Moderate sedation induced by general anaesthetics disrupts audio-spatial feature binding with sustained P3 components in healthy humans.

Feature binding is considered to be the basis for conscious stimulus perception, while anaesthetics exert a gradient effect on the loss of consciousness (LOC). By integrating these two streams of research, the present study assessed the effect of two anaesthetic agents (i.e. propofol and midazolam) on audio-spatial feature binding. We also recorded the electrophysiological activity of the frontal channels. Using pharmacokinetic simulation, we determined the effect-site concentration (Ce) of the anaesthetics at loss of response to verbal command and eyelash reflex. We subsequently adjusted Ce to 75%, 50% and 25% of Ce-LOC to achieve deep, moderate and light sedation, respectively. Behavioural results showed that moderate sedation selectively disrupted feature binding. The frontal channels showed a P3 component (350-600 ms peristimulus period) following the presentation of audio-spatial stimuli at baseline and under moderate and light sedations. Critically, the late event-related potential component (600-1000 ms) returned to the pre-activated level (0-350 ms) at baseline and under light sedation but was sustained under moderate sedation. We propose that audio-spatial feature binding may require the presence of a P3 component and its subsequent and sufficient decline, as under anaesthetic-induced moderate sedation the P3 component was sustained and featured binding was impaired.

Effects of sedation on subjective perception of pain intensity and autonomic nervous responses to pain: A preliminary study.

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.

Obesity, epidural analgesia, and subcostal incision are risk factors for postoperative desaturation.

PURPOSE: This study was designed to determine the incidence of oxygen desaturation after upper abdominal surgery during the first 48 hr on general surgical wards and also to identify risk factors for oxygen desaturation. METHODS: This descriptive study was conducted in 206 patients not expected to receive supplemental oxygen postoperatively who were undergoing upper abdominal surgery in a tertiary care university hospital. Desaturation was classified either as constant, i.e., oxygen saturation < 90% for > three minutes or < 85% once or as episodic, i.e., when oxygen saturation is decreased by > or = 5% below baseline for one to two minutes ten times or more during the night period. Possible risk factors were elderly patients (> 70 yr), obesity (body mass index [BMI] > 25 kg.m(-2)), smoking, surgical time > 180 min, postoperative pain control methods, intraoperative blood loss, and site of incision. RESULTS: Of the 206 patients enrolled, 171 were retained for analysis. Desaturation occurred in 65 patients (38%). Forty-eight of these had constant hypoxemia with nadir oxygen saturation values ranging from 71-89%. The remaining 17 patients experienced nocturnal episodic hypoxemia. After multivariate analysis, the three factors that correlated with postoperative desaturation were BMI > 25 (adjusted odds ratio [OR] 3.06; 95% confidence interval (CI) 1.38-6.79; P = 0.006), subcostal incision (OR 2.68; 95%CI 1.34-5.38; P = 0.005), and neuraxial opioids (OR 2.44; 95%CI 1.21-4.91; P = 0.013). CONCLUSION: Oxygen desaturation is common after upper abdominal surgery, and the risk factors are obesity, a subcostal incision, and neuraxial opioid administration.