Why does infection decrease the effectiveness of local anesthetic?

• A. It accelerates the onset of action
• B. It increases tissue buffering capacity
• C. It reduces tissue vascularity
• D. It removes the buffering ability of the tissues which is needed for the anesthetic to be active

Answer: (D)

Infection lowers tissue pH, creating an acidic environment. Local anesthetics are weak bases and must exist in a non-ionized (un-charged) form to cross the nerve membrane and reach the sodium channels. In acidic tissue, more of the drug becomes ionized and cannot diffuse into the nerve easily, so its onset is delayed and its effectiveness reduced. Tissue buffering helps keep pH closer to normal; when infection overwhelms this buffering capacity, the pH drops further and the non-ionized fraction of the anesthetic decreases, undermining its activity.