Abstract
Rather than relying solely on subjective pain evaluation using means such as the visual analogue scale (VAS), in clinical situations it is possible to observe evoked responses of the autonomic nervous system (ANS) as objective indicators. Few studies, however, have reported these relationships under finely controlled sedation. 16 healthy male participants were administrated in intravenous sedation with either propofol or midazolam randomly. We initially determined, using pharmacokinetic simulation, the effect-site concentration (Ce) of anaesthetic at loss of response to verbal command and eyelash reflex (Ce-LOR). Then subsequently adjusted Ce to 75%, 50%, and 25% of Ce-LOR to achieve deep, moderate, and light sedation. At awake control state and each sedation level, a noxious electrical stimulation was applied three times at the right forearm, an average pain intensity of the three stimuli was rated on a VAS (0–10). Changes in the peripheral perfusion index measured by oximetry were used as an indicator of ANS response. We analyzed the influence of sedation level on VAS and ANS responses compared to the awake control state. While ANS responses were similar in all conditions, VAS was statistically significantly lower in moderate (5.6±0.6, p <0.005) or deep (5.3±0.6, p <0.001) sedation than in the awake state (7.2±0.4). This study revealed that even when the ANS responds similarly to the same stimulation, subjective pain perception is attenuated by sedation. A cerebral mechanism other than that of the brainstem might determine subjective pain intensity.
Highlights
Intravenous sedation is widely performed in various clinical procedures both to reduce anxiety and pain and to ensure that the patient has no awareness or memory of the procedure
perfusion index (PI)% change would have indicated changes in autonomic nervous system (ANS) response during the stimulation but, when comparing results between any depth of sedation or awake state, no statistically significant differences were found in ANS values (Fig 5)
We investigated how sedation affected perceived pain intensity (VAS scores) and ANS changes (PI%) when noxious stimulation was applied; we found that while visual analogue scale (VAS) scores declined under sedation with propofol or midazolam, PI% remained at the same level
Summary
Intravenous sedation is widely performed in various clinical procedures both to reduce anxiety and pain and to ensure that the patient has no awareness or memory of the procedure. Patients are unable to express pain perception in real time, autonomic nervous system (ANS) responses, such as changes in heart rate, blood pressure, and peripheral perfusion [1,2,3,4], are commonly used as surrogate indicators. Peripheral perfusion index (PI), analyzed by pulse oximetry, is reported as an effective indicator of sympathetic nervous activity in anesthesia procedure [5] or in experimental pain study with alert consciousness [6]. These autonomic responses are useful both because they are objective measures and because their changes correlate well with alert-state SPI scores. Few studies have investigated correlations between changes in SPI and ANS in sedated subjects
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