Why does infection decrease the effectiveness of local anesthetic?

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)

Local anesthetics must cross the nerve membrane in their non-ionized form to block conduction. The amount of non-ionized drug depends on the tissue pH: higher pH favors the non-ionized form, while acidic conditions push the drug toward the ionized form, which cannot easily penetrate the nerve.

Infection creates an acidic environment in the tissues and can overwhelm local buffering. This keeps the tissue pH low, shifting the anesthetic toward its charged form and reducing its ability to diffuse into nerves. As a result, onset is slower and the block is less effective. If the tissue pH could be maintained or raised (buffered), more of the drug would remain non-ionized and active. That’s why infection decreases anesthetic effectiveness by diminishing the buffering that helps maintain a pH favorable to the active form.