Distribution

The systemic distribution of local anesthetics can be described sufficiently by a two-compartment model.^[102] The rapid disappearance phase is believed to be related to uptake by rapidly equilibrating tissues (i.e., tissues that have high vascular perfusion). The slower phase of disappearance from blood is mainly a function of the particular compound ( Table 14-10 ).^[103] The half-lives of lidocaine and mepivacaine are similar, but a comparison of bupivacaine and etidocaine reveals that etidocaine has a more rapid rate of tissue redistribution and biotransformation than bupivacaine does.^[104]

Local anesthetics are distributed throughout all body tissues, but the relative concentration in different tissues varies. In general, more highly perfused organs show higher concentrations of local anesthetic than less well perfused organs do. Local anesthetics are rapidly extracted by lung tissue, so the whole-blood concentration of local anesthetics decreases markedly as they pass through the pulmonary vasculature.^{[105] [106]} The highest percentage of an injected dose of a local anesthetic is found in skeletal muscle. Although this tissue does not have any particular affinity for this class of drugs, the mass of skeletal muscle makes it the largest reservoir for local anesthetics.