Describe the pharmacokinetics of local anesthetics.
The blood concentration of local anesthetics is
determined by the following: the amount injected, the rate of absorption from
the site of injection, the rate of tissue distribution, and the rate of
biotransformation and excre-tion of the drug. Local absorption is a function of
the site selected, dose injected, and blood flowing through the area. Of all
the traditional locations, absorption of local anesthetics is most rapid
following application to airway epithelium (especially alveoli). Other types of
blockade have been assessed with regard to associated blood levels of local
anesthetic. In decreasing order of associated local anesthetic blood
concentrations, they are intrapleural, intercostal, lumbar epidural, brachial
plexus, and subcuta-neous. Consequently, local anesthetic administered into the
airway is more likely to result in dangerously high blood levels than the same
dose injected subcutaneously. Absorption is, also, dependent on perfusion of
the injection site. As blood flow past the area is diminished, absorption of
local anesthetic is decreased. For this reason, vasoconstrictors are often
added to local anesthetic solutions. Epinephrine, 5 μg/ml (1:200,000), is frequently mixed with lidocaine or mepivacaine
regardless of the site of injection. Although epinephrine will reduce
absorption of bupivacaine and etidocaine during major nerve blocks, it does not
significantly alter plasma concentrations following lumbar epidural blockade.
Absorbed local anesthetics are distributed
first to highly perfused tissues, such as lung, brain, and heart. This is the α-elimination phase. A short α-elimination phase indi-cates rapid
distribution from blood to tissues. Residual local anesthetics in plasma are
next distributed to poorly perfused areas (β-elimination phase) as well as being
metabolized and excreted (γ-elimination phase).
Ester local anesthetics are hydrolyzed by
plasma pseudocholinesterases. Qualitative or quantitative pseudo-cholinesterase
deficiencies may impair ester local anesthetic metabolism. Para-aminobenzoic
acid is an important ester degradation product because it is a highly
allergenic molecule. Amides are metabolized in the liver to a variety of
by-products. Sources of impaired hepatic function such as extremes of age,
congestive heart failure, and hepatitis will decrease amide biotransformation.
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