Malignant hyperthermia

Surgical approach (if relevant)

• Not an operation; a pharmacogenetic anaesthetic emergency.
• If MH occurs intra-op: surgery should be stopped/expedited and wound made safe as soon as feasible.
  • Communicate clearly: “suspected MH—stop triggers, call for dantrolene, stop surgery if possible”.

Anaesthetic management (typical context)

• Type of anaesthesia
  • For MH-susceptible patient: trigger-free GA or regional (both acceptable).
  • Avoid: volatile agents and suxamethonium.
• Airway
  • ETT or SGA depending on surgery; choose to optimise ventilation/temperature monitoring if high risk.
• Duration
  • Any duration; MH can occur early (minutes) or later, and may present in recovery/ICU.
• How painful
  • Procedure-dependent; MH management prioritises resuscitation over analgesia. Use multimodal/RA where appropriate.
• Monitoring emphasis
  • Continuous capnography (earliest sign often rising ETCO₂), core temperature, ECG, invasive BP if unstable, urine output.

Definition and overview

• A life-threatening hypermetabolic crisis of skeletal muscle triggered by certain anaesthetic drugs in genetically susceptible individuals.
• Key concept: uncontrolled intracellular Ca²⁺ release → sustained muscle contraction + massively increased metabolism.
• Most commonly associated genes: RYR1 (ryanodine receptor) and CACNA1S (DHPR).

Triggers and non-triggers

• Triggers
  • Volatile inhalational agents (e.g. sevoflurane, isoflurane, desflurane; historically halothane).
  • Suxamethonium (especially rapid onset rigidity/hyperkalaemia).
• Not triggers (safe in MH-susceptible patients)
  • IV induction agents (propofol, thiopentone, etomidate, ketamine), opioids, benzodiazepines.
  • Non-depolarising NMBs, local anaesthetics, nitrous oxide.
  • Neostigmine/sugammadex, antiemetics, antibiotics.

Pathophysiology (what to say in a viva)

• Trigger exposure → abnormal RYR1/DHPR coupling → excessive Ca²⁺ release from sarcoplasmic reticulum.
• Consequences
  • Sustained contraction/rigidity → ↑ CO₂ production and ↑ O₂ consumption.
  • Heat generation → hyperthermia (often late sign).
  • Rhabdomyolysis → hyperkalaemia, CK rise, myoglobinuria → AKI risk.
  • Metabolic + respiratory acidosis; sympathetic surge; arrhythmias; DIC in severe cases.

Clinical features (recognition)

• Earliest/most sensitive intra-op sign: unexpected rise in ETCO₂ despite increased minute ventilation.
• Other early signs
  • Unexplained tachycardia, arrhythmias, rising O₂ consumption, sweating, mottling.
  • Muscle rigidity (generalised or masseter spasm), especially after suxamethonium.
• Later signs
  • Rapidly rising core temperature (can increase 1–2°C every 5 min in fulminant cases).
  • Dark urine (myoglobin), bleeding/DIC, cardiovascular collapse.
• Blood gas/lab pattern
  • Mixed acidosis, hyperkalaemia, rising lactate, raised CK, raised phosphate, low/normal calcium, myoglobinaemia/uria.

Differential diagnosis (important in FRCA)

• Causes of rising ETCO₂/tachycardia
  • Hypoventilation, circuit/valve malfunction, exhausted CO₂ absorber, increased dead space, rebreathing.
  • CO₂ insufflation (laparoscopy), tourniquet release, bicarbonate administration.
  • Sepsis, thyroid storm, pheochromocytoma crisis, neuroleptic malignant syndrome, serotonin syndrome.
• Hyperthermia causes
  • Overwarming, transfusion reaction, sepsis, heat stroke, anticholinergic toxicity.

