What are the clinical differences between the ester and amide local
anesthetics?
The important differences between ester and
amide local anesthetics relate to the mechanisms by which they are metabolized
and their potential to produce allergic reactions.
Ester local anesthetics undergo rapid
hydrolysis by plasma pseudocholinesterases and to a lesser extent in the liver
with resulting pharmacologically inactive metabolites. However,
para-aminobenzoic acid is one of the metabolites that has been associated with
allergic reactions in a small percentage of patients. Cerebrospinal fluid lacks
the pseudocholinesterase enzyme, so ester local anesthetics administered
intrathecally will persist until they have been absorbed by the bloodstream.
Patients with atypical plasma pseudocholinesterase are at increased risk for
toxic side-effects because of impaired metabolism. Plasma pseudocholinesterase activity
may also be decreased in patients with liver disease or those taking certain
chemother-apeutic drugs. Cocaine is the only ester local anesthetic not
metabolized in the plasma; rather it undergoes significant liver metabolism.
Amide local anesthetics undergo enzymatic
degradation in the liver, which in general is much slower than ester
hydrolysis. Patients with decreased liver function (e.g., liver cirrhosis) or
decreased liver blood flow (e.g., congestive heart failure) are predisposed to
systemic toxicity from impaired metabolism. The amide local anesthetics are not
metabolized to para-aminobenzoic acid, and allergic reactions are extremely
rare. However, solutions of these drugs may contain preservatives (paraben
family, whose structure is similar to para-aminobenzoic acid) or additives
(sodium bisulfite) that are the frequent culprits of adverse reactions.
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