## Local Anesthetic Pharmacokinetics **Key Point:** Bupivacaine is an amide local anesthetic with the longest duration of action (4–8 hours) among commonly used agents, owing to its high lipophilicity and extensive protein binding (95%). ### Duration Determinants Duration of local anesthetics depends on: 1. **Lipophilicity** — higher lipophilicity → longer tissue penetration and retention 2. **Protein binding** — higher binding → slower systemic absorption and prolonged action 3. **pKa** — lower pKa → faster onset but does not directly determine duration 4. **Vascularity of injection site** — increased blood flow shortens duration ### Comparison Table | Local Anesthetic | Type | Lipophilicity | Protein Binding | Duration | pKa | | --- | --- | --- | --- | --- | --- | | Procaine | Ester | Low | 6% | 30–60 min | 8.9 | | Lidocaine | Amide | Moderate | 70% | 1–2 hrs | 7.9 | | Mepivacaine | Amide | Moderate | 75% | 2–3 hrs | 7.6 | | Bupivacaine | Amide | High | 95% | 4–8 hrs | 8.1 | **High-Yield:** Bupivacaine's high protein binding (95%) and lipophilicity make it ideal for peripheral nerve blocks and epidural anesthesia, but this same property increases CNS and cardiac toxicity risk if overdosed. **Clinical Pearl:** Ropivacaine is a newer amide with similar duration to bupivacaine but slightly lower cardiac toxicity; however, bupivacaine remains the gold standard for long-acting regional blocks.
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