Malignant hyperthermia: an oral boards study guide

Malignant hyperthermia is a crisis triggered by MH-triggering anesthetics — MHAUS names the volatile agents isoflurane, desflurane and sevoflurane — and the depolarizing muscle relaxant succinylcholine (MHAUS).

MHAUS lists the general signs as a rising heart rate, greatly increased body metabolism, muscle rigidity and/or fever, with muscle breakdown and increased blood acidity (What is MH?). Muscle cells release large amounts of potassium, which can be followed by ventricular arrhythmias, and myoglobin, which can injure the kidneys (MHAUS). MHAUS advises continuous end-tidal CO2 monitoring; in its treatment steps, falling end-tidal CO2 is one of the signs that dantrolene is working (Managing a crisis).

On the oral boards: say what you see, name MH as your concern, and start the crisis steps out loud.

MHAUS says to stop triggers, get help and dantrolene, hyperventilate with oxygen, and give dantrolene as soon as possible. Delegate these tasks together; do not wait to finish one before starting the next. The list below is a study summary.

• Get help and the MH cart. Call for help, get the dantrolene/MH cart (call 911 at a surgicenter), and call the MHAUS hotline.
• Stop the triggers. Tell the surgeon to stop the procedure. Stop volatile agents and succinylcholine. If surgery has to continue, switch to IV non-triggering anesthetics.
• Hyperventilate. 100% oxygen at flows of 10 L/min to flush volatile agent and lower end-tidal CO2. Use activated charcoal filters if available, replaced hourly.
• Give dantrolene — see the dosing below.
• Cool if core temperature is above 39°C or rising rapidly, and stop cooling once it is below 38°C.
• Treat hyperkalemia, acidosis and arrhythmias, and monitor closely (below).

MHAUS MH Hotline (emergencies only): 1-800-644-9737; outside North America 001-209-417-3722 (MHAUS).

• Dose: 2.5 mg/kg IV rapidly. Repeat as needed until end-tidal CO2 falls, rigidity eases, or heart rate slows (MHAUS).
• More may be needed: large doses (more than 10 mg/kg) may be required for persistent contractures or rigidity. If more than 10 mg/kg doesn’t resolve it, MHAUS says to consider other diagnoses (MHAUS). Repeat as frequently as needed for the clinical response; do not treat a fixed time interval as a reason to delay treatment.
• Dantrium / Revonto: reconstitute each 20 mg vial with 60 mL sterile water (no bacteriostatic agent) and shake until clear (MHAUS).
• Ryanodex: reconstitute each 250 mg vial with 5 mL sterile water for injection (without a bacteriostatic agent) and shake to an orange, uniform, opaque suspension (MHAUS).

On the oral boards: name the dose by weight, say you’d assign someone to mix it, and say what you’re watching to decide whether to re-dose.

From MHAUS, for potassium above 5.9 mEq/L, or a lower value with ECG changes:

• For life-threatening hyperkalemia: calcium chloride 10 mg/kg (max 2,000 mg) or calcium gluconate 10–50 mg/kg (max 3,000 mg)
• Sodium bicarbonate 1–2 mEq/kg IV
• Insulin and glucose — adult: 10 units regular insulin IV with 50 mL of 50% dextrose; pediatric: 0.1 units/kg regular insulin IV with 2 mL/kg of 25% dextrose. Check glucose hourly.
• Refractory: consider albuterol or another beta-agonist, dialysis, or ECMO if in cardiac arrest

Treat arrhythmias with standard drugs but avoid calcium channel blockers. Use the blood gas and the MHAUS crisis protocol to guide acidosis treatment with expert help.

MHAUS recommends a venous or arterial blood gas, core temperature, a bladder catheter for urine output, and considering arterial and/or central venous monitoring. Follow heart rate, core temperature, end-tidal CO2, minute ventilation, blood gases, potassium, CK, urine myoglobin and coagulation studies as clinically warranted. MHAUS gives an acute urine-output goal above 1 mL/kg/hr. For suspected muscle breakdown and myoglobin in the urine, follow its post-acute guidance for kidney protection, urine output, and pH and electrolyte monitoring.

• Where: once stable, PACU or ICU for at least 24 hours (MHAUS).
• Dantrolene: 1 mg/kg IV every 4–6 hours for at least 24 hours, longer if clinically indicated. MHAUS says it can be stopped, or spaced to every 8 or 12 hours, once the patient has been metabolically stable for 24 hours, core temperature is below 38°C, CK is falling, there is no myoglobinuria, and muscle is no longer rigid (post acute phase).
• Watch for relapse: MHAUS says 25% of MH events relapse, and relapses can be fatal. Signs include increasing rigidity without shivering, inappropriate hypercarbia, unexplained metabolic acidosis, and an inappropriate temperature rise (MHAUS).
• Complications to look for include altered consciousness, dysrhythmias, pulmonary edema, kidney injury, DIC, liver dysfunction, weakness and compartment syndrome (MHAUS).
• Follow-through: MHAUS recommends genetic screening of RYR1 after a fulminant episode, alerting family members and referring them to an MH diagnostic testing center, and reporting the event to the North American MH Registry (MHAUS).

Practice idea: rehearse your response to rising potassium, an arrhythmia, a relapse in PACU, and questions from the family. For the format of the exam itself, see the ABA APPLIED Exam, explained.