Local Anesthetics

Department of the Navy
Bureau of Medicine and Surgery

Local anesthetics are drugs that block the generation and propagation of impulses in excitable tissues, most notably the spinal cord, spinal nerve roots, and peripheral nerves, as well as skeletal muscle, cardiac muscle, and the brain. This section will briefly cover the pharmacology of commonly available local anesthetics, review the maximum recommended doses, and discuss the potentially catastrophic allergic and toxic responses.

** Dose for 70 kg male. Use only as a general guide. PNB = Peripheral Nerve Block, EPI = Epinephrine

Although there are many local anesthetics available, the clinician should remember that lidocaine is frequently the safest overall choice and the standard by which all other local anesthetics are compared. When in doubt, use lidocaine.

• Aminoamides are cleared from the plasma by hepatic metabolism. Active metabolites of lidocaine can contribute to toxicity even when plasma levels of lidocaine are in a therapeutic range.

• Aminoesters are rapidly cleared from the plasma by plasma and liver cholinesterases. Plasma levels of these local anesthetics may be elevated in patients with deficient or atypical cholinesterase enzyme.

Toxicity of Local Anesthetics

Prevention is the best solution for avoiding systemic toxicity with local anesthetics. Meticulous attention to technique and to recognition of intravascular injections with appropriate monitoring is indicated. Signs and symptoms of local anesthetic toxicity include tinnitus, perioral numbness, metallic taste in mouth, slurring of speech, and mental status changes. Oxygen should be administered at the first sign of toxicity. Should symptoms progress, maintain the patient's airway and follow advanced cardiac life support (ACLS) guidelines. Be prepared to treat for seizures. (Bretylium being used in preference to lidocaine.)

Manifestations of Systemic Epinephrine Absorption

• Increased heart rate.

• Increased cardiac output.

• Decreased systemic vascular resistance.

Contraindications to the Addition of Epinephrine to Local Anesthetics

• Unstable angina.

• Cardiac dysrhythmias.

• Uncontrolled hypertension.

• Treatment with monoamine oxidase (MAO) inhibitors or tricyclic antidepressants.

• Uteroplacental insufficiency.

• Peripheral nerve blocks in areas that may lack collateral blood flow (penis, digits).

• Intravenous (IV) regional anesthesia.

Solutions of epinephrine containing 5 mcg/ml (1:200,000) appear to be optimal for the reduction of surgical bleeding and systemic absorption of local anesthetics. Solutions of local anesthetics may be adjusted to contain epinephrine for vasoconstriction by manually adding 5 mcg of epinephrine to 1 ml of anesthetic solution to produce a concentration of 1:200,000.