Adding sodium bicarbonate to a lidocaine solution immediately before infiltration shortens the time to surgical anaesthesia. What is the mechanism?

• A It increases the ionised cation fraction available to bind sodium channels
• B It enhances vasoconstriction, raising tissue drug concentrations
• C It directly blocks potassium channels, accelerating membrane stabilisation
• D It raises the pH, increasing the unionised base fraction that crosses the nerve membrane ✓

Explanation

Commercial local anaesthetic solutions are acidified to improve shelf stability, which leaves most drug in the charged cation form. Alkalinisation with sodium bicarbonate shifts the Henderson-Hasselbalch equilibrium toward the uncharged free base, increasing the fraction able to diffuse across lipid membranes to reach the intracellular site of action on the sodium channel. Onset is faster and injection less painful. Vasoconstrictor additives slow onset by reducing uptake, and channel blocking requires the charged form acting from inside the cell after diffusion.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.