malignant hyperthermia catherine maw 24/10/2012. outline define and discuss aetiology of thermal...
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Malignant HyperthermiaCatherine Maw24/10/2012
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OUTLINE• Define and discuss aetiology of thermal disorders• Relevance to ICU• Clinical Presentation of MH• Differential diagnosis and pitfalls• Treatment in theatre and ICU• Subsequent management
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Thermoregulation• Balance between heat production and loss• Hypothalamic thermoregulatory centre• “Pyrexia” = resetting of thermoregulatory set point to a higher
level by activation of heat conserving mechanisms• “Hyperthermia” = failure of effector mechanisms to maintain
the normal set point
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Fever in the ICU• Regulated hyperthermia• Endogenous pyrogens (IL6 and PGE2) act on the hypothalamus• Reset the thermoregulatory set point to higher temp• Effector organs prevent heat loss• May be protective • When pyrogens decrease, set point decreases• Deleterious effects (↑CO, O2 consumption, CO2 production)
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Hyperthermia• Failure of effector mechanisms to maintain the hypothalamic
set point (core ≥ 40°C)• Heat stroke • Drug induced hyperthermias (MH, NMS, Serotonin syndrome,
sympathomimetic syndrome, anticholinergic syndrome)• Heat injury is the insult• Protein denaturation and lipid dissolution at 42°C (core)
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Why is it fatal?• Direct cellular damage• Increases membrane permeability• Activation of Na-K-ATPase pump• ATP depletion• Tissue oedema• Cytokine activation, coagulation cascade activation • Cellular death (lactate, hyperkalaemia, acidosis)• Similar picture to sepsis
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Why?• Metabolic acidosis• Hyperkalaemia• Rhabdomyolysis• Renal failure• DIC• Liver failure• Death
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Australian History• 1960: Dr Jim Villiers at Royal Melbourne Hospital• Patient with 10 family members who died under GA• Patient had malignant hyperthermia (MH)• Villiers presented the successful anaesthetic outcome• 1972: Lancet. Denborough and Lovell.• Royal Melbourne (one of 3) centres for MH
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Definition and Aetiology• Pharmacological disease of skeletal muscle• Hypermetabolic crisis• Induced by exposure to volatile anaesthetic agents or
Suxamethonium• Loss of normal calcium homeostasis• Unregulated release of Calcium form the sarcoplasmic
reticulum• Myocyte hypermetabolism
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Relevance • Anaesthetic complication• Ongoing patient care will always involve ICU• Insidious versus acute• True MH rare• Hyperthermia differentials more common
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Epidemiology• 1 in 10,000 to 1 in 30,000 anaesthetics• Young adults (45-55% of cases in <19 years)• More frequent in minor ops• Male > Female 2:1• Mortality previously 70-80%• Reduced to 2-3% now
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Genetics of MH• Majority of MH susceptible patients have mutations on RYR1
or DHP genes• Inherited or spontaneous• 50% Autosomal Dominant• 200 mutations identified• 29 have causality
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Pathophysiology ctd• Sustained muscle contraction due to high levels of myoplasmic
calcium• Heat generated (initial insult)• Cascade similar to sepsis/systemic inflammation• Initial aerobic metabolism generating CO2 and → cellular
acidosis• Then Oxygen and ATP depletion → worsening acidosis and
lactate production• Depleted energy → muscle death and rhabdomyolysis
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Risk Factors• Positive family history• Previous exposure to Suxamethonium or volatiles• Exertional heat stroke • Exercise induced rhabdomyolysis• Central core disease• Scoliosis• Strabismus surgery
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Diagnosis
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Early• Prolonged masseter muscle spasm after Suxamethonium• Inappropriately ↑ ETCO2 or tachypnoea during
spontaneous respiration (ETCO2 >60)• Inappropriately ↑ ETCO2 (ETCO2 >55) during controlled
ventilation• Inappropriate tachycardia• Cardiac arrhythmias, especially ventricular ectopics
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Developing• Developing rise in temperature (0.5 ◦C per 15 mins)• Progressive respiratory and later metabolic acidosis• Hyperkalaemia• Profuse sweating• Cardiovascular instability• Desaturation• Generalised muscle rigidity
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Late• Myoglobinuria• Myalgia• Grossly elevated CK• Coagulopathy• Cardiac arrest
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Differential diagnosis• Inadequate anaesthesia / machine issue / patient factor• Sepsis• Intracerebral infection or bleed• Recreational drugs• Neuroleptic malignant syndrome• Thyroid storm• Phaeochromocytoma
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Management• ANZCA suggest MH Resource kit• Link to mhanz• Task cards based on the aviation safety model• If diagnosis is suspected:• Declare Emergency• Call for HELP and send for MH resource kit• Turn off the volatile and remove vaporisers• Hyperventilate on >15l/min fresh gas flows with 100% O2• TIVA
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Ongoing Care• ICU for ventilatory support, haemodynamic monitoring, renal
support• CK peaks at 14 hours• Dantrolene does not effect cardiac or smooth muscle• Recrudescence in 25%• 1mg/kg Dantrolene every 6 hours for 48 hours
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MH Susceptibility Testing• Gold standard is the contracture test• In vitro response of a fresh sample of muscle tissue to Caffeine
or Halothane• Muscle strip in physiological solution is attached to a strain
gauge and electrically stimulated to measure baseline tension• Repeat in Halothane and Caffeine• High sensitivity and specificity• Expensive and specialist referral needed• Genetic testing cheaper but sensitivity 30-50%
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?
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