The differential sensory-to-motor block ratio that makes bupivacaine preferred for labour analgesia at low concentrations relates primarily to which property?
- A Lower pKa (7.9 vs 8.1 for lidocaine) producing more un-ionised drug at physiological pH
- B Shorter duration of action allowing dose titration without motor toxicity
- C Greater lipid solubility producing faster neuronal penetration in thin unmyelinated fibres
- D Higher protein binding favouring slow kinetics and differential Aδ/C fibre over Aα/Aβ blockade at low concentrations ✓
Explanation
Bupivacaine's very high protein binding (95–96%) and high lipid solubility create frequency-dependent and concentration-dependent differential fibre blockade. At low epidural concentrations (0.0625–0.125%), bupivacaine preferentially blocks small-diameter, slowly conducting nociceptive Aδ and C fibres (pain) while sparing larger Aα/Aβ fibres (motor and proprioception). This produces effective analgesia with minimal motor block — ideal for walking epidurals in labour. Lidocaine's lower protein binding and intermediate duration provide less differential blockade. Bupivacaine's pKa of 8.1 (not 7.9) actually means more ionised drug at pH 7.4, slightly slowing onset compared to agents with lower pKa.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.