Background: Sciatic nerve blocks are used for many orthopaedic procedures on the knee, lower leg, foot and ankle.However, as nerve block durations vary considerably, timing of supplemental analgesia is challenging. Therefore, knowledge on the effect of local anaesthetic (LA) dose on block duration is important to outweigh the benefits of increasing LA dose against the risk of LA systemic toxicity. In this randomised, double-blind trial, we aimed to explore the relationship between the volume of ropivacaine 0.2% and sciatic nerve block duration. We hypothesised that increasing LA volume would prolong block duration.
Methods: We randomised 60 healthy volunteers to receive one of five volumes of ropivacaine 0.2%: 5, 10, 15, 20, or 30 ml. We used an ultrasound-guided, catheter-based technique targeting the sciatic nerve in the infragluteal region. Primary outcome was sensory block duration defined as time of insensitivity to a cold stimulus. Intergroup differences were tested using one-way ANOVA.
Results: Mean (SD) sensory block durations for the tibial nerve(TN)with increasing volume were: 9.3 h (1.7), 10.4 h (1.6), 9.7 h (2.9), 10.7 h (2.8), 9.9 h (2.6). Mean (SD) sensory block durations for the common peroneal nerve(CPN) were: 10.6 h (2.7), 11.9 h (1.5), 11.0 h (3.3), 13.2 h (3.7), 13.5 h (6.1). There were no intergroup differences (P =0.67 (TN); P = 0.25 (CPN)).
Conclusion: We found no effect of increasing the volume of ropivacaine 0.2% from 5 ml to 30 ml on sensory sciatic nerve block duration.