BACKGROUND: Among the various factors influencing the success rate, onset time, and duration of peripheral nerve blocks, the role of local anesthetics concentration remains uncertain. In this prospective, randomized, single-blinded study, we evaluated whether varying the dilution of a fixed dose of mepivacaine solution influenced onset time and duration of sciatic nerve block.
METHODS: Ninety ASA physical status I to II patients scheduled for foot surgery were randomly allocated to receive a double-injection Labat sciatic nerve block with 12 mL mepivacaine 2% (group concentration I = 45 patients) or 24 mL of mepivacaine 1% (group volume II = 45 patients). The nerve stimulator was initially set at 2 Hz, 0.1 millisecond, 1 mA. The total amount of local anesthetic (240 mg) was kept constant and equally divided between the peroneal and tibial nerves. All patients also received an ultrasound-guided popliteal sciatic nerve catheter for postoperative analgesia. Times to readiness for surgery, performance, and offset of local anesthetic were recorded. Our primary end point was to determine a possible difference in offset time between groups. Continuous variables were expressed as median (IQR) and compared with the Wilcoxon-Mann-Whitney U test; WMWodds are reported together with their 95% confidence interval.
RESULTS: The overall success rate of sciatic nerve block was 99%. Time of performance was shorter in group I, 120 seconds (90–150 seconds), than that in group II, 150 seconds (120–180 seconds) (P = 0.0048; WMWodds 2.26 [1.35–4.34]). The onset time of sensory and motor sciatic nerve block was 4 minutes (2–9 minutes) in group I and 6 minutes (4–10 minutes) in group II (P = 0.41; WMWodds 1.21 [0.77–1.95]), while the duration of sensory block was 235 minutes (203–250 minutes) in group I, and 240 minutes (218–247 minutes) in group II respectively (P = 0.51; WMWodds 1.20 [0.69–2.16]).
CONCLUSIONS: We found no evidence that varying volume and concentration while maintaining a fixed total dose of mepivacaine alters the onset time and duration of double-injection sciatic nerve block. Considering our WMWodds results, possible differences in onset time and duration comparable to differences in the performance time between groups cannot be excluded.