and cold stimulation) from other sympathetic stimuli (stimulation by noise and painful images). Twenty-three healthy volunteers participated in this prospective observational study. The number of fluctuations in skin conductance (NFSC) and normalized skin conductance level (nSCL) were measured and compared with pain scores on a self-reported pain scale (numerical pain scale [NPS]). Both NFSC and nSCL increased during mechanical stimulation. Further, nSCL, but not NFSC, well reflected heat stimulus intensity, suggesting its ability to quantitatively evaluate pain. nSCLs during physical pain stimulation were greater than those during other sympathetic stimulations. However, NFSC was not able to completely distinguish between the stimuli. These results suggest that nSCL could better differentiate physical pain stimuli from other sympathetic stimuli than NFSC. In comparisons between subjective and objective pain assessment in the same individual, nSCL correlated better with NPS score, indicating the possibility of being able to monitor the transition of pain. Monitoring changes in skin conductance using nSCL might be useful for objectively detecting physical pain. Psychometric response scales, such as the visual analogue scale, have been widely used for assessment of pain 1. This seems to be reasonable because the sensation of pain is usually affected by psychogenic factors, including mood, attention, anxiety, expectation, hypnosis and empathy 2. This fact is reflected in the following definition of pain by the International Association for the Study of Pain: An unpleasant sensory and emotional experience with actual or potential tissue damage, or described in terms of such damage 3. However, an objective pain assessment tool is still required, because verbal expression of pain is difficult for certain individuals, such as children, mentally-handicapped persons and intubated and sedated patients. Numerous studies regarding objective pain assessment have been conducted, and several physiological markers of pain, such as heart rate variability and pupillary reflexes, were proposed. However, none of them has been established as a validated marker for pain assessment so far 4,5. Among the different markers proposed, changes in skin conductance might be promising. Activation of sympathetic nerves in the skin following the experience of pain sensation and/or certain emotions, including fear and excitement, results in sweating in the palmar and plantar areas, which in turn increases skin conductance in these areas. Hence, the rationale for development of the skin conductance monitor was to detect changes in skin conductance over time 6. The principle of the skin conductance monitor raised the question r...

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