In infants, amide local anesthetics are not metabolized at a regular rate. This is especially important to remember when running infusions either intravenously or via regional or neuraxial anesthesia.
Ester local anesthetics are metabolized by pseudocholinesterases in the plasma. Amide local anesthetics are metabolized in P450 dependent pathways.
Maximum dose of local anesthetic varies based on the local anesthetics and sometimes the addition of epinephrine.
Treatment of local anesthetic systemic toxicity has two arms—one is supportive care and another is treatment with intralipid 20% at 1.5 mL/Kg.
Systemic absorption depends on location of injection, with intercostal injection leading to the highest blood levels.
HISTORY OF LOCAL ANESTHETICS
Cocaine is the first local anesthetic to be discovered. It remains the only naturally occurring local anesthetic. In regions where the coca leaf grows, such as Peru, the leaf has a long history of being dried and chewed. Reports of cocaine “making the tongue numb” existed. Carl Koller, an ophthalmologist, had sampled this and believed that cocaine could potentially be applied to the eye for surgery. He studied this on animals in the laboratory, then upon his own eye and eventually on patients. In 1884, he used cocaine to provide local anesthesia for glaucoma surgery, the first report of use of a local anesthetic for surgical anesthesia.1 Until 1905, cocaine was the only available local anesthetic. In 1905, Alfred Einhorn synthesized procaine, another ester local anesthetic.
William Halstead is another pioneer in the area of local anesthetics. He is credited with performing the first regional block, a dental nerve block. He performed many blocks and held weekly teaching sessions of regional anesthesia with the use of cocaine. This led to him becoming a habitual user of cocaine and much of his work was not reported.
Following this, the use of local anesthetic developed in the hands of pioneers such as Leonard Corning, Heinrich Quincke, and Karl Bier.
to understand how local anesthetics exert their action it is important to understand nerve cell conduction and to have a basic understanding of the nerve itself. Nerve fibers consist of axons that are encased in endoneurium. Nerve fibers are then collected into fascicles, which are surrounded by specialized connective tissues known as perineurium. Finally, the fascicles are grouped and bound by another layer of connective tissue known as the epineurium (Figure 8-1).2 An important difference between the epineurium and the perineurium is that the perineurium is capable of protecting neurons from chemical injury.3