Local Anesthetic pH Adjustment
Local anesthetic solutions have an acidic pH for chemical stability and bacteriostasis. Local anesthetic solutions that are formulated with epinephrine by the manufacturer are more acidic than the “plain” solu-tions that do not contain epinephrine. Because they are weak bases, they exist primarily in the ionic form in commercial preparations. The onset of neural block requires permeation of lipid barriers by the uncharged form of the local anesthetic. Increasing the pH of the solutions increases the fraction of the uncharged form of the local anesthetic. Addition of sodium bicarbon-ate (1 mEq/10 mL of local anesthetic) immediately before injection may therefore accelerate the onset of the neural blockade. This approach is most useful for lidocaine, mepivacaine, and chloroprocaine. Sodium bicarbonate is typically not added to bupivacaine, which precipitates above a pH of 6.8.
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