Intravenous lidocaine increases the depth of anaesthesia of propofol for skin incision--a randomised controlled trial.

ABSTRACT

BACKGROUND: The anaesthetic potency of intravenous propofol is quantified by its Cp50 value, which is defined as the plasma concentration required to prevent movement response in 50% of patients to surgical stimuli. We hypothesised that, in addition to propofol anaesthesia, an intravenous bolus of lidocaine 1.5 mg/kg will decrease the Cp50 value of propofol during anaesthesia. METHODS: We enrolled 54 elective surgical patients undergoing propofol-based anaesthesia, and randomised them to either lidocaine 1.5 mg/kg, lidocaine 0.5 mg/kg or placebo (NaCl 0.9%) 3 min before skin incision. The propofol Cp50 value was then calculated using the 'up-and-down' method of Dixon and Massey. RESULTS: There was no significant reduction in propofol requirements after the administration of 0.5 mg/kg lidocaine from 8.5 µg/ml [confidence interval (CI) 6.0-11.625] to 8.25 µg/ml (CI 6.75-9.76); however, a bolus of 1.5 mg/kg lidocaine decreased the Cp50 value of propofol by 42% from 8.5 µg/ml (CI 6.0-11.625) to 4.92 µg/ml (CI 4.5-5.78) (P < 0.05). CONCLUSION: An intravenous bolus injection of 1.5 mg/kg lidocaine 2% caused a significant reduction of the propofol Cp50 value.