Which factor primarily influences the onset time of a local anesthetic?

Which factor primarily influences the onset time of a local anesthetic?

• A. PKa of the anesthetic
• B. Color of the syringe
• C. Patient's favorite food
• D. Time since last dental cleaning

Answer: (A)

The onset time hinges on how much of the local anesthetic is in the uncharged, lipid-soluble form that can cross the nerve membrane to reach the sodium channels. This depends on the drug’s pKa: the pH at which half the molecules are ionized. When the tissue pH is close to the drug’s pKa, a larger fraction is uncharged at the injection site, so diffusion into the nerve is faster and the block occurs more quickly. In inflamed tissue, where the pH is lower, more of the drug is ionized, slowing onset. The other factors listed don’t affect this diffusion and ion-channel blockade.

The onset time hinges on how much of the local anesthetic is in the uncharged, lipid-soluble form that can cross the nerve membrane to reach the sodium channels. This depends on the drug’s pKa: the pH at which half the molecules are ionized. When the tissue pH is close to the drug’s pKa, a larger fraction is uncharged at the injection site, so diffusion into the nerve is faster and the block occurs more quickly. In inflamed tissue, where the pH is lower, more of the drug is ionized, slowing onset. The other factors listed don’t affect this diffusion and ion-channel blockade.