The vehicle for LAs is sterile water. Some LAs contain citric acid, an antioxidant; edetate calcium disodium, a stabilizer; and sodium chloride to produce isotonicity. Sodium hydroxide and/or hydrochloric acid are added to adjust the pH, to a pH range of 3.4 to 6.5, favoring the formation of stable water-soluble LA salts in the solution. Once injected into the interstitial space, the buffering capacity of the extracellular fluid (physiological pH of 7.4) favors the formation of free base LA and greater diffusion.
Most LA formulations include epinephrine for vasoconstriction.13-16 It decreases the rate of LAs’ systemic absorption, thereby, reducing the risk of LAs’ systemic toxicity; it localizes LAs, thereby, prolonging LAs’ duration of action; and, with infiltration anesthesia, reduces superficial bleeding from arterioles and capillaries in the operative field.13-17 The vasoconstrictor in the mepivacaine 2% formulation is levonordefrin, a derivative of norepinephrine.16
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