Adding sodium bicarbonate to lidocaine immediately before infiltration speeds the onset of block. What is the mechanism?
- A It directly opens sodium channels and sensitises them to the blocker
- B It increases the fraction of drug in the unionised base form, which crosses the nerve membrane faster ✓
- C It increases plasma protein binding and traps the drug near the nerve
- D It vasodilates the injection site and improves delivery to the nerve
Correct answer: B. It increases the fraction of drug in the unionised base form, which crosses the nerve membrane faster
Explanation
Local anaesthetics cross the axonal membrane only in their unionised form, then re-ionise inside the axoplasm to block the channel from the cytoplasmic side. Commercial solutions are acidic to keep the drug soluble. Raising the pH with bicarbonate shifts the equilibrium towards the free base, enlarging the membrane-permeable fraction and shortening latency. The effect is modest and the solution must be prepared freshly because precipitation occurs on standing.
Reference: Miller's Anesthesia, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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