APPROACH TO THE PATIENT WITH SUSPECTED LOCAL ANESTHETIC SENSITIVITY

Abstract

Local anesthetics prevent the generation and conduction of nerve impulses. This result is accomplished by decreasing or blocking the increase in the permeability of excitable membranes to sodium that normally is produced by a slight depolarization of the membrane. 32 This action of local anesthetics is caused by their direct interaction with one or more sites in the voltage-gated sodium channels. The degree of block produced by a given concentration of local anesthetic depends on how frequently the nerve is stimulated and on its resting membrane potential. These frequency- and voltage-dependent effects occur because the local anesthetic molecule in its charged, active form gains access to its binding site(s) in the sodium channel only when it is in an open state. 12 The binding characteristics vary among local anesthetics depending on their dissociation constant (pKa), lipid solubility, pH at the injection site, protein binding, and molecular size. Local anesthetics that have a low pKa or higher lipid solubility have a faster onset of action. 47 Duration of anesthesia, on the other hand, is directly proportional to plasma protein binding. The addition of vasoconstrictors (epinephrine) to local anesthetics limits systemic absorption, promotes local hemostasis, and prolongs duration of action. Although variation exists, the differential rate and type of nerve block follows a predictable pattern. Generally, small unmyelinated C fibers and autonomic fibers (mediating pain sensations), and small myelinated Aδ fibers (mediating pain and temperature sensations) are blocked before larger myelinated Aγ, Aβ, and Aα fibers (carrying postural, touch, pressure, and motor information). 12,32,61 Systemic absorption of local anesthetics depresses the conduction and other electrical functions of the myocardium and cardiac contractility, and can induce peripheral vasodilation in a dose-dependent fashion, which can lead to decreased cardiac output, severe hypotension, or cardiac arrest. Drug effects on the central nervous system include stimulation, depression, or both. 38