Which physicochemical property of a local anaesthetic agent most strongly determines its speed of onset after tissue infiltration?
- A Degree of protein binding
- B Lipid solubility
- C pKa relative to tissue pH ✓
- D Molecular weight
Correct answer: C. pKa relative to tissue pH
Explanation
Only the uncharged (base) form of a local anaesthetic diffuses across the nerve sheath and membrane. The fraction present as free base depends on the pKa: agents whose pKa lies closer to physiological pH 7.4 have a larger unionised fraction at tissue pH and therefore act faster. Lidocaine (pKa 7.9) has a faster onset than bupivacaine (pKa 8.1). Lipid solubility determines potency and protein binding determines duration, not onset.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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