If the tissue pH is low due to infection, what happens to LA effectiveness?

If the tissue pH is low due to infection, what happens to LA effectiveness?

• A. Less free base available, decreased diffusion
• B. Greater diffusion
• C. Increased pH in tissue
• D. LA becomes stronger

Answer: (A)

When a local anesthetic is a weak base, its ability to diffuse into nerves depends on its uncharged, free-base form. In infected tissue, the environment becomes acidic, pushing more of the drug into the ionized, charged form. The Henderson-Hasselbalch relationship explains that at a lower pH (below the drug’s pKa), the balance shifts toward the ionized form, so there is less free base available to cross the nerve membrane. With less uncharged drug able to diffuse into the nerve, fewer molecules reach the site to block sodium channels, leading to slower onset and reduced overall effectiveness. So, the best statement is that there is less free base available, resulting in decreased diffusion. The other options don’t fit because diffusion wouldn’t be greater in acidic tissue, pH would not increase, and the anesthetic would not become stronger.

When a local anesthetic is a weak base, its ability to diffuse into nerves depends on its uncharged, free-base form. In infected tissue, the environment becomes acidic, pushing more of the drug into the ionized, charged form. The Henderson-Hasselbalch relationship explains that at a lower pH (below the drug’s pKa), the balance shifts toward the ionized form, so there is less free base available to cross the nerve membrane. With less uncharged drug able to diffuse into the nerve, fewer molecules reach the site to block sodium channels, leading to slower onset and reduced overall effectiveness. So, the best statement is that there is less free base available, resulting in decreased diffusion. The other options don’t fit because diffusion wouldn’t be greater in acidic tissue, pH would not increase, and the anesthetic would not become stronger.