Amides

Lidocaine hydrochloride (Xylocaine) is the most commonly used local anesthetic. It is well tolerated, and in addition to its use in infiltration and regional nerve blocks, it is commonly used for spinal and topical anesthesia and as an antiarrhythmic agent (see Chapter 16). Lidocaine has a more rapidly occurring, more intense, and more prolonged duration of action than does pro-caine.

Bupivacaine hydrochloride (Marcaine, Sensorcaine) has particularly long action, and some nerve blocks last more than 24 hours; this is often an advantage for postoperative analgesia. Its use for epidural anesthesia in obstetrics has attracted interest because it can relieve the pain of labor at concentrations as low as 0.125% while permitting some motor activity of abdominal muscles to aid in expelling the fetus. The lower concentration minimizes the possibility of cardiac toxicity. Fetal drug concentrations remain low, and drug-induced neurobehavioral changes are not observed in the newborn. Bupivacaine also is approved for spinal anesthesia and is approximately four times more potent and more toxic than mepivacaine and lidocaine. It can be used with or without epinephrine.

Levobupivacaine hydrochloride (Chirocaine) is the S-enantiomer of bupivacaine. It too has long action. Animal studies show that it has less CNS and cardiac toxicity than does bupivacaine. It also is slightly more motor sparing than is bupivacaine.

Ropivacaine (Naropin) is a recently developed long-acting amide-linked local anesthetic. Its duration of action is similar to that of bupivacaine, but it is slightly less potent and requires higher concentrations to achieve the same degree of block. Its primary advantage over bupi-vacaine is its lesser degree of cardiotoxicity.

Etidocaine hydrochloride (Duranest), although chemically similar to lidocaine, has a more prolonged action. It is used for regional blocks, including epidural anesthesia. It exhibits a preference for motor rather than sensory block; therefore, its use in obstetrics is limited, although fetal drug concentrations remain low. It can be used with or without epinephrine.

Mepivacaine hydrochloride (Carbocaine) is longer acting than lidocaine and has a more rapid onset of action (3-5 minutes). Topical application is not effective. It has been widely used in obstetrics, but its use has declined recently because of the early transient neurobe-havioral effects it produces. Adverse reactions associated with mepivacaine are generally similar to those produced by other local anesthetics. It can be used with epinephrine or levonordefrin (dental use only).

Prilocaine hydrochloride (Citanest) is an amide anesthetic whose onset of action is slightly longer than that of lidocaine; its duration of action is comparable. Prilocaine is 40% less toxic acutely than lidocaine, making it especially suitable for regional anesthetic techniques. It is metabolized by the liver to orthotoluidine, which when it accumulates, can cause conversion of hemoglobin (HB++) to methemoglobin (HB+++). Oxygen transport is impaired in the presence of methemoglo-binemia. Treatment involves the use of reducing agents, such as methylene blue, given intravenously, to reconvert methemoglobin to hemoglobin.

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