Anesthetics

Amide-Based

Local anesthetics can be either ester- or amide-based. Ester local anesthetics are generally unstable in solution and fast-acting, are rapidly metabolised by cholinesterases in the blood plasma and liver, and more commonly induce allergic reactions. Amide local anesthetics are generally heat-stable, with a long shelf life (around two years). Amides have a slower onset and longer half-life than ester anesthetics, and are usually racemic mixtures, with the exception of levobupivacaine (which is S(-) -bupivacaine) and ropivacaine (S(-)-ropivacaine). Although general rules exist for onset and duration of anesthesia between ester- or amide-based local anesthetics, these are properties are ultimately dependent on myriad factors including the lipid solubility of the agent, the concentration of the solution, and the pK_a. Amides are generally used within regional and epidural or spinal techniques, due to their longer duration of action, which provides adequate analgesia for surgery, labor, and symptomatic relief. Some esters, such as benzocaine and tetracaine, are found in topical formulations to be absorbed through the skin.