Malignant hyperthermia
Why examiners ask this
Malignant hyperthermia is the one crisis where the examiner expects a complete, ordered protocol delivered without hesitation, and then expects you to distinguish it from its mimics. It tests organization under pressure and application of knowledge about dantrolene, hyperkalemia, and the pharmacology of triggering agents. It also appears in disguise: a child with a muscular dystrophy, a masseter spasm after succinylcholine, or a family history mentioned in passing in the stem.
The 60-second answer
The first sign is usually an unexplained, rising end-tidal CO2 that outpaces increases in minute ventilation, with tachycardia, rigidity and, later, a rising temperature. I call for help and the MH cart, stop the volatile agent and succinylcholine, change to total intravenous anesthesia, hyperventilate with 100% oxygen at high flows, and tell the surgeon to finish or abort. I give dantrolene 2.5 mg/kg IV rapidly and repeat it as often as needed until end-tidal CO2, rigidity and tachycardia resolve; doses above 10 mg/kg are sometimes required, but if I reach that without response I reconsider the diagnosis.1 At the same time I treat the consequences: bicarbonate for acidosis, calcium, insulin and glucose for hyperkalemia, active cooling that stops once the temperature falls below 38 °C, and antiarrhythmics other than calcium-channel blockers.1 I place an arterial line, send gases, potassium, CK and coagulation studies, and keep the patient in the ICU on dantrolene for at least 24 hours.1
Decision points and the "why"
- Dantrolene before anything else — it is the only specific treatment; it blocks calcium release from the sarcoplasmic reticulum. Every other step buys time. I delegate mixing it because each 20 mg vial needs 60 mL of sterile water; the 250 mg formulation needs only 5 mL.2
- Hyperventilate with 100% oxygen at 10 L/min fresh gas flow — the CO2 production is enormous;1 I do not need to change the circuit or machine, because flushing with high flows is faster than swapping equipment, though I remove the vaporizers and add charcoal filters if available.
- Treat hyperkalemia aggressively — rhabdomyolysis releases potassium, and hyperkalemic arrest is what kills these patients; calcium chloride 10 mg/kg (maximum 2 g), insulin 10 units with 50 mL of 50% dextrose in adults, bicarbonate 1–2 mEq/kg, and hyperventilation.1
- No calcium-channel blockers — with dantrolene they cause hyperkalemia and cardiovascular collapse; I use amiodarone, lidocaine or beta-blockers for arrhythmias.
- Stop cooling at 38 °C — after-drop hypothermia is a real complication; external cooling first (circulating-water mattress, ice packs), then 20 mL/kg of chilled IV fluid, and lavage of an already-open cavity only if needed.3
- Watch for recrudescence — about one in five recurs, 80% of those within 16 hours; dantrolene 1 mg/kg every 4–6 hours (bolus or infusion) for at least 24 hours, and I follow CK, potassium and urine output.4
- Differential — I state it early: light anesthesia, hypoventilation or CO2 absorbent exhaustion, sepsis, thyroid storm, pheochromocytoma, neuroleptic malignant syndrome, serotonin syndrome, and anaphylaxis. A rising end-tidal CO2 that does not fall with increased ventilation points to MH.
The complication they will inject
The patient goes into ventricular tachycardia with a potassium of 7.2. I start ACLS, with the caveat that calcium-channel blockers are out and that the arrest is driven by hyperkalemia, so calcium chloride, insulin with dextrose, bicarbonate, and continued hyperventilation are the treatment alongside the rhythm management. I continue dantrolene, because the hyperkalemia will not resolve while the muscle is still releasing. Alternatively the examiner moves to the aftermath: the patient is stable and the family asks what happens next. I explain that this is an inherited condition, that first-degree relatives should be considered susceptible until tested, that caffeine-halothane contracture testing or genetic testing for RYR1 is available through MHAUS centers, and that the patient needs a medical alert identifier and a letter for future anesthetics.
Pitfalls that cost points
- Starting with cooling or bicarbonate before dantrolene.
- Stating the dantrolene dose without saying you repeat it to effect.
- Forgetting to change to a non-triggering anesthetic while the surgery continues.
- Omitting the ICU and the 24-hour recrudescence risk.
- Confusing masseter spasm with established MH; masseter rigidity alone is an indication to abort elective surgery and monitor, not to give dantrolene reflexively.
Rapid-why
Q: Why does end-tidal CO2 rise before temperature? A: Uncontrolled calcium release drives aerobic and anaerobic metabolism; CO2 production rises immediately, whereas heat accumulates more slowly.
Q: Which drugs are safe? A: Propofol, opioids, benzodiazepines, ketamine, etomidate, nitrous oxide, local anesthetics and all non-depolarizing relaxants.
Q: How do you prepare a machine for a known-susceptible patient? A: Remove the vaporizers, change the CO2 absorbent and circuit, and flush at high flow for the manufacturer's recommended time (10 to more than 90 minutes), or use activated charcoal filters, which are effective after a 90-second high-flow flush.5
Q: Which myopathies are linked to MH? A: Central core disease, multiminicore disease and King-Denborough syndrome; Duchenne and Becker dystrophies are not MH-susceptible but have a separate succinylcholine-induced hyperkalemic rhabdomyolysis risk.
Numbers to know
| Item | Value |
|---|---|
| Dantrolene initial | 2.5 mg/kg IV, repeat to effect1 |
| Dantrolene maintenance | 1 mg/kg every 4–6 h for ≥24 h4 |
| Vial reconstitution | 20 mg in 60 mL sterile water; 250 mg in 5 mL (Ryanodex)2 |
| Cooling endpoint | Stop below 38 °C3 |
| Recrudescence | ~20%, mostly within 16 h4 |
| Charcoal filters | Effective after a 90 s high-flow flush5 |
| MHAUS hotline | 1-800-644-97372 |
Links
- succinylcholine, Volatile agents and nitrous oxide: when and when not, hyperkalemia, Muscular dystrophies and myotonias (coming), ACLS adapted to the operating room
- Atlas: Capnography: waveforms and sudden loss of ETCO2; Monitor atlas: other perioperative and peripartum events (coming)
- MHAUS treatment protocol (current version)
Sources
- 1.Managing a Crisis (MH treatment protocol) · MHAUS · mhaus.org
- 2.What should be on an MH cart? · MHAUS · mhaus.org
- 3.Evidence-based interventions to alleviate hyperthermia associated with MH · MHAUS · mhaus.org
- 4.Dantrolene Administration After an MH Event · MHAUS · mhaus.orgRecrudescence 20% of 308 patients (Burkman, Anesthesiology 2007).
- 5.Preparation of Anesthesia Workstations to Anesthetize MH-Susceptible Patients · MHAUS · mhaus.org
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.