Effects of adjuvants to local anaesthetics on their duration. I. Studies of dextrans of widely varying molecular weight and adrenaline in rat infraorbital nerve block.

Local anaesthetics of the amide type were studied in a modified rat infraorbital nerve block model, with which it was possible to determine varying degrees of sensory block. Of the agents investigated, 0.5% bupivacaine tended to give a longer duration of block than 2% prilocaine or 2% lidocaine, while 0.5% etidocaine had the shortest duration. The duration of prilocaine was prolonged by addition of adrenaline, 5 micrograms/ml, more than that of the other agents. Addition of dextrans of Mw 40-110 X 10(3) did not cause any prolongation of block induced by bupivacaine. When mixed with dextrans over a wide range of Mw (40-4900 X 10(3), prilocaine exhibited significant prolongations of its action by up to 200%. The extent of prolongation was dependent on the degree of block, the concentration of dextrans in the local anaesthetic solution, and the Mw of the dextran although in a less uniform way. An increase in the relative viscosity of the solutions might be a factor of importance for the prolonging effect of addition of dextran to local anaesthetics. Since a formulation providing analgesia of a long duration would be of clinical value, further studies on combinations of the comparatively low-toxicity agent prilocaine and macromolecular substances are of interest.