LA Failure in Infection — NDEB AFK MCQ
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Correct answer: A — Infected tissue has a lower pH (acidic), reducing the proportion of non-ionised (base) form available to penetrate the nerve membrane
Local anaesthetics are weak bases (pKa 7.6-8.1). In normal tissue (pH 7.4), a sufficient proportion exists in the non-ionised (lipophilic) base form to penetrate the lipid nerve membrane and reach the intracellular binding site on the sodium channel. In infected/inflamed tissue, the pH is lower (6.0-6.5), shifting the equilibrium toward the ionised (charged) form, which cannot cross the nerve membrane. This explains why local anaesthesia is often ineffective in areas of acute infection (acidosis trapping).
Reference: Malamed – Handbook of Local Anesthesia, 7th ed., 2020