A 38-year-old woman with post-operative pain is given intravenous fentanyl 100 mcg. Thirty minutes later, she develops severe muscle rigidity affecting the chest wall and abdomen, with difficulty breathing (RR 6/min, SpO₂ 88%). She is conscious and alert. What is the most appropriate immediate management?

Explanation

## Opioid-Induced Muscle Rigidity (Opioid-Induced Truncal Rigidity) ### Clinical Recognition **Key Point:** Opioid-induced muscle rigidity is a dose-dependent phenomenon that occurs with **potent synthetic opioids** (especially fentanyl, sufentanil, remifentanil) and is **NOT a sign of malignant hyperthermia**. **Mnemonic: FENTANYL RIGIDITY — F**requency increases with **dose**, **E**specially chest wall, **N**ot malignant hyperthermia, **T**reatment is naloxone + neuromuscular blockade, **A**cute onset (minutes), **N**eed airway management, **Y**ield to reversal, **L**arge doses cause severe rigidity. ### Pathophysiology - Fentanyl binds to opioid receptors in the **locus coeruleus, nucleus raphe pontis, and striatum** - Causes **increased muscle tone** via central mechanisms (NOT peripheral muscle contraction) - Rigidity is **dose-dependent** and **time-dependent** (onset 5–30 minutes) - Affects **chest wall, abdomen, and limbs** — can be severe enough to prevent ventilation ### Why Naloxone + Neuromuscular Blockade is Correct **High-Yield:** Management algorithm for opioid-induced rigidity: ```mermaid flowchart TD A[Opioid-induced rigidity + respiratory compromise]:::outcome --> B{Can ventilate with bag-mask?}:::decision B -->|Yes| C[Administer naloxone 0.4-0.8 mg IV]:::action B -->|No| D[Prepare for intubation]:::action C --> E[Reassess rigidity and RR]:::action D --> F[Administer neuromuscular blocker<br/>succinylcholine 1-1.5 mg/kg IV<br/>OR rocuronium 1.2 mg/kg IV]:::action F --> G[Intubate and mechanically ventilate]:::action G --> H[Administer naloxone 0.4-0.8 mg IV]:::action H --> I[Maintain sedation + analgesia<br/>Avoid repeated opioid dosing]:::action ``` 1. **Naloxone administration:** Competitively antagonizes fentanyl at opioid receptors, reversing rigidity and respiratory depression within 1–2 minutes 2. **Neuromuscular blockade:** If naloxone alone insufficient or patient cannot be ventilated, succinylcholine (depolarizing) or rocuronium (non-depolarizing) allows mechanical ventilation 3. **Mechanical ventilation:** Supports oxygenation and CO₂ elimination until naloxone effect wears off (then monitor for re-rigidity) **Clinical Pearl:** Opioid-induced rigidity is **NOT malignant hyperthermia**: - No fever (initially) - No rhabdomyolysis or myoglobinuria - No hyperkalemia or cardiac dysrhythmias (unless severe hypoxia) - **Dantrolene is NOT indicated** — it does not reverse opioid effects ### Monitoring After Naloxone - Continuous pulse oximetry, capnography, and cardiac monitoring - Reassess muscle rigidity every 15–30 minutes - Risk of **re-rigidity** if naloxone wears off before fentanyl is metabolized (fentanyl t½ = 3–4 hours, naloxone t½ = 60–90 min) - May require naloxone infusion (0.4–0.8 mg/hr) or repeated boluses [cite:Harrison 21e Ch 467; Stoelting's Pharmacology in Anesthesia Ch 8]