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Comparison of the concentration-dependent effect of sevoflurane on the spinal H-reflex and the EEG in humans

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It has been shown that spinal reflexes such as the H-reflex predict motor responses to painful stimuli better than cortical parameters derived from the EEG. The precise concentration-dependence of H-reflex suppression by anaesthetics, however, is not known. Here we investigated this concentration-response relationship and the equilibration between the alveolar and the effect compartment for sevoflurane. Methods: 

In 26 patients, the H-reflex was recorded at a frequency of 0.1 Hz while anaesthesia was induced and maintained with sevoflurane at increasing and decreasing concentrations. Population pharmacodynamic modelling was performed using the NONMEM software package, yielding population mean parameters as well as indicators of interindividual variability. Results: 

Suppression of H-reflex amplitude occurred at lower concentrations (mean EC50 1.04 ± 0.10 vol%, SE of NONMEM estimate) than the effect on either BIS or SEF95 of the EEG (mean EC50 1.55 ± 0.08 and 1.72 ± 0.18 vol%, respectively), and exhibited a higher interindividual variability. The concentration-response function for the H-reflex was also steeper (mean ë 2.83 ± 0.25). In addition, the equilibration between alveolar and effect compartment was slower for the H-reflex (mean ke0 0.15 ± 0.01 min−1) than for BIS or SEF95 (mean ke0 0.22 ± 0.02 and 0.41 ± 0.05 min−1). Conclusion: 

The differences in EC50 and slope of the concentration-response relationships for H-reflex suppression and the EEG parameters point to different underlying mechanisms. In addition, the differences in time constant for equilibration between alveolar and effect compartment confirm the notion that immobility is caused at a different anatomic site than suppression of the EEG.
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Keywords: Anaesthetic mechanism; H-reflex; population pharmacodynamics; sevoflurane; spinal reflex

Document Type: Research Article

Affiliations: 1: Department of Anaesthesiology, Charité Campus Mitte, Berlin, Germany, 2: Department of Anaesthesiology, University of Berne, Switzerland, and 3: Department of Anaesthesiology, University of Bonn, Germany

Publication date: 2004-05-01

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