## Opioid Pharmacokinetics: Duration of Action **Key Point:** Remifentanil is an ultra-short-acting opioid metabolized by plasma esterases, making it ideal for rapid offset and continuous infusion during anesthesia. ### Remifentanil: Unique Pharmacokinetics - **Metabolism:** Rapid hydrolysis by non-specific plasma and tissue esterases (NOT hepatic) - **Elimination half-life:** 10–20 minutes - **Context-sensitive half-time:** ~3–4 minutes (remains short even after prolonged infusion) - **Onset:** 1–3 minutes (IV) - **Offset:** 5–10 minutes after infusion cessation - **Advantage:** Predictable, rapid recovery independent of infusion duration ### Comparison with Other Opioids | Opioid | Metabolism | Half-life | Duration | Clinical Use | |--------|-----------|-----------|----------|---------------| | **Remifentanil** | Plasma esterase | 10–20 min | Ultra-short | Induction, infusion | | **Sufentanil** | Hepatic | 2–3 hrs | Intermediate | Induction, maintenance | | **Fentanyl** | Hepatic | 2–4 hrs | Intermediate | Induction, maintenance, patches | | **Morphine** | Hepatic (glucuronidation) | 2–4 hrs | Long | Postoperative analgesia | **High-Yield:** Remifentanil is the ONLY opioid with esterase metabolism — all others undergo hepatic metabolism. This makes remifentanil ideal when rapid offset is essential (e.g., airway assessment, short procedures, continuous infusion titration). **Clinical Pearl:** Remifentanil must be given as a continuous infusion or repeated boluses; single-dose duration is deceptively short, but context-sensitive half-time remains minimal even after hours of infusion — a major advantage over fentanyl. **Warning:** Remifentanil does NOT provide postoperative analgesia due to its rapid offset. Transition to longer-acting opioids or multimodal analgesia before infusion cessation. **Mnemonic:** **REMI** = **R**apid **E**sterase **M**etabolism = **I**deal for induction.
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