Anesthesia · single question
A tissue at the injection site has a pH of 3.5 (acidic, as in an infected area). How does this affect local anesthetic onset?
Show answer and explanation
Correct answer: B. Lower (slower) onset — anesthesia is more difficult to achieve
B) Lower/slower onset — harder to achieve anesthesia — CORRECT.
Local anesthetics work in their unionized (free base) form to penetrate nerve membranes. At low pH (acidic environment, e.g., infection/abscess), the equilibrium shifts toward the ionized form (BH+) — fewer unionized molecules available to cross the nerve membrane. This is why anesthesia is difficult to achieve in infected tissues: reduced unionized drug = reduced nerve penetration = poor or slow anesthesia.
A) Faster onset: The opposite is true in acidic environments.
C) Same onset: pH directly affects the ionization equilibrium (Henderson-Hasselbalch) of local anesthetics.
D) Faster due to ionization: Increased ionization means LESS drug available for membrane penetration — slower, not faster.
Local anesthetics work in their unionized (free base) form to penetrate nerve membranes. At low pH (acidic environment, e.g., infection/abscess), the equilibrium shifts toward the ionized form (BH+) — fewer unionized molecules available to cross the nerve membrane. This is why anesthesia is difficult to achieve in infected tissues: reduced unionized drug = reduced nerve penetration = poor or slow anesthesia.
A) Faster onset: The opposite is true in acidic environments.
C) Same onset: pH directly affects the ionization equilibrium (Henderson-Hasselbalch) of local anesthetics.
D) Faster due to ionization: Increased ionization means LESS drug available for membrane penetration — slower, not faster.