The effect of ropivacaine concentration on common peroneal nerve block duration using a fixed dose: A randomised, double-blind trial in healthy volunteers.

BACKGROUND: The effect of local anaesthetic concentration on peripheral nerve block duration is unclear. Recent clinical trials found nerve blocks of equivalent duration despite changing local anaesthetic concentration but with a fixed local anaesthetic dose. A criticism of these studies is that the local anaesthetic doses used were above the proposed local anaesthetic dose-duration ceiling level, masking any potential effect of different local anaesthetic concentrations on nerve block duration. OBJECTIVE: We investigated the effect of local anaesthetic concentration on nerve block duration using a fixed local anaesthetic dose below the local anaesthetic dose-duration ceiling level. We hypothesised that changing local anaesthetic concentration would affect nerve block duration. DESIGN: Randomised, double-blind trial. SETTING: Single-centre, academic hospital. PARTICIPANTS: Healthy volunteers. INTERVENTIONS: Each participant received an ultrasound-guided common peroneal nerve block with a fixed dose of 10 mg of ropivacaine dissolved in either 2.5, 5, 10, 15 or 20 ml of 0.9% saline according to group allocation, yielding local anaesthetic concentrations of 4, 2, 1, 0.67 and 0.5 mg ml. MAIN OUTCOME MEASURES: The primary outcome was duration of sensory block defined as altered or no sensitivity to a cold stimulus. The secondary outcome was duration of motor block defined as either paresis or paralysis. Intergroup differences were tested using one-way Analysis of variance . RESULTS: All participants had sensory block, and 56 out of 60 participants had motor block. From the highest to the lowest concentration groups, mean ±â€ŠSD sensory block durations were 13.1 ±â€Š2.7, 13.4 ±â€Š3.3, 12.6 ±â€Š3.9, 10.4 ±â€Š2.9 and 11.0 ±â€Š2.1 h (P = 0.073), and mean ±â€ŠSD motor block durations were 8.5 ±â€Š2.0, 7.9 ±â€Š3.0, 6.1 ±â€Š3.1, 5.9 ±â€Š3.5, 4.0 1.9 h (P = 0.002). CONCLUSION: In contrast to our hypothesis, we found no changes in mean sensory nerve block duration. However, local anaesthetic dilution resulted in reduced motor block duration. TRIAL REGISTRATION: ClinicalTrials.gov, NCT03326609.

Intra- and inter-Individual variability in nerve block duration: A randomized cross-over trial in the common peroneal nerve of healthy volunteers.

BACKGROUND: The reported variation in nerve block duration is considerable. To individualize nerve block therapy, knowledge of the intra- vs inter-individual variability is essential. We investigated the relative contribution of these 2 parameters to the overall nerve block duration variability. METHODS: With ethics committee approval, we conducted a randomized cross-over trial where 20 healthy volunteers received 8 common peroneal nerve blockades with lidocaine 0.5% on 4 consecutive days. Allocations were 5 mL to either the right or left side and 10 mL to the opposite side on day 1 and 2 and vice versa on day 3 and 4. With fixed needle entry and nerve target, we repeated local anaesthetic deposition for each blockade. The primary outcome was variation in duration of sensory nerve block defined as insensitivity to a cold stimulus. Data were analysed using linear mixed model regression. RESULTS: The mean sensory block duration of 380 (95% CI = [342; 418]) minutes on day one was 55 [33; 77] minutes longer than on day two (P < .001), but there were no differences in mean duration between days 2, 3 and 4. The ratios with 2.5; 97.5 percentiles between inter- and intra-individual variation were 2.4 [0.8; 5.2] for the 5 mL blockades and 3.0 [0.9; 6.7] for the 10 mL blockades. The probabilities of inter- to intra-individual variation-ratios >1 were 96% and 97%. CONCLUSION: The intra-individual variability is a substantially minor contributor to the overall variability in sensory nerve block duration compared with the inter-individual variability.