Minimal local anaesthetic volumes for sciatic nerve block: evaluation of ED99 in volunteers

Abstract

This randomized, double-blinded volunteer study was designed to evaluate the ED(99) volume of local anaesthetic for sciatic nerve blocks using a step-up/step-down methodology. A maximum of 20 volunteers were included to receive an ultrasound-guided sciatic nerve block with mepivacaine 1.5% and a starting volume of 0.2 ml mm(-2) cross-sectional nerve area. In cases of a complete sensory block, the volume was reduced by 0.02 ml mm(-2) cross-sectional nerve area until the first block failed. Thereafter, the volume of local anaesthetic was increased by 0.02 ml mm(-2) cross-sectional nerve area. After three cycles of successful/failed blocks, the ED(99) volume of local anaesthetic could be calculated by a probability function. The influence of the volumes of local anaesthetics on sensory onset times and duration of sensory block was evaluated by linear regression. The ED(99) volume of local anaesthetic for sciatic nerve block was calculated with 0.10 ml mm(-2) cross-sectional nerve area. The correlation between the volume of local anaesthetic and the sensory onset time was weak (r=0.14), whereas the correlation between the volume of local anaesthetic and the duration of sensory block was moderate (r=0.65). This is the first study where an ED(99) volume of local anaesthetic for sciatic nerve block has been evaluated. The resulting local anaesthetic volume of 0.10 ml mm(-2) cross-sectional nerve area seems to have no impact on sensory onset time, whereas the duration of sensory block is shorter.