A 32-year-old male from rural Maharashtra is undergoing elective open reduction and internal fixation of a femur fracture. Fifteen minutes after induction with propofol and succinylcholine, the operating room team notices masseter muscle rigidity, followed by a rapid rise in end-tidal CO₂ (ETCO₂ from 35 to 60 mmHg within 5 minutes), core temperature rising at 1°C per minute, and dark-colored urine in the catheter. What is the most appropriate immediate next step in management?

Explanation

## Recognition and Immediate Management of Malignant Hyperthermia ### Clinical Presentation in This Case The constellation of signs—masseter rigidity (early sign), hypercarbia (ETCO₂ spike), rapid temperature rise, and myoglobinuria (dark urine)—is pathognomonic for malignant hyperthermia (MH). Early recognition and immediate action are life-saving. **Key Point:** Masseter muscle rigidity after succinylcholine is an early and highly specific warning sign of MH susceptibility; it may precede fever by 30–60 minutes. ### Immediate Management Algorithm ```mermaid flowchart TD A[MH suspected: Masseter rigidity + ETCO₂ rise + Temperature rise]:::outcome A --> B[STOP all triggering agents immediately]:::urgent B --> C[Hyperventilate with 100% O₂]:::action C --> D[Administer dantrolene 2.5 mg/kg IV bolus]:::action D --> E[Repeat dantrolene q5-10 min if signs persist]:::action E --> F[Active cooling: Cold IV saline, ice packs, cold peritoneal lavage]:::action F --> G[Monitor: Core temp, ETCO₂, K⁺, CK, urine myoglobin, coagulation]:::action G --> H[Admit to ICU for 24-48 hrs post-op surveillance]:::outcome ``` ### Why Dantrolene Is the Definitive Treatment Dantrolene sodium acts on the sarcoplasmic reticulum to inhibit Ca²⁺ release from the ryanodine receptor (RYR1), directly addressing the underlying pathophysiology of MH. It is the **only specific treatment** and must be given immediately—not after laboratory confirmation. **High-Yield:** Dantrolene is reconstituted with sterile water (20 mg vial requires 60 mL water); prepare multiple vials in advance. Each 2.5 mg/kg bolus takes ~1 minute to infuse. ### Concurrent Supportive Measures | Intervention | Rationale | | --- | --- | | Discontinue succinylcholine and volatile anesthetics | Both are triggering agents; volatile anesthetics are the most potent triggers | | Hyperventilate with 100% O₂ | Reduces ETCO₂, improves oxygenation, aids cooling | | Cold IV saline (4°C) | Core cooling; target temp < 38.5°C | | Monitor K⁺, CK, urine myoglobin | Rhabdomyolysis causes hyperkalemia (risk of VF), myoglobinuria (acute kidney injury) | | Maintain urine output > 200 mL/hr | Prevents myoglobin precipitation in renal tubules | **Clinical Pearl:** Hyperkalemia from muscle breakdown can cause fatal arrhythmias. If K⁺ > 6.5 mEq/L and ECG changes present, give calcium gluconate, insulin + dextrose, and sodium bicarbonate immediately. **Warning:** Do NOT delay dantrolene administration to obtain lab results. MH mortality without dantrolene approaches 50%; with prompt dantrolene, it is < 5%. ### Post-Operative Management - Admit to ICU for 24–48 hours - Repeat dantrolene 1 mg/kg IV q4–6h for 24 hours to prevent recrudescence - Arrange MH susceptibility testing (caffeine halothane contracture test or genetic testing for RYR1/CACNA1S mutations) after recovery - Counsel patient and first-degree relatives; provide MedAlert bracelet [cite:Miller's Anesthesia 8e Ch 91]