Which local anesthetic has an onset around 5 minutes and a duration of about 6 hours?

Which local anesthetic has an onset around 5 minutes and a duration of about 6 hours?

• A. Lidocaine
• B. Prilocaine
• C. Bupivacaine
• D. Mepivacaine

Answer: (C)

Understanding onset and duration of local anesthetics hinges on two properties: how quickly the drug can cross nerve membranes to block sodium channels, and how long the drug remains active in the tissue. The speed of onset relates to the proportion of the molecule that is non-ionized at physiological pH—the more non-ionized, the faster it penetrates nerve membranes. The duration of action relates to how lipid-soluble the drug is and how tightly it binds to tissue proteins; greater lipid solubility and binding mean the drug stays around longer to keep the nerve blocked. Bupivacaine has a slower onset compared with some other agents because its pKa leads to a smaller fraction of non-ionized molecule at body pH, so it takes a bit longer to achieve effective nerve blockade—around five minutes in typical scenarios. But once it acts, its high lipid solubility and strong protein binding give it a long duration, commonly around six hours, especially when used with a vasoconstrictor to slow systemic absorption. In contrast, lidocaine tends to work quickly but wears off in roughly 1–2 hours, while prilocaine and mepivacaine have intermediate onset and shorter durations. Thus, the described profile most closely matches bupivacaine.

Understanding onset and duration of local anesthetics hinges on two properties: how quickly the drug can cross nerve membranes to block sodium channels, and how long the drug remains active in the tissue. The speed of onset relates to the proportion of the molecule that is non-ionized at physiological pH—the more non-ionized, the faster it penetrates nerve membranes. The duration of action relates to how lipid-soluble the drug is and how tightly it binds to tissue proteins; greater lipid solubility and binding mean the drug stays around longer to keep the nerve blocked.

Bupivacaine has a slower onset compared with some other agents because its pKa leads to a smaller fraction of non-ionized molecule at body pH, so it takes a bit longer to achieve effective nerve blockade—around five minutes in typical scenarios. But once it acts, its high lipid solubility and strong protein binding give it a long duration, commonly around six hours, especially when used with a vasoconstrictor to slow systemic absorption. In contrast, lidocaine tends to work quickly but wears off in roughly 1–2 hours, while prilocaine and mepivacaine have intermediate onset and shorter durations. Thus, the described profile most closely matches bupivacaine.