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(-1). 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.