Which factor primarily determines the duration of action of local anesthetics?

Which factor primarily determines the duration of action of local anesthetics?

• A. Lipid solubility
• B. pKa
• C. Protein binding
• D. Concentration

Answer: (C)

Duration of local anesthesia is determined mainly by how long the drug remains at the nerve site, anchored by binding to tissue proteins. When the anesthetic diffuses to a nerve, binding to tissue proteins creates a reservoir that releases drug slowly, so the sodium channels stay blocked longer even as plasma levels fall. Lipid solubility affects how well the drug penetrates membranes and its potency, influencing onset and depth, but it doesn’t set how long the block lasts. The pKa controls the proportion in the nonionized form and thus how quickly the drug begins acting, not the duration. Concentration can alter the amount available, but the persistence of effect is driven by tissue protein binding.

Duration of local anesthesia is determined mainly by how long the drug remains at the nerve site, anchored by binding to tissue proteins. When the anesthetic diffuses to a nerve, binding to tissue proteins creates a reservoir that releases drug slowly, so the sodium channels stay blocked longer even as plasma levels fall. Lipid solubility affects how well the drug penetrates membranes and its potency, influencing onset and depth, but it doesn’t set how long the block lasts. The pKa controls the proportion in the nonionized form and thus how quickly the drug begins acting, not the duration. Concentration can alter the amount available, but the persistence of effect is driven by tissue protein binding.