THE DURATION OF ACTION OF BUPIVACAINE, LEVOBUPIVACAINE, ROPIVACAINE AND PETHIDINE IN PERIPHERAL-NERVE BLOCK IN THE RAT

Background: There is a current interest in local anaesthetic drugs/for mulations exhibiting long durations of sensory bled; and minor motor-b locking effects. Objectives: To compare the duration of sensory and mo tor blockade in peripheral nerve blocks induced by the new agents ropi vacaine and levobupivacaine, with that of racemic bupivacaine and peth idine. Methods: Groups of 8 male Sprague-Dawley rats were subjected to infraorbital (IONB) or sciatic nerve block (SNB) employing 0.2 mi of differently concentrated solutions of bupivacaine, levobupivacaine, ro pivacaine or pethidine. The sensory blocking effect in IONB is express ed as (i) the time to elicitation of an abdominal jerk by electrical s timulation at arbitrarily chosen (threshold) intensities (IONB degree 3, 5, 8 and 10), and as (ii) the area under the curve (AUG, threshold intensities vs time). The duration of motor block in SNB is given as t he time from injection to regained ability to walk and grip normally w ith the toes. Comparisons of the dose-effect relationships for the inv estigated agents were made by analysis of covariance. Results: In IONB the log (dose)-log (effect) lines for bupivacaine, levobupivacaine an d ropivacaine did not deviate from parallelism. The duration of sensor y block induced by equimolar doses of these agents was similar, althou gh bupivacaine exerted more pronounced effects than levobupivacaine (A UG 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 f ound not to deviate from parallelism, both agents exerting similar dur ations of action. The motor-blocking effects of ropivacaine showed an inverse dose-duration relationship (P=0.019). Conclusions: Equimolar d oses of the investigated local anaesthetics exerted similar durations of sensory blockade in a peripheral nerve block model in the rat.