Local Anesthetics: Common Agents and Their Applications

Topical local anesthetics like dyclonine and benzocaine are rapidly absorbed through mucous membranes and skin, producing short-lived anesthesia suitable for over-the-counter applications. Nerve-blocking agents such as lidocaine and bupivacaine provide prolonged anesthesia and pain control for medical procedures but are used at low concentrations due to transient neurologic pain.

Tissue vascularity directly determines anesthesia duration in ophthalmic applications. Normal cornea has the longest duration, while inflamed conjunctiva has the shortest, requiring repeated applications of agents like proparacaine and tetracaine. Higher blood flow in inflamed tissue increases drug absorption and reduces anesthetic effect.

Lidocaine readily escapes into systemic circulation from muscles, gastrointestinal, and respiratory tracts. Adding vasoconstrictors like epinephrine constricts blood vessels, reducing absorption and systemic toxicity. This combination allows lidocaine to maintain localized anesthetic effects while minimizing adverse systemic exposure and improving clinical efficacy.

Bupivacaine is an amide-linked local anesthetic that preferentially blocks sensory fibers over motor fibers, making it ideal for extended pain control. It can be continuously infused through catheters to provide adequate anesthesia for several days, making it valuable for procedures requiring sustained anesthetic coverage.

Most topical anesthetics are irritants and cannot be directly applied to sensitive areas like the eyes. Specialized ophthalmic agents such as proparacaine and tetracaine are formulated specifically for ocular use and applied as single drops to avoid tissue damage and irritation.

Topical local anesthetics with low aqueous solubility, such as benzocaine, can be applied directly to wounds where they remain localized and produce sustained effects. Their poor water solubility prevents rapid absorption through tissue, allowing prolonged local anesthetic action at the application site.

Cocaine is an ester of benzoic acid and methylecgogine that provides both anesthesia and vasoconstriction locally. It is particularly beneficial for upper respiratory tract surgeries, where it anesthetizes tissue while shrinking the mucosa. Available as 1%, 4%, and 10% solutions, the lower concentrations are preferred to reduce toxicity.