Levobupivacaine versus ropivacaine for sciatic nerve block in the rat.

Background And Objectives: Ropivacaine and levobupivacaine, both single S- enantiomers, are being promoted as safer alternatives to racemic bupivacaine. To determine whether levobupivacaine produces a more potent and longer lasting peripheral nerve block than ropivacaine, we compared functional blockade of sciatic nerve in the rat at several doses with these 2 agents.

Methods: Percutaneous sciatic nerve blocks were performed in 6 groups of rats (n = 8) with 0.1 mL of 0.0625%, 0.125%, and 0.25% of levobupivacaine or ropivacaine, with and without 1:200,000 epinephrine. In the 2 different hindlegs of the same 8 rats, we measured the degree and duration of deficit of 3 functions of peripheral nerve (proprioception, motor function, nociception) effected by the 2 respective drugs at the same concentration, using a blinded cross-over design.

Results: In general, both degree and duration of functional deficits produced by levobupivacaine were slightly greater than those from ropivacaine at the same dose. At the lowest concentration (0.0625%), no functions were completely blocked and the only significant difference was a longer motor impairment by levobupivacaine. At the highest concentration (0.25%), all formulations completely blocked the 3 functions, all of which lasted longer with levobupivacaine than ropivacaine. There were no significant differences in degree and duration of deficits between the 2 drugs at midconcentration (0.125%). The actions of both drugs were generally potentiated by epinephrine, but the rank order of potency was not altered.

Conclusions: At the lowest concentration (0.0625%), levobupivacaine produces a greater degree of motor impairment and a longer duration of proprioceptive impairment relative to ropivacaine. At the middle concentration (0.125%), there no differences between the 2 drugs. At the higher concentration (0.25%), which is within the range used clinically for peripheral nerve block (0.25%-0.5%), levobupivacaine produces approximately a 30% longer duration of complete block in each modality compared with that by ropivacaine.