8.5:
Local Anesthetics: Common Agents and Their Applications
Local anesthetics or LAs are commonly used for various applications ranging from topical pain relievers such as cough lozenges to intravenous injections.
Topical agents like dyclonine and pramocaine are rapidly absorbed through the mucous membranes and the skin. Although the anesthesia is short-lived, these drugs have popular over-the-counter applications as lozenges, sprays, patches, and gels.
Topical LAs with low aqueous solubility, such as benzocaine, can be applied directly to wounds, where they remain localized and produce a sustained effect.
Most topical anesthetics are irritants and cannot be directly applied to sensitive areas like the eyes. LAs for ophthalmic use, such as proparacaine and tetracaine, are applied as a single drop at a time.
The duration of anesthesia depends on tissue vascularity. It is longest for normal cornea and shortest for inflamed conjunctiva, which requires repeated applications.
Nerve-blocking LAs such as lidocaine and bupivacaine are used for prolonged anesthesia and pain control. Because they cause severe transient neurologic pain, these are only used at low concentrations.
8.5:
Local Anesthetics: Common Agents and Their Applications
Local anesthetics (LAs) are commonly used for various applications in medical and dental procedures. Some of the common agents used are cocaine, lidocaine, and bupivacaine.
Cocaine is an ester of benzoic acid and methylecgogine. It is used to anesthetize and vasoconstrict locally. Currently, it is used primarily for topical applications. It is beneficial for surgeries on the upper respiratory tract, providing anesthesia and shrinking the mucosa. Cocaine in the form of cocaine hydrochloride is available in the market as 1%, 4% and 10% solutions. The 1% and 4% solutions are most commonly used as it reduces toxicity. High toxicity and abuse of cocaine have led to its reduced use in the clinic.
Lidocaine is an amide-linked LA with a wide range of applications. It can be formulated for injections as well as topical, ophthalmic, mucosal, and transdermal uses. Lidocaine can be used as a single agent or in combination with other agents such as prilocaine and tetracaine. It produces faster and long-lasting anesthesia. Although lidocaine is effective and can be administered without adding vasoconstrictors, it readily escapes into the systemic circulation from the muscles, GI and respiratory tracts. Adding vasoconstrictors like epinephrine can constrict the blood vessels, reducing absorption and systemic toxicity.
Bupivacaine is also an amide-linked LA with a wide range of applications. It is used in applications where a prolonged anesthesia duration is required. Bupivacaine tends to block the sensory fibers more than the motor fibers. It can be infused continuously through catheters to provide adequate anesthesia for several days.