## Local Anesthetic Duration Classification **Key Point:** Duration of action of local anesthetics is determined by their lipophilicity, protein binding, and pKa. Bupivacaine has the longest duration among commonly used agents. ### Duration Ranking | Local Anesthetic | pKa | Protein Binding (%) | Lipophilicity | Duration | | --- | --- | --- | --- | --- | | Procaine | 8.9 | 6 | Low | Short (30–60 min) | | Lidocaine | 7.9 | 70 | Moderate | Intermediate (60–120 min) | | Mepivacaine | 7.6 | 75 | Moderate | Intermediate (90–180 min) | | Bupivacaine | 8.1 | 95 | High | Long (180–720 min) | **High-Yield:** Bupivacaine's high protein binding (95%) and lipophilicity allow it to remain bound to nerve tissue longer, providing duration of 4–12 hours in peripheral nerve blocks. This makes it the gold standard for peripheral nerve blocks requiring prolonged anesthesia. **Clinical Pearl:** Ropivacaine, a newer amide local anesthetic, has duration similar to bupivacaine but with a better safety profile (lower cardiotoxicity). However, bupivacaine remains the longest-acting among the options listed. **Mnemonic:** **LLMP** — Lipophilicity, Long protein binding, Metabolism (amide vs. ester), Potency → all correlate with duration. Bupivacaine scores highest on all four. ### Why Bupivacaine Dominates 1. Highest protein binding (95%) → prolonged tissue depot effect 2. Highest lipophilicity → penetrates nerve sheath readily and persists 3. Amide structure → slow hepatic metabolism 4. Provides 4–12 hours of anesthesia in peripheral blocks (vs. 1–2 hours for lidocaine) 
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