What is local anesthesia?
Local anesthesia is the application of numbing agents to temporarily reduce or remove transmission of nerve sensations for short surgery or other localized procedures. A secondary use is continuous infusion administration for temporary relief of acute or chronic pain conditions. Local anesthesia in its truest form is limited to small body areas; conduction, or regional, anesthesia simply extends localized administration to a larger body area. The anesthetic agents can be applied to the skin topically or can be injected under the skin into tissue directly around a nerve ending. Both methods provide short-term blockage of sensations between peripheral nerve endings or bundles and the brain by interacting with sodium ion channels around the nerve-conduction pathways; the local anesthetics alter ion gradients across cell walls at the site to prevent nerves from conducting sensory information. Local anesthetics in any form do not provide sedation or whole-body effects, because they affect the peripheral nervous system, in contrast with the sedative effects of general anesthesia on the central nervous system. The duration of the nerve block may be proportional to the amount or rate of drug administered and the potency of the anesthetic selected; however, the intensity and duration of effect also may vary depending on type of drug, administration site and method (for example, topical administration is less intense than tissue injection), size of the nerve sheaths affected (for example, smaller sheaths or individual, rather than bundled, nerves may react more intensely to similar doses), and interactions with other drugs (for example, antihypertensive medications) or conditions.
The two main classes of local anesthetics are the esters and the amides, which have similar aromatic and amine groups in the chemical structures but differ in the intermediate group. Esters, which include procaine (Novocaine) and benzocaine, are hydrolyzed during breakdown, whereas amides, which include lidocaine and bupivicaine, are broken down by cytochrome enzymes in the liver. Both types come as sprays, patches, creams or lotions, and injections that generally have half-lives of less than two hours. Both can be given with vasoconstrictors, such as epinephrine, to slow blood-vessel distribution of the anesthetic away from local tissue and to improve the duration of the numbing effect at the application site.
In a surgical context, local anesthesia is most often used for dental, minor surgical, and emergency procedures. Emergency-department techniques such as sutures may require topical agents or an injection into the tissue for deeper or longer suturing. Many types of invasive dental procedures, such as cavity fillings and root canals, require anesthetic injection over a large area of nerve bundles in the oral cavity. Dermatologic procedures such as mole removal require topical or injected anesthetic nerve block at nerve endings. Topical skin numbing prior to drug injections is also common, such as topical lidocaine/prilocaine cream applied before vaccines given to children. Spinal anesthesia procedures block the peripheral nervous system conduction directly where the peripheral and central nervous systems meet to prevent sensation during cesarean section deliveries, cytoscopies, and other pelvic procedures for which general anesthesia is not required. After the anesthetic is administered by skin absorption or injection, nerve block typically occurs within approximately fifteen minutes and ranges from blockage of pain sensations only to full blockage of pain and temperature sensitivity. The extent of numbness is proportional to the potency and dose administered, with pain inhibition followed sequentially by touch, heat, and muscle-control inhibition.
Use of local anesthesia for pain is less common. Continuous catheter infusion with low doses of local anesthetics provides relief of acute pain, such as during treatment of a patient who has experienced trauma, and may have fewer side effects than analgesic treatments. Chronic pain may be successfully numbed by similar use of anesthesia, but as yet there is no evidence of long-term effects beyond the time of administration.
Although topical or local injected applications are safer than generalized anesthesia, or sedation, risks are still present. Allergy to para-aminobenzoic acid (PABA) can cause a cross-reaction to ester anesthetics, because hydrolysis of an ester anesthetic releases PABA as a breakdown product. Although rare, allergy to amide anesthetics is also possible; both allergies can manifest as a rash, wheezing, or even anaphylactic shock. Common side effects of both anesthetic drug classes are shallow breathing, altered heart rate, anxiety, tremors, dizziness, prolonged numbness, and tinnitus (ringing sensation in the ear). Although unlikely, central nervous system depression with associated bradycardia and cardiac depression are possible, especially with extremely high doses or rates of administration. Potentially irreversible nerve-conduction block can occur within five minutes of toxic doses of anesthesia, and methemoglobinemia—evidenced by shortness of breath, fatigue, dizziness, and weakness—has occurred with benzocaine in particular. Such extreme side effects are more likely to occur if the patient has preexisting renal or liver problems that prevent adequate drug clearance, is pregnant, or is very young or very old. Improper injection into the vascular system or directly into a nerve sheath can also lead to these toxicities. Typically, however, nerve block from the correct application of local anesthetics will reverse on its own within a few hours.
Since the isolation of cocaine from coca plants in the late 1800s, interest in using chemical agents to reduce sensory effects without sedation has grown substantially. Procaine was derived from cocaine in 1904 by Alfred Einhorn to reduce toxicity associated with cocaine use; the more concentrated amide drug lidocaine, still one of the most widely used local anesthetic agents, followed in 1943, and others in the amide class have improved upon the potency of lidocaine. Local anesthetics have since played an expanding role in medicine, from large-area nerve blocks for cesarean deliveries to short-term relief as a treatment for chronic pain via catheter infusion. In the twenty-first century, efforts to standardize office-based anesthesia are developing because of the prevalence of local anesthesia administration for routine outpatient skin, dental, and minor surgical procedures.
Long-term local anesthesia is being developed that could provide numbing effects for as long as two to three days, with research focused mainly on the natural agent saxitoxin. Such extended localized numbness would provide pain relief throughout a procedure and afterward during the period of most acute pain and recovery time. Saxitoxin is unrelated to cocaine and the other amide and ester agents; it is found in many varieties of fish and works by blocking transmission at extracellular, rather than intracellular, sodium channels.
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