Local anesthetics are tertiary amines and are divided into esters and amides. Esters are metabolized by plasma cholinesterases, and neonates and infants up to 6 months of age have one-half of the adult levels of this enzyme. Amides are metabolized by the liver and are bound by plasma proteins, and neonates and infants up to 3 months of age have a reduced hepatic blood flow and immature degradation pathways. Consequently, a larger amount of the drug remains unmetabolized and active in children than it does in adults. Neonates and infants also are at greater risk of toxic effects due to lower levels of albumin and α1-glycoprotein. In addition, because the pediatric nerve fibers are small and the degree of myelination is not complete, the minimum concentration necessary to obtain nerve block may be reduced, and lower concentrations of local anesthetic are required. The toxic effects of local anesthetics are dependent on the total dose of drug administered and on the rapidity of absorption into the bloodstream.

Although bupivacaine has been the local anesthetic of choice for continuous infusion techniques, its toxicity, especially in the case of continuous infusions, represents an increased concern that has led to the introduction of ropivacaine and levobupivacaine, which have less cardiotoxicity and neurotoxicity and produce a preferential sensory block (children may be emotionally affected by the inability to move their limbs). In children, most of the guidelines of continuous local anesthetic infusion techniques are still largely
based on experience developed with neuroaxial techniques. As in adult patients, continuous nerve blocks are mostly indicated for controlling pain of moderate to severe intensity that is expected to last at least 24 hours prior to or after upper or lower extremity surgery.

Lidocaine is an amide local anesthetic with a fast onset and an intermediate duration of action. It has a favorable toxicity profile.

Mepivacaine is an amide local anesthetic, rapidly metabolized into the liver and excreted through the kidneys. Mepivacaine has a short to intermediate action. It is mostly indicated for single blocks and is rarely used for continuous blocks.

Bupivacaine is a long-lasting local anesthetic whose efficacy is well documented. Its duration of action has made bupivacaine the drug of choice in postoperative pain control for years, but its cardiotoxicity (ventricular arrhythmias, myocardial depression) or neurotoxicity (convulsions) represents an important limitation, especially for continuous blocks. It is well established that prolonged infusions of bupivacaine in children represent a major risk for the development of toxic plasma levels (≥2 mg/mL). This is accentuated in newborns and infants less than 4 months old because of their low serum albumin and α1-glycoprotein levels. Pharmacokinetic studies as well as clinical use of bupivacaine in children have led to guidelines for central block loading doses of 0.25% bupivacaine 2.0 to 2.5 mg/kg, followed by a continuous infusion of 0.125% bupivacaine 0.4 to 0.5 mg/kg/hour in children and 0.2 mg/kg/hour in newborns and infants for 24 to 48 hours.

The plasma concentrations of bupivacaine and its main metabolite after continuous fascia iliaca compartment block in children are as follows: 0.25% bupivacaine 1.56 mg/kg with epinephrine followed by 0.1% bupivacaine 0.135 mg/kg/hour for 48 hours. This technique provides adequate analgesia in most cases and shows no severe adverse effects.

For continuous infusion in brachial plexus, the maximum recommended dose of bupivacaine is 0.3 to 0.4 mg/kg/hour in children and 0.20 to 0.25 mg/kg/hour in infants and neonates.

Continuous femoral blocks are indicated for postoperative pain management of femoral shaft fractures (0.2% bupivacaine 0.15 mL/kg/hour).

Ropivacaine is a pure S-enantiomer local anesthetic that has rapidly gained widespread acceptance not only for adults but also for regional anesthesia in children. The main reason for its increased use, even for continuous infusion techniques, is its better safety profile, with a wider therapeutic window and reduced risks for central nervous system toxicity and cardiotoxicity. Furthermore, ropivacaine has shown a preferential sensory/motor block discrimination compared with bupivacaine in adults as well as in children.