Chapter 105

Cocaine appeared in the first published reports of an effective local anesthetic agent in 1884 and 1885.1 Halstead’s experiments with cocaine demonstrated its effectiveness in more than 1000 minor surgical cases.1 Modern local anesthetic agents have supplanted cocaine as the agent of choice as they are more efficacious and safer to use.2 They are all synthetic derivatives of cocaine. The Emergency Physician will routinely be confronted with acute wounds, abscesses, and other injuries requiring local anesthesia. An expert knowledge of local anesthesia is a required piece of our armamentarium. One must know the agents available, the techniques for optimum pain relief, and the methods to avoid and treat adverse reactions.

The molecule that comprises the structure of any commercially available local anesthetic agent has three major components. They consist of a lipophilic aromatic chain joined by either an amide or an ester linkage to a hydrophilic tertiary amide.3 Local anesthetics are classified according to their intermediate chain as either esters or amides. Procaine was the first available injectable ester local anesthetic agent and was synthesized in 1905.4 Lidocaine was the first amide anesthetic agent and was first produced in 1945.4 Researchers have produced agents with varied anesthetic properties based upon potency, onset, and duration of action by making substitutions at different sites on the basic molecule.4 An appreciation of these different properties permits the Emergency Physician to make logical choices regarding the optimal use of these agents.

Neural depolarization results from a rapid influx of sodium ions through special sodium channels within the nerve cell membrane.3,5,6 Local anesthetic agents function by reversibly binding to specific protein receptors within these sodium channels.3,5,6 The local anesthetic agent impedes sodium influx and blocks depolarization.4,5,7,8 Local anesthetic agents with longer protein binding to the sodium channel results in a longer duration of blockade and anesthesia. Small diameter nerve fibers responsible for pain and temperature sensation are blocked preferentially before fibers that propagate touch, motor function, and proprioception.3,6,9

Local anesthetic potency is determined by the lipid solubility of the agent.8,10 Highly lipid-soluble agents more readily traverse the lipoprotein nerve cell membrane and are more effective at inducing blockade.

The primary determinant of a local anesthetic’s onset of action is its pKa.11 The pKa is the pH at which a given drug exists in equal proportions as ionized and unionized molecules. The unionized molecules more readily cross the nerve cell membrane. A portion of the molecule becomes charged once inside the neuron. It is this ionized portion that binds most completely to receptor proteins within the sodium channels.3,10 Commercially available local anesthetic agents are weak bases with pKa’s of 7.6 to 8.9.3 Agents with a lower pKa at physiologic pH (7.4) will ...

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