Objectives: To compare the second differential index (SDI) calculated from the auditory evoked potential (AEP) and electroencephalogram (EEG) parameters: median frequency (MF), spectral edge frequency (SEF) and burst suppression rate (BSR) determined at four equivalent minimum alveolar concentrations (MAC) of isoflurane or halothane.
Animals: Twelve male Wistar rats weighing 418 g (SD +/- 18.4 g).
Methods: Auditory evoked potentials and EEG responses were recorded in animals implanted with electrodes at established anaesthetic concentrations. Depth of anaesthesia was assessed using the strength of the pedal withdrawal reflex (PWR), and data were analysed using repeated measures anova and paired t-tests.
Results: The SEF tended to decrease with increasing depth of halothane anaesthesia (F = 4.198, p = 0.05), but not with isoflurane. The MF and SDI were significantly higher during halothane than with isoflurane (F = 5.82, p = 0.036 and F = 5.263, p = 0.045, respectively) at equivalent depths of anaesthesia, and EEG burst suppression occurred at deeper planes of isoflurane but not halothane anaesthesia.
Conclusions: The study demonstrated that EEG and AEP characteristics recorded at MAC equivalent concentrations were suppressed to a greater degree by isoflurane than by halothane. These findings have strong implications for research projects where EEG recordings are collected, and also cast more general doubts upon the value of such parameters for evaluating depth of isoflurane anaesthesia in rats.