Which factor causes a local anesthetic to have faster onset?

Which factor causes a local anesthetic to have faster onset?

• A. Lower pKa
• B. Higher pKa
• C. Higher molecular weight
• D. Higher protein binding

Answer: (A)

The speed at which a local anesthetic begins to work depends on how quickly the drug can cross the nerve membrane to reach its site of action inside the neuron. Local anesthetics are weak bases that exist in balance between a non-ionized (lipid-soluble) form and an ionized form. Only the non-ionized form readily diffuses through the lipid nerve membrane, so the faster the fraction in the non-ionized form at physiological pH, the quicker the onset. A lower pKa means more of the drug is non-ionized at body pH (~7.4), leading to faster membrane diffusion and a quicker onset. Conversely, a higher pKa would favor the ionized form at the same pH, slowing onset. Factors like higher molecular weight or greater protein binding hinder diffusion and prolong onset, so they don’t produce the faster onset seen with a lower pKa.

The speed at which a local anesthetic begins to work depends on how quickly the drug can cross the nerve membrane to reach its site of action inside the neuron. Local anesthetics are weak bases that exist in balance between a non-ionized (lipid-soluble) form and an ionized form. Only the non-ionized form readily diffuses through the lipid nerve membrane, so the faster the fraction in the non-ionized form at physiological pH, the quicker the onset. A lower pKa means more of the drug is non-ionized at body pH (~7.4), leading to faster membrane diffusion and a quicker onset. Conversely, a higher pKa would favor the ionized form at the same pH, slowing onset. Factors like higher molecular weight or greater protein binding hinder diffusion and prolong onset, so they don’t produce the faster onset seen with a lower pKa.