Dextran-Local Anesthetic Interactions

Abstract

A number of clinical investigators have found that low molecular weight (LMW) dextran prolongs the action of local anesthetics on peripheral nerve. Other reports indicate no significant change in duration with these combinations. We performed a series of experiments to determine if LMW dextran interacted with lidocaine or bupivacaine so that the duration of block would be enhanced. Millipore filtration, equilibrium dialysis, and membrane ultrafiltration were performed with 14C-lidocaine in the presence and absence of LMW dextran to determine whether any binding existed between LMW dextran and lidocaine which might prolong a block by retarding the egress of the local anesthetic. In all cases, the lidocaine crossed the 8,000 to 10,000 molecular weight barriers as quickly with LMW dextran as without it. To confirm these findings in vivo, 14C-lidocaine, with and without 5% LMW dextran, was infused into the infraorbital nerve of rats. Thirty minutes after the block was established, the animals were killed and the nerves removed and sectioned. The amount of lidocaine still present was the same whether or not LMW dextran was added. If the amount of lidocaine present is not altered, then its effect must be enhanced if a prolonged block is to be produced. Increased refractory period would increase the effect of a given concentration of lidocaine or other local anesthetic on the transmission of noxious stimuli. LMW dextran 5% did not increase the block of 0.25 mM lidocaine on frog sciatic in vitro at frequencies from 1 to 100 Hz. Direct assessment of the effect of LMW dextran was made by injecting 1 mg of bupivacaine or 2 mg of lidocaine with or without 5% LMW dextran into the rat maxillary nerve and assessing the duration of nerve block. In both cases, block duration was the same whether or not dextran was added. These data suggest that LMW dextran does not affect the kinetics of lidocaine nor does it enhance the action of lidocaine or bupivacaine in other ways, and thus these data support the findings of those who see no prolongation of action when LMW dextran is added to lidocaine or bupivacaine.