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Background: The spinal cord is an important site of anaesthetic action because it mediates surgical immobility. During anaesthesia with volatile anaesthetics, it has been shown that the suppression of the spinal H-reflex correlates with surgical immobility. To evaluate whether the H-reflex could also be a possible candidate for monitoring immobility during propofol anaesthesia, this study assessed the concentration-dependent suppression of the H-reflex by propofol. To discriminate different effect sites, the individual concentration response-curves and the t1/2ke0 of the H-reflex have been compared with those of two EEG parameters. Methods: In 18 patients, anaesthesia was induced and maintained with propofol infused using a target-controlled infusion pump at stepwise increasing and decreasing plasma concentrations between 0.5 and 4.5 mg/l. The H-reflex of the soleus muscle was recorded at a frequency of 0.1 Hz. Calculated propofol concentrations and H-reflex amplitude were analysed in terms of a pharmacokinetic-pharmacodynamic (PKPD) model with a sigmoid concentration-response function. Results: For slowly increasing propofol concentrations, computer fits of the PKPD model for H-reflex suppression by propofol yielded the following median parameters: EC50 1.1 (0.8–1.7) mg/l, slope parameter 2.4 (2.0–3.7), and a t1/2ke0 of 6.7 (2.8–7.5, 25–75% quantiles) min. For the bispectral index, the t1/2ke0 was 2.2 (1.8–3.1) min and for the spectral edge frequency at the 95th percentile of the power spectrum 2.8 (1.9–3.2) min. Conclusions: Propofol, unlike sevoflurane, suppresses the spinal H-reflex at concentrations far lower than the C50 skin incision. The differences in t1/2ke0-values indicate the presence of different effect compartments for effects on the H-reflex and the EEG. [ABSTRACT FROM AUTHOR]