Specific Agents

Lidocaine is an amide anesthetic available in 0.5 percent, 1.0 percent, and 2 percent concentrations and is the most commonly used anesthetic in the ED because of its excellent efficacy and low toxicity profile. Decreased metabolism occurs with hepatic failure and decreased hepatic blood flow (e.g., congestive cardiac failure) and in these circumstances can lead to high plasma levels. Its onset of action is within 2 to 5 min and its duration of action is 1 to 2 h. Lidocaine can be combined with epinephrine and bicarbonate.

Bupivacaine (0.25 percent) is an amide anesthetic and is highly protein bound. It has a longer duration of action than lidocaine, 4 to 6 h, and is more cardiotoxic. Bupivacaine is preferred for prolonged procedures (such as ingrown toenail removal), when longer postprocedural analgesia is desired, and for non-IV regional blocks. Bupivacaine is contraindicated for IV regional blocks due to its cardiac toxicity, from which fatalities have been reported. Similar caution regarding conditions affecting hepatic metabolism should be followed as for lidocaine. The onset of action of bupivacaine is similar to that of lidocaine, but some studies suggest its injection is more painful.

Prilocaine, an amide LA with a lower cardiac toxicity profile than lidocaine or bupivacaine, has similar anesthetic potency, milligram for milligram. After intravenous injection, its CNS toxicity is less than lidocaine due to a lower blood level because of differences in its distribution and peripheral uptake. It is also broken down by amidases in the liver more rapidly than lidocaine, resulting in a shorter duration of toxic effects. Prilocaine and tetracaine are the active agents in EMLA cream (eutectic mixture of local anesthetics) for topical use on intact skin. Prilocaine may lead to methemoglobinemia after a large intravenous bolus (or total dose > 600 mg). Due to its lower toxicity, it is commonly used in Australia for intravenous regional arm blocks.

Procaine is an ester LA and can be used for patients who are allergic to the amide anesthetics (e.g., lidocaine). It has a rapid onset but short duration of action. Tetracaine is an ester LA and is more lipid soluble and longer acting than other LAs in its class. It is commonly used for topical LA preparations.

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