Background: Growing evidence suggests that intravenous lidocaine as a component of multimodal analgesia improves recovery after major colorectal surgery. There is little published data regarding ideal dosing and target plasma concentration in this context, and we wanted to establish our dosing schedule was safe by measuring blood levels of lidocaine.
Methods: We measured the plasma lidocaine concentration of 32 patients at 30 min, 6 h and 12 h after starting intravenous lidocaine infusion for analgesia after major colorectal surgery. Patients received a bolus of 1.5 mg kg-1 over 20 min at the time of induction of anaesthesia. This was followed by a continuous infusion of 2% w/v lidocaine at 3 ml hr-1 (60 mg hr-1) for patients weighing up to 70 kg and 6 ml hr-1 (120 mg hr-1) for patients weighing over 70 kg, using actual body weight.
Results: The overall mean plasma lidocaine concentration was 4.0 μg ml-1 (range 0.6-12.3 μg ml-1). In patients treated with the higher infusion dose, the mean concentration was 4.6 μg ml-1 compared to 3.2 μg ml-1 in those patients on the lower dose. Mean levels were higher at 6 h than 30 min and higher again at 12 h. There were no adverse events or reports of symptoms of local anaesthetic toxicity.
Conclusions: Whilst there were no signs or symptoms of lidocaine toxicity in our patients, there was a wide range of plasma concentrations including some over 10 μg ml-1; a level above which symptoms of toxicity may be expected. We have changed our dosing protocol to using ideal rather than actual body weight based on these results.