Why is sodium bicarbonate added to local anesthetic solutions?

Sodium bicarbonate is commonly added to local anesthetic solutions primarily to adjust pH. The pH of the local anesthetic solution affects its effectiveness, particularly the ability of the anesthetic to diffuse across nerve membranes. When the pH is increased (made more basic) by adding sodium bicarbonate, it helps to convert more of the local anesthetic into its free base form, which is the form that can more readily penetrate nerve membranes to exert its anesthetic effects.

Adding sodium bicarbonate can also enhance the onset of anesthesia, as a more neutral pH close to physiological levels reduces the discomfort associated with acidic local anesthetic solutions. This practice is essential, especially when a vasoconstrictor is present, as it balances the acidity that can occur from the vasoconstrictor and helps to increase the availability of the anesthetic to its target site.

While increasing viscosity and enhancing anesthetic potency are considerations in the formulation of local anesthetics, they are not primary purposes of adding sodium bicarbonate. Adding sodium bicarbonate is less about enhancing potency in a direct manner and more about ensuring the anesthetic can function effectively and quickly where it's needed.