Injectable Local Anesthetics: Types, Mechanisms, and Clinical Selection

Classification & Common Agents

Amide Anesthetics (Most Used)

• Lidocaine (Xylocaine): Fast onset (2–5 min), intermediate duration (1–2 h); 1–2% with/without epi. Workhorse for infiltration, nerve blocks, IV regional.
• Bupivacaine (Marcaine): Slow onset (5–10 min), long duration (4–8 h); 0.25–0.5%. Preferred for postoperative analgesia, epidurals, spinals.
• Ropivacaine (Naropin): Similar to bupivacaine, less cardiotoxic; 0.2–0.75%.
• Mepivacaine (Carbocaine): Rapid onset, intermediate (2–3 h); 1–2%; less vasodilation—good without epi.
• Prilocaine (Citanest): Intermediate; metabolite can cause methemoglobinemia at high doses.
• Levobupivacaine (Chirocaine): S-enantiomer of bupivacaine; lower CNS/cardio toxicity.

Ester Anesthetics (Less Common)

• Procaine (Novocain): Very short (15–30 min); dental, infiltration.
• Tetracaine (Pontocaine): Long-acting; spinal, topical.
• Chloroprocaine (Nesacaine): Ultra-short; epidural, caudal.
• Cocaine: Only topical anesthetic with vasoconstriction; ENT use; high abuse potential.
• Benzocaine: Topical only (sprays, lozenges).

Mechanism of Action

Bind voltage-gated Na⁺ channels intracellularly → prevent depolarization → block action potential propagation. Use-dependent (bind better to active nerves).

Clinical Selection Factors

• Desired onset/duration.
• Vasoconstrictor need (epinephrine 1:100k–1:200k prolongs, reduces systemic absorption).
• Patient factors: allergy (true amide allergy rare; ester allergy more common—PABA metabolite), cardiac disease, liver dysfunction (amide metabolism), pregnancy (lidocaine preferred).
• Site: infiltration, peripheral block, neuraxial, IV regional (Bier block).

Toxicity Prevention

• Know max doses (mg/kg): Lidocaine 4.5 (w/ epi 7), Bupivacaine 2.5, Ropivacaine 3.
• Aspirate before injection; inject slowly; fractionate doses.
• Monitor for CNS (tinnitus, metallic taste, seizures) & CVS (arrhythmia, hypotension) signs.
• Lipid emulsion therapy (Intralipid 20%) for LAST (Local Anesthetic Systemic Toxicity).