Figure 1 shows the chemical structure of frequently used local anesthetics for selective injections. All these drugs consist of a lipophilic group (aromatic ring) connected by an amide intermediate chain to a ionizable group (usually a tertiary amine) (8,9). The lipophilic domain determines the anesthetic potential and toxicity of the drug. Because local anesthetics can penetrate only as an uncharged (non-ionized) fraction into cells, the ionization potential of the amine group correlates
FIGURE 1 Chemical structure of selected local anesthetics used for selective injections.
with the speed of onset of the drug. The amine link determines another pharmacological property of these agents. The amine links are less susceptible to hydrolysis by plasma esterase than ester links (as in procaine), which result in a longer duration of action (8,9).
The clinical characteristics of the most commonly used local anesthetics for selective injections are displayed in Table 1. For selective injections, these local anesthetics are usually injected without epinephrine. The drugs vary with respect to potency, speed of onset, diffus-ibility, duration of action, and relative toxicity. Basic requirements for selective blocks for diagnostic purposes include a rapid speed of onset and short duration of action. For therapeutic use, long duration is more desirable. Robivacaine requires special attention for selective injections in musculoskeletal radiology. Robivacaine is a potent long-duration local anesthetic similar to bupivacaine. However, robivacaine is characterized by a less pronounced effect on motor nerve fibers.
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