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

Acta Anaesthesiol Scand. 2006 Feb;50(2):193-200. doi: 10.1111/j.1399-6576.2006.00923.x.

Abstract

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 t(1/2ke0) 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 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.

Conclusions: 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.

Publication types

  • Clinical Trial

MeSH terms

  • Analysis of Variance
  • Anesthetics, Intravenous / blood
  • Anesthetics, Intravenous / pharmacokinetics
  • Anesthetics, Intravenous / pharmacology*
  • Dose-Response Relationship, Drug
  • Electric Stimulation / methods
  • Electroencephalography / methods
  • Female
  • H-Reflex / drug effects*
  • Humans
  • Male
  • Middle Aged
  • Propofol / blood
  • Propofol / pharmacokinetics
  • Propofol / pharmacology*
  • Spinal Cord / drug effects*
  • Time Factors

Substances

  • Anesthetics, Intravenous
  • Propofol