Makindo Medical Notes.com

Related Subjects: Acute Kidney Injury |Acute Rhabdomyolysis |Hyperkalaemia

Thanks to the dramatic progress in understanding the clinical manifestation and pathophysiology of the syndrome, mortality has dropped from over 80% thirty years ago to less than 5%.

About

• Usually precipitated by drugs in those genetically susceptible however previous uneventful anaesthesia with triggering agents does not preclude MH
• Hypermetabolism and acidosis rhabdomyolysis and hyperkalaemia and untreated it is invariably fatal

Aetiology

• Usually Autosomal dominant inherited predisposition seen in 1 in 20,000
• At least four chromosomal locations have been identified
• Best defined is the Ryanodine receptor on 19q13
• Acute rise in intracellular calcium causes rigidity and rhabdomyolysis

Pathogenesis

Causes

• Halothane, sevoflurane, desflurane
• Methoxyflurane, Suxamethonium
• Depolarizing muscle relaxant succinylcholine
• Vigorous exercise and heat.

Clinical

• Increased ETCO₂ is one of the earliest, most sensitive and specific signs of MH
• Trunk or body rigidity
• Masseter muscle spasm or trismus may be an early sign
• Hyperