Malignant Hyperthermia

OVERVIEW

  • Malignant Hyperthermia = pharmacogenetic disease of skeletal muscle induced by exposure to certain anaesthetic agents
  • incidence 1:5,000 -> 1:65,000 anaesthetics (suspected)
  • mutation in the gene coding for the ryanodine receptor
  • autosomal dominant
  • gene on chromosome 19
  • thymidine instead of cytosine
  • produces a cysteine for arginine substitution at position 615 of the receptor.

PATHOPHYSIOLOGY

  • excess Ca2+ release during muscle contraction -> increased musle metabolism + heat production.
  • prolonged and intensified interation between actin and myosin.
  • enhanced aerobic metabolism -> lactic acidosis -> accumulation of intra-mitochondrial calicum -> deconjugation of oxidative phosphorylation -> cytolysis.

TRIGGERS

  • stress (in pigs)
  • all volatile agents (except N2O)
  • suxamethonium
    -> these all either enhance Ca2+ influx or slowing its efflux
  • some may have tolerated the same agents previously
  • rare in barbiturate-N2O-opiate-tranquiliser-non-depolarising muscle relaxant anaesthesia
  • sux potent trigger (first exposure)
  • volatiles (median exposure till fulminant -> 3)

CLINICAL

  • in lower North Island of NZ trigger names = Harvey, Harwere & Cook
  • history of Central Core Disease

(1) increased ETCO2
(2) tachycardia
(3) tachypnoea
(4) masseter spasm (if develops this convert to a MH safe anaesthetic)
(5) muscle rigidity
(6) temp increase (late) – 1 C\15min

  • intra-op & 4\24 post op
  • tachyarrhythmia
  • difficulty ventilation
  • hypertension
  • sweating
  • DIC
  • hyperkalaemia
  • cardiac arrest

INVESTIGATIONS

  • PaCO2 >60mmHg
  • PvCO2 >90mmHg and increasing
  • BE -5 and falling
  • metabolic acidosis
  • CK >50,000 IU/L
  • K+ increases
  • Na+ increases
  • myoglobinuria

ACUTE MANAGEMENT

  • call for help
  • discontinue all anesthetic agents
  • maintain anaesthesia with hypnotics and opioids.
  • muscle relaxation with NDNMBD
  • terminate surgery
  • hyperventilate
  • 100% O2
  • cool (N/S stomach lavage)
  • maintain urine output
  • inotropes as needed
  • HCO3 2-4mEq/kg
  • Dantrolene 2.5mg/kg every 5min (total dose 10mg/kg/day – continuous infusion or treat recurrence (25%))
  • cardiac arrhythmias -> beta blockers & lignocaine.
  • high K+ -> glucose-insulin & frusemide
  • watch for DICCK Q6hrly

PROGNOSIS

  • mortality without dantrolene = 70%
  • mortality with dantrolene = 5%

LONG-TERM MANAGEMENT

  • referral to an MH centre
  • warn patient and family of impending consequences
  • if uncertain about diagnosis -> must screen
  • IVCT
  • subsequent gene mutations studies if positive + family screening
  • if blood test negative -> must have a muscle biopsy
  • medic alert and appropriate documentation

PROPHYLACTIC MANAGEMENT

  • take history
  • decrease anxiety with midazolam
  • machine -> remove vapourisers, flush with O2 @ 10 L\min for 20min
  • new circuit and airway devices
  • ETCO2 monitoring
  • nasal temp probe
  • dantrolene available

OBSTETRIC PATIENTS

  • baby = 50% chance of having MH
  • planned delivery with early anaesthetic advice
  • anticipate airway problems -> AFOI
  • RA safe and preferred
  • MH safe drugs
  • sux doesn’t cross the placenta

CCC Toxicology Series

CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the  Clinician Educator Incubator programme, and a CICM First Part Examiner.

He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.

His one great achievement is being the father of three amazing children.

On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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