Local anesthesia is a reversible sensory loss in a defined area of the body associated with transient inhibition of peripheral nerve conduction. The use of a local anesthetic agent should be followed by complete recovery, i.e., without evidence of structural or functional nerve damage. The ideal local anesthetic agent should provide profound, reversible local anesthesia with rapid onset and satisfactory duration of action, and minimal adverse effects.19
The vehicle for LAs is sterile water. Some formulations contain citric acid, an antioxidant; and edetate calcium disodium, a stabilizer; and sodium chloride is added to produce isotonicity. Sodium hydroxide and/or hydrochloric acid are added to adjust the pH; at a pH range of 3.4 to 6.5 they form stable water-soluble salts with LAs. Once injected, the buffering capacity of the extracellular fluid (physiological pH of 7.4) favors free base formation and greater neuronal penetration.
Epinephrine bitartrate or levonordefrin is included in most LA formulations. They cause vascular smooth muscle contraction at the site of LA administration, slow the rate of LA absorption into the systemic circulation, and enhance the duration of local anesthetic action. To minimize oxidation of the vasoconstrictor, sodium or potassium metabisulfite is included in these formulations. Note that epinephrine 1:100,000 is physiologically equivalent to levonordefrin 1:20,000.
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