Halothane and propofol differentially affect electroencephalographic responses to noxious stimulation

Background. Anaesthetics blunt neuronal responses to noxious stimulation, including effects on electroencephalographic (EEG) responses. It is unclear how anaesthetics differ in their ability to modulate noxious stimulation-evoked EEG activation. We investigated the actions of propofol and halothane on EEG responses to noxious stimuli, including repetitive electrical C-fibre stimulation, which normally evokes neuronal wind-up. Methods. Rats were anaesthetized with halothane (n = 8) or propofol (n = 8), at 0.8x or 1.2x the amount required to produce immobility in response to tail clamping [minimum alveolar concentration (MAC) for halothane and median effective dose (ED50) for propofol]. We recorded EEG responses to repetitive electrical stimulus trains (delivered to the tail at 0.1, 1 and 3 Hz) as well as supramaximal noxious tail stimulation (clamp; 50 Hz electrical stimulus, each for 30 s). Results. Under halothane anaesthesia, noxious stimuli evoked an EEG activation response manifested by increased spectral edge frequency (SEF) and median edge frequency (MEF). At 0.8 MAC halothane, the tail clamp increased the MEF from approximate to 6 to approximate to 8.5 Hz, and the SEF from approximate to 25.5 to approximate to 27 Hz. At both 0.8 and 1.2 MAC halothane, similar patterns of EEG activation were observed with the 1 Hz, 3 Hz and tetanic stimulus trains, but not with 0.1 Hz stimulation, which does not evoke wind-up. Under propofol anaesthesia, noxious stimuli were generally ineffective in causing EEG activation. At 0.8 ED50 propofol, only the tail clamp and 1 Hz stimuli increased MEF (approximate to 8 to approximate to 10-10.5 Hz). At the higher propofol infusion rate (1.2 ED50) the repetitive electrical stimuli did not evoke an EEG response, but the tetanic stimulus and the tail clamp paradoxically decreased SEF (from approximate to 23 to approximate to 21.5 Hz). Conclusions. Propofol has a more significant blunting effect on EEG responses to noxious stimulation compared with halothane.