How does an ACIDIC tissue environment, such as that produced by acute inflammation or infection, affect the action of a local anesthetic?

Correct answer: D. Onset is delayed and the anesthetic is less effective

• A. The reverse is true — acidity reduces the proportion of lipid-soluble base available to cross the membrane, so onset is slower, not faster.
• B. Duration is not prolonged. Inflammation also brings vasodilation and increased blood flow, which washes anesthetic away and SHORTENS duration.
• C. The anesthetic does not become neurotoxic in an acidic field; it simply fails to penetrate adequately.
• D. CORRECT. Local anesthetics are weak bases supplied as acidic hydrochloride salts. Only the UNCHARGED (base) form can cross the lipid nerve membrane; once inside, it is the CHARGED form that blocks the sodium channel. In an acidic inflamed field, the equilibrium shifts toward the charged form, so far less drug penetrates the nerve. The result is slow, incomplete or failed anesthesia — the everyday clinical frustration of trying to anesthetize a hot tooth.

Note: This is why you use a REGIONAL BLOCK proximal to the inflamed field rather than infiltrating directly into it. Buffering the cartridge with sodium bicarbonate raises the pH and speeds onset for the same reason.