Investigation of the EEG effects of intravenous lidocaine during halothane anaesthesia in ponies

OBJECTIVE: To record the electroencephalographic changes during castration in ponies anaesthetized with halothane and given intravenous (IV) lidocaine by infusion. The hypothesis tested was that in ponies, IV lidocaine is antinociceptive and would therefore obtund EEG changes during castration. ANIMALS: Ten Welsh mountain ponies referred to the Department of Clinical Veterinary Medicine, Cambridge for castration under general anaesthesia. MATERIALS AND METHODS: Following pre-anaesthetic medication with intramuscular acepromazine (0.02 mg kg(-1)) anaesthesia was induced with IV guaiphenesin (60 mg kg(-1)) and thiopental (9 mg kg(-1)) and maintained with halothane at an end-tidal concentration (FE'HAL) of 1.2%. A constant rate infusion of IV lidocaine (100 microg kg(-1) minute(-1)) was administered throughout anaesthesia. The electroencephalogram (EEG) was recorded continuously using subcutaneous needle electrodes. All animals were castrated using a closed technique. The raw EEG signal was analysed after completion of each investigation, and the mean values of EEG variables (median frequency, spectral edge frequency, total amplitude) recorded during a baseline period (before surgery began) and the removal of each testicle were compared using anova for repeated measures. RESULTS: Spectral edge frequency (SEF) 95% decreased during removal of the second testicle compared with baseline recordings. No other significant EEG changes during castration were measured. CONCLUSIONS: Lidocaine obtunded the EEG changes identified during castration in a previous control study, providing indirect evidence that lidocaine administered peri-operatively was antinociceptive and contributed to anaesthesia during castration. CLINICAL RELEVANCE: The antinociceptive effect of lidocaine combined with its minimal cardiovascular effects indicate a potential use for systemic lidocaine in clinical anaesthetic techniques.