Local Anesthesia

Local anesthetics are divided into ester- and amide-type agents. Ester agents include procaine, chloroprocaine, cocaine, and tetracaine. Amide-type agents include lidocaine, bupivacaine, mepivacaine, and etidocaine. Ester agents are hydrolyzed by plasma pseudocholinesterase. Amides undergo transformation through aromatic hydroxylation in the liver (3).

The mechanism of action of local anesthetics is by diffusion of the base form of the agent across the nerve sheath and membrane. Binding of the agent to the nerve receptor site results in inhibition of Na2+ transfer. The fastest-acting local anesthetics include chloroprocaine, mepivacaine, lidocaine, and etidocaine. Etidocaine is the strongest, mepivacaine is the weakest, and lidocaine is intermediate in potency. Bupivacaine is an amide agent similar to lidocaine, but lasts up to three times longer and is up to four times more potent (4).

Allergies to the local anesthetics are rare and are usually specific to ester-type agents. These allergic patients can usually be given amide-type agents because cross-reactivity between local anesthetics is rare. A true history of allergy will include the development of hives, wheezing, cardiac arrest, or shock. Most patients who claim a local allergy often have really had an adverse reaction. Symptoms of palpitations and nervousness may be a response to an additive such as Paraben or epinephrine. Patients may be describing the sequelae of inadvertent intervascular injections (4).

Lidocaine is the most commonly used amide-type agent. The maximum subcutaneous dosage of lidocaine is 7 mg/kg or less than 500 mg in a healthy adult. Two percent lidocaine contains 20 mg/mL; therefore, the maximum dose in the 70-kg adult is 25 mL of 2%, or 50 mL of 1% lidocaine. As a general rule, 2% lidocaine is preferred except when the clinical question is infection, where 1% is utilized. The lidocaine effects usually last for approximately 0.5 to 1 hour, but may be prolonged in elderly patients or those with liver failure (5).

Local anesthetics work best for soft tissue analgesia, but do not work well for intra-medullary processes. The pain in this latter situation is caused by focal, local increase in intramedullary pressure. This occurs because when a needle enters the medullary cavity, it displaces local tissues. To ease this situation, a large amount of local anesthetic is given to the periosteum by scraping the local needle against the surface of the bone, as well as intravenous (IV) analgesia and patient reassurance.

Local anesthesia should be given prior to making the dermatomy. Before creating a skin wheal, anesthesia should be given a little deeper because the creation of the wheal is in itself somewhat painful. To test the adequacy of the skin anesthesia, we probe with the tip of the scalpel, touching the skin's surface without causing bleeding.

When using local anesthetics in areas of wounds, particularly devitalized areas, the lower local tissue pH decreases the lidocaine efficacy. This should be taken into consideration when using local anesthesia for percutaneous biopsies of suspected osteomyelitis in devitalized patients (6).

One should also pay careful attention to the indication for biopsy and the potential bacteriostatic or bactericidal effect of local anesthetics. This is particularly true when the indication for a biopsy is potential infection. Although the dose causing a bactericidal effect is quite large, and even the doses required for a bacteriostatic effect are fairly large, when doing a procedure for possible infection, IV sedation should be maximized and the use of local anesthesia should be minimized. That is also why 1% lidocaine is preferred when biopsying potential infections (7).

Epinephrine may be added to local anesthetics to cause local tissue vasoconstriction and consequently limit the uptake of the local anesthetic. Epinephrine should not be used in a patient who has a history of unstable angina pectoris and cardiac arrhythmia, or when performing a peripheral procedure on the fingers or toes.

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