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

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 D ixon and M assey. 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.