Suppression of the human spinal H-reflex by propofol: a quantitative analysis

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 t(1/2ke0) of the H-reflex have been compared with those of two EEG parameters.

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.

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 t(1/2ke0) of 6.7 (2.8-7.5, 25-75% quantiles) min. For the bispectral index, the t(1/2ke0) 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.

Propofol, unlike sevoflurane, suppresses the spinal H-reflex at concentrations far lower than the C50 skin incision. The differences in t(1/2ke0)-values indicate the presence of different effect compartments for effects on the H-reflex and the EEG.