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 ± SD sensory block durations were 13.1 ± 2.7, 13.4 ± 3.3, 12.6 ± 3.9, 10.4 ± 2.9 and 11.0 ± 2.1 h (P = 0.073), and mean ± SD motor block durations were 8.5 ± 2.0, 7.9 ± 3.0, 6.1 ± 3.1, 5.9 ± 3.5, 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.