Closed-loop control of anaesthesia

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Prediction of responses to various stimuli during sedation: a comparison of three EEG variables

OBJECTIVE: To compare the ability of the aepEX, a derivative of auditory evoked potentials, the bispectral index, and SEF95% to predict responses to various stimuli. DESIGN: Prospective clinical study. SETTING: General Intensive Care Unit in a university hospital. PATIENTS AND PARTICIPANTS: Forty postsurgical, mechanically ventilated patients. INTERVENTIONS: Target concentrations of blood propofol were randomly set at 0.5 microg/ml, 1.0 microg/ml, 1.5 microg/ml, and 2.0 microg/ml, with a fixed fentanyl infusion rate between 0.5 microg.kg(-1).h(-1) and 1.5 microg.kg(-1).h(-1). MEASUREMENTS: Depth of sedation was subjectively assessed with the Ramsay Sedation Score. The aepEX was recorded using an auditory evoked potentials system. The bispectral index and SEF95% were measured using an Aspect A-1000 monitor. RESULTS: The aepEX, bispectral index, and SEF95% correlated with the Ramsay Sedation Score, the Pk value being greatest for the aepEX, followed by the bispectral index. All three variables could predict opening of the eyes in response to verbal commands or a glabellar tap, the aepEX being a better predictor than the bispectral index or SEF95%. All three EEG variables had Pk values >0.5 in predicting coughing or movement in response to tracheal suction, but they were unable to predict increases in heart rate or systolic blood pressure. CONCLUSIONS: The aepEX was the best predictor, followed by bispectral index. Although in most intensive care patients subjective sedation scales are sufficient to assess levels of sedation, the aepEX and bispectral index were potential alternatives to subjective scales when they do not work well in the setting of neuromuscular blockade or may not be sufficiently sensitive to evaluate very deep sedation

Kinetics of Light-Induced Concentration Patterns in Transparent Polymer Solutions

When exposed to weak visible laser light, solutions of common polymers like poly­(isoprene) and poly­(butadiene) respond by local concentration variations, which in turn lead to refractive index changes. Various micropatterns have been recently reported, depending mostly on the solvent environment and the irradiation conditions. Here, we focused on the simpler case of single polymer-rich filaments and we employed phase contrast microscopy to systematically investigate the influence of laser illumination and material parameters on the kinetics of the optically induced local concentration increase in the polydiene solutions. The refractive index contrast of the formed filaments increased exponentially with the laser illumination time. The growth rate exhibited linear dependence on the laser power and increased with polymer chain length in semidilute solutions in good solvents. On the contrary, the kinetics of the formed filaments appeared to be rather insensitive to the polymer concentration. Albeit the origin of the peculiar light field-polymer concentration coupling remains yet elusive, the new phenomenology is considered necessary for the elucidation of its mechanism