Local anesthetics are weak bases and are usually made available clinically as salts to increase solubility and stability. In the body, they exist either as the uncharged base or as a cation (see Chapter 1, Ionization of Weak Acids and Weak Bases). The relative proportions of these two forms are governed by their pKa and the pH of the body fluids according to the Henderson-Hasselbalch equation, which can be expressed as:
If the concentration of base and conjugate acid are equal, the second portion of the right side of the equation drops out, as log 1 = 0, leaving:
TABLE 26–1 Structure and properties of some ester and amide local anesthetics.1
Structure Potency (Procaine = 1) Duration of Action
Mepivacaine (Carbocaine, Isocaine) CH3CH3ONHCNCH32 Medium
Bupivacaine (Marcaine), 1Other chemical types are available including ethers (pramoxine), ketones (dyclonine), and phenetidin derivatives (phenacaine). 2Data not found.