What is the role of local anesthetics in therapeutic injections for pain management?

Updated: Jun 19, 2018
  • Author: Anthony H Wheeler, MD; Chief Editor: Meda Raghavendra (Raghu), MD  more...
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The application of any injectable substance can lead to allergic, idiosyncratic, or adverse side effects. Previous suspicious or unfavorable responses may be verified through prior hospital or office records. In some cases, a small amount of the substance in question may be injected subcutaneously to test the patient's reaction to exposure.

Safe and effective use of local or regional anesthesia requires thorough knowledge of the pharmacology of local anesthetics (LAs). Local infiltration for neural blockade can be accomplished by using dilute concentrations of LAs, as they rapidly penetrate the various tissues around targeted nerve endings. When large-diameter nerves are targeted, the quantity of drug reaching the central axonal core is reduced because of incomplete penetration of surrounding epineurium, perineurium, endoneurium, fat, blood vessels, and lymphatics, which can constitute as much as 40% of the peripheral nerve diameter.

Some of the injected substance is absorbed by local blood during its diffusion, which acts as another important mechanism for reducing the amount of drug that actually reaches the nerve axon. Higher concentration of LAs may cause local vasomotor paralysis, which increases local blood flow and enhances systemic absorption. Blood flow through injected tissues can be reduced by using an LA solution mixed with epinephrine, which decreases systemic absorption and improves penetration of the anesthetic to its target. Therefore, the vascularity of various tissues should be considered when deciding on the LA concentration and amount of injectate. Absorption into the bloodstream not only reduces potency of the injected material at the target site, but also increases systemic side effects. Low concentrations of LAs typically are used to block smaller, lightly myelinated and unmyelinated nerve fibers, such as C, A-delta, and B-preganglionic sympathetic fibers.

Several clinical characteristics should be considered when choosing an LA. The latency of onset of anesthetic action is an important clinical property; however, concentration, total dose, distance between the injection site and target, and relative penetrance of the compound also should be considered. Penetrance depends on target-tissue characteristics, including the thickness of superimposed, fibrous, and other intervening tissues. Tissue penetrance of specific LAs determines latency of onset and intensity of induced anesthesia. Duration of LA action depends on pharmacodynamic properties of the anesthetic, concentration, total dose, and vascularity of the region under scrutiny. LA toxicity relates to all of these factors and also to biotransformation.

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