Anesthesia · single question
What happens when a local anesthetic solution is buffered?
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Correct answer: C. Increase pH of local anesthetic
Buffering local anesthetic (typically adding sodium bicarbonate) INCREASES the pH of the acidic commercial LA solution (typically pH 3.5-4.5 for LA with vasoconstrictor) toward physiologic pH (~7.4). This increases the proportion of the unionized (free base) form of the LA, which crosses nerve membranes more readily. Result: FASTER onset of action, less pain on injection, and potentially longer duration. The buffering raises the pH of the solution.
- A. INCORRECT. Buffering does not decrease the duration of action. The duration depends on tissue binding, vasoconstrictor, and blood flow — not initial pH.
- B. INCORRECT. Buffering DECREASES (speeds up) the onset of pulpal anesthesia, not increases it. More unionized free base = faster membrane penetration = faster onset.
- C. CORRECT. Buffering with sodium bicarbonate INCREASES the pH of the LA solution from its acidic commercial form (~3.5-4.5) toward physiologic pH. This is the primary chemical change: higher pH = more free base form = faster onset and less injection pain.