The suppression of spinal F-waves by propofol does not predict immobility to painful stimuli in humans

Abstract

Background. The immobilizing effects of volatile anaesthetics are primarily mediated at the spinal level. A suppression of recurrent spinal responses (F-waves), which reflect spinal excitability, has been shown for propofol. We have assessed the concentration-dependent F-wave suppression by propofol and related it to the logistic regression curve for suppression of movement to noxious stimuli and the effect on the bispectral index™ (BIS™). The predictive power of drug effects on F-waves and BIS for movement responses to noxious stimuli was tested. Methods. In 24 patients anaesthesia was induced and maintained with propofol infused by a target controlled infusion pump at stepwise increasing and decreasing plasma concentrations between 0.5 and 4.5 mg litre-1. The F-waves of the abductor hallucis muscle were recorded at a frequency of 0.2 Hz. BIS values were recorded continuously. Calculated propofol concentrations and F-wave amplitude and persistence were analyzed in terms of a pharmacokinetic-pharmacodynamic (PK/PD) model with a simple sigmoid concentration-response function. Motor responses to tetanic electrical stimulation (50 Hz, 60 mA, 5 s, volar forearm) were tested and the EC50tetanus was calculated using logistic regression. Results. For slowly increasing propofol concentrations, computer fits of the PK/PD model for the suppression by propofol yielded a median EC50 of 1.26 (0.4-2.3) and 1.9 (1.0-2.8) mg litre-1 for the F-wave amplitude and persistence, respectively. These values are far lower than the calculated EC50 for noxious electrical stimulation of 3.75 mg litre-1. This difference results in a poor prediction probability of movement to noxious stimuli of 0.59 for the F-wave amplitude. Conclusions. F-waves are almost completely suppressed at subclinical propofol concentrations and they are therefore not suitable for prediction of motor responses to noxious stimuli under propofol mono-anaesthesia. © The Board of Management and Trustees of the British Journal of Anaesthesia 2005. All rights reserved.