Immediate management (intra-operative MH algorithm)

• 1) Call for help + declare MH + get MH trolley/dantrolene; allocate roles.
• 2) Stop triggers immediately
  • Turn off vapourisers, disconnect vaporiser if possible; stop suxamethonium/volatiles.
  • Switch to 100% O₂ with high fresh gas flows (e.g. ≥10 L/min) and hyperventilate.
• 3) Change anaesthetic technique
  • Maintain anaesthesia with TIVA (propofol/opioid) and non-depolarising NMB if needed.
• 4) Give dantrolene early
  • Initial dose 2.5 mg/kg IV, repeat as needed until ETCO₂/rigidity/temperature and haemodynamics improve (large cumulative doses may be required).
  • Continue with ongoing dosing/infusion per local protocol due to recrudescence risk (commonly for 24–48 h).
• 5) Active cooling (if hyperthermic)
  • Stop warming devices; expose patient; cool IV fluids; ice packs to axilla/groin/neck; consider cold lavage (gastric/bladder) if severe.
  • Aim ~38°C then stop aggressive cooling to avoid overshoot hypothermia.
• 6) Treat acidosis and hyperkalaemia
  • ABG frequently; consider sodium bicarbonate if severe metabolic acidosis (treat cause first).
  • Hyperkalaemia: calcium chloride/gluconate, insulin+dextrose, salbutamol, consider dialysis/haemofiltration if refractory.
  • Avoid calcium channel blockers with dantrolene (risk of hyperkalaemia/cardiovascular collapse).
• 7) Manage arrhythmias and haemodynamics
  • Treat per ALS; correct hyperkalaemia/acidosis first; use amiodarone/lidocaine as appropriate.
  • Invasive monitoring, vasopressors/inotropes as needed; consider arterial line early.
• 8) Protect kidneys / manage rhabdomyolysis
  • Aim urine output ~1–2 mL/kg/h with IV fluids; consider mannitol/furosemide depending on local practice and volume status.
  • Monitor CK, creatinine, electrolytes; check urine for myoglobin.
• 9) Coagulation and complications
  • Check coagulation profile; treat DIC/bleeding with blood products guided by labs/TEG/ROTEM.
• 10) Post-crisis care
  • ICU transfer for monitoring and ongoing dantrolene; document; inform patient/family; refer to MH unit for definitive testing.

Dantrolene: key facts

• Mechanism: inhibits RYR1-mediated Ca²⁺ release from sarcoplasmic reticulum → reduces muscle metabolism and rigidity.
• Adverse effects
  • Muscle weakness, sedation, nausea; phlebitis/extravasation; hepatotoxicity (more with chronic use).
• Practical points
  • Reconstitution can be time-consuming; ensure staff know location and mixing instructions; use dedicated runner/mixer.
  • Large volumes may be required depending on formulation; monitor fluid balance.

Investigations and diagnosis

• During event
  • ABG/VBG: pH, PaCO₂, lactate; U&E (K⁺), glucose; CK; FBC; LFT; coagulation; myoglobin; troponin if indicated.
  • Continuous ETCO₂ and core temperature; urine output and colour; ECG for arrhythmias.
• Definitive diagnosis (after recovery)
  • In vitro contracture test (IVCT) / caffeine–halothane contracture test: gold standard in many systems.
  • Genetic testing (RYR1/CACNA1S): useful, but a negative result does not always exclude susceptibility.

Anaesthesia for the MH-susceptible patient (elective plan)

• Pre-op
  • Clarify history: personal MH episode, family history, unexplained peri-op death/fever, rhabdomyolysis, exertional heat illness; previous anaesthetics and agents used.
  • If uncertain and surgery urgent: treat as MH-susceptible (trigger-free).
• Machine preparation (trigger-free)
  • Remove/disable vapourisers; change breathing circuit and CO₂ absorber; flush with high fresh gas flows per local policy OR use activated charcoal filters if available.
  • Use dedicated “clean” machine if available.
• Intra-op technique
  • Regional anaesthesia where appropriate; otherwise TIVA with propofol/opioid ± non-depolarising NMB.
  • Avoid suxamethonium; plan RSI alternatives (e.g. rocuronium + sugammadex strategy).
  • Standard monitoring plus continuous capnography and core temperature for GA.
• Post-op
  • Routine recovery is acceptable after uneventful trigger-free anaesthesia; maintain vigilance for unexpected hypermetabolism.
  • If any concern: extended monitoring, check CK/U&E, consider ICU.

Special scenario: masseter muscle rigidity after suxamethonium

• Consider MH, especially if accompanied by rising ETCO₂, tachycardia, acidosis, hyperkalaemia, or generalised rigidity.
• Immediate actions
  • Stop triggers; switch to trigger-free technique; monitor ETCO₂ and temperature closely; obtain ABG, K⁺, CK; consider early dantrolene if other signs evolve.
• Airway implications
  • May make laryngoscopy difficult; consider waking if safe and surgery non-urgent; if proceed, ensure experienced help and alternative airway plans.