Plasma prilocaine and mepivacaine concentrations after combined lumbosacral plexus block

In a pharmacokinetic study of combined sciatic/3-in-1 block for lower limb surgery, the two moderate-acting local anaesthetics prilocaine and mepivacaine were compared. The mean maximum venous plasma concentrations of mepivacaine were more than twice as high as when prilocaine was used as anaesthetic (5.1 micrograms/ml vs. 2.37 micrograms/ml). When used in combination with the former, ornipressin did not reduce plasma concentrations of mepivacaine to values which were below the threshold for toxic symptoms (5-6 mg/ml). The peak plasma concentrations exceeded the threshold of 5 micrograms/ml in four of the nine patients of the mepivacaine group (maximum value 7.21 mg/ml) and in two of the nine patients of the mepivacaine+ornipressin group (maximum value 8.61 micrograms/ml).