Onset of Action

The structural domain of LAs responsible for their hydrophilicity is the amine group. In aqueous solution, the primary determinant of the ratio of ionized to unionized LA molecules is the LA’s pKa or the dissociation constant.2-10 A LA with a pKa closer to 7.4 (physiologic pH) will have a greater fraction of neutral molecules at the site of drug administration that can diffuse across neuronal membranes, and, therefore, a faster onset of action (Table 3).

Table 3. A LA’s pKa determines the fraction of free base at the site of drug administration, which is reflected in the drug’s onset of action.
  Lidocaine 2% Mepivacaine 2% Prilocaine 4% Articaine 4% Bupivacaine 0.5%
pKa (Dissociation constant) 7.9 7.7 7.9 7.8 8.1
Fraction of free base at pH 7.4 25% 33% 25% 29% 1.7%
Onset of action 2-4 min. 2.4 min. 2.4 min. 2-4 min. 4.8 min.
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