The duration of action of bupivacaine, levobupivacaine, ropivacaine and pethidine in peripheral nerve block in the rat

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Abstract
Background: There is a current interest in local anaesthetic drugs/formulations exhibiting long durations of sensory block and minor motor–blocking effects. Objectives: To compare the duration of sensory and motor blockade in peripheral nerve blocks induced by the new agents ropivacaine and levobupivacaine, with that of racemic bupivacaine and pethidine. Methods: Groups of 8 male Sprague–Dawley rats were subjected to infraorbital (IONB) or sciatic nerve block (SNB) employing 0.2 ml of differently concentrated solutions of bupivacaine, levobupivacaine, ropivacaine or pethidine. The sensory blocking effect in IONB is expressed as (i) the time to elicitation of an abdominal jerk by electrical stimulation at arbitrarily chosen (threshold) intensities (IONB degree 3, 5, 8 and 10), and as (ii) the area under the curve (AUC, threshold intensities vs time). The duration of motor block in SNB is given as the time from injection to regained ability to walk and grip normally with the toes. Comparisons of the dose–effect relationships for the investigated agents were made by analysis of covariance. Results: In IONB the log (dose)–log (effect) lines for bupivacaine, levobupivacaine and ropivacaine did not deviate from parallelism. The duration of sensory block induced by equimolar doses of these agents was similar, although bupivacaine exerted more pronounced effects than levobupivacaine (AUC by 25%, P=0.001; IONB degree 3 by 14%, P=0.03). In SNB only the log (dose)–log (duration) lines for bupivacaine vs levobupivacaine were found not to deviate from parallelism, both agents exerting similar durations of action. The motor–blocking effects of ropivacaine showed an inverse dose–duration relationship (P=0.019). Conclusions: Equimolar doses of the investigated local anaesthetics exerted similar durations of sensory blockade in a peripheral nerve block model in the rat.