Clonidine Enhances the Effects of Lidocaine on C-Fiber Action Potential

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Abstract
Degree of C-fiber inhibition, corresponding to an AP area under the curve of 75.8% ± 9.4% (mean ± SE) and 82.2% ± 5.9% of control, respectively. Concentrations of clonidine less than 500 μM did not inhibit C-fiber AP. Clonidine, added in concentrations of 500 nM, 500 /.LM, and 5 mM to a 500 μM lidocaine perfusion, caused a significant decrease in fiber blockade of 18%, 20%, and 54%, respectively, as compared with clonidine added to Locke perfusion (P < 0.05). The sodium channel blocker tetrodotoxin (3 μM) decreased the AP area to 9.3% ± 1.3% of control. The remaining tetrodotoxin-resistant AP was almost completely blocked by clonidine (500 μM) and lidocaine (500 μM), indicating a higher susceptibility of tetrodotoxin-resistant fibers to the two drugs than the C-fiber population as a whole. The enhancing effect of a low dose of clonidine (500 μM) on lidocaineinduced (500 μM) inhibition of C-fiber AP might explain the clinical observation that clonidine, at approximately 1000-fold lower concentrations than lidocaine, prolongs the action of lidocaine in peripheral nerve block. Address correspondence to Dr. Gaumann, Department of Anesthesiology, University Hospital Geneva, 1211 Geneva 4, Switzerland. Accepted for publication January 27, 1992. © 1992 International Anesthesia Research Society...

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