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Mechanism

Understanding the mechanism, dosage, and toxicity of local anesthetics is important for safe and effective anesthesia. Local anesthetics are synthetic cocaine derivatives. These agents work by binding to the receptor sites of sodium channels. By blocking ion movement across nerve cell membranes, the drug prevents build up of action potential and subsequent sensory nerve transmission.

Recall that lidocaine is also classified as a Class Ib antidysrhythmic medication. Class Ib agents are defined by their inhibition of fast sodium channels, which shortens cell repolarization as well as action potential duration and refractoriness.

 

Anesthetic Groups

Local anesthetics are of two groups: aminoamides and aminoesters. A way to distinguish the two groups: amides contain two or more letter “I’s”, while esters contain only one.

 

Aminoramides

Aminoramides include lidocaine, bupivicaine, mepivicaine, and prilocaine. Lidocaine (Xylocaine) and bupivicaine (Marcaine) are the most commonly used agents for local infiltration and field and nerve blocks. Amides are metabolized in the liver.

 

Aminoesters

Aminoesters include tetracaine, procaine, and chlorprocaine. The most commonly used ester, tetracaine, is used as a topical anesthetic. Esters are metabolized by cholinesterase inhibitors in the plasma.

Lidocaine

Lidocaine is the most commonly used local anesthetic for a wide range of procedures. It is typically used in 1% solution; a 0.5% solution can be used when larger volumes are desired, and 2% solution, common in dentistry, is also often available in emergency departments. The maximum dose of lidocaine 1% is 4.5 mg/kg. There are 10 mg of lidocaine in 1 ml of 1% solution, so the maximum volume of 1% solution is 0.45 ml/kg, with a maximum dose 30 ml in a 70-kg adult. Lidocaine's duration of action is typically up to 1 hour.

 

Lidocaine is available with and without epinephrine (1:100,000). Epinephrine causes local vasoconstriction which limit local spread of the drug and therefore allowing a larger dose of drug to be infiltrated without causing systemic toxicity. The maximum dose of lidocaine 1% with epinephrine is 7 mg/kg, (0.7 ml/kg, maximum dose of 50 ml in a 70-lb adult).

Bupivicaine

Bupivicaine is a longer acting amide anesthetic and is useful for longer procedures. Bupivicaine is typically found in 0.25% and 0.5% solutions with and without epinephrine. The maximum dose of 0.25% solution is 2 mg/kg (0.8 ml/kg, maximum dose of 40 ml in a 70.kg adult) Duration of action for bupivicaine is 2-6 hours.

 

Bupivicaine is not approved for children <12 years old. Toxic effects of bupivicaine are similar to those lidocaine but bupivicaine can be more cardiotoxic. There is higher risk of toxicity from bupivicaine in infants because the primary serum protein it is bound by, alpha1-acid-glycoprotein, is found at lower levels in infants (Hsu).

Additives and adjuncts

Epinephrine

Most local anesthetics have a vasodilatory effect. Addition of a vasoconstrictor such as epinephrine (usually 1/200,000) limits local spread of the agent, achieving a higher local concentration of anesthetic, longer duration of effect, reduced systemic absorption, and hemostasis for improved visualization. Because vasoconstriction can cause tissue ischemia and necrosis in end organ, epinephrine is contraindicated in the nose, pina, fingers, toes, or penis, and should be also avoided in avulsed flaps with tenuous blood supply. If epinephrine is inadvertently injected in these areas, it can be reversed by local infusion or intravascular injection of 1.5-5 mg of phentolamine. Note that anesthetics containing epinephrine are usually packaged with a red top and red lettering on the label.

 

Sodium bicarbonate (NaHCO3)

Local anesthetics are weak bases, but are marketed as hydrochloride salts with pHs of 4 to 6 to increase shelf life and solubility. Local anesthetics cross the cell membrane in an unionized form, but must return to the cationic state to bind to receptor sites. Buffering with NaHCO3 increases the amount of unionized drug, speeds its penetration to the nerve tissue, and in clinical setting has been shown to decrease the initial sting of infiltration compared to unbuffered lidocaine (3). A 1:10 ratio of 8.4% sodium bicarbonate to 1% lidocaine is recommended. Bupivicaine can be buffered with a 1:100 ratio (0.1 ml of sodium 8.4% sodium bicarbonate mixed with 10 ml of 0.5% bupivicaine), but caution must be taken because with this less soluble agent precipitation can occur (5).

 

Diphenhydramine (Benadryl)

Patients requiring local anesthetics for wound management may state they are allergic to lidocaine or other “caines”. Though allergy to amide or ester-based local anesthetics is rare, in a busy ED it may be difficult to determine whether the patient has a true allergy to these agents. Substitute agents can be useful in these situations. A 1% solution of diphenhydramine has been shown to be equally effective as lidocaine 1%, but can be more painful to infiltrate (4). A 1% solution (1 gram/100 mL) can be made by mixing 50 mg (1 cc) of the drug with 4 cc sterile saline. In addition to being more painful when injected, diphenhydramine can cause local skin reaction, and can be sedating (2,4).

 

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