What is the effect on local anesthetic (LA) solution when it is buffered by adding sodium bicarbonate?

Commercial local anesthetic solutions are acidic (pH approximately 3.5 to 6.5) to maintain stability and solubility of the vasoconstrictor. When buffered with sodium bicarbonate (typically 1 mEq/10 mL of LA), the pH is raised to near physiologic (pH 7.0 to 7.4). A higher pH shifts the equilibrium toward the free base (uncharged) form of the LA molecule, which is the lipid-soluble form that penetrates nerve membranes. The benefits of buffering include: faster onset of anesthesia (reduced latency), reduced injection pain (acidic solutions are more painful), and improved efficacy in inflamed tissue (which is acidic and normally reduces LA effectiveness). Buffered LA does NOT reduce anesthesia duration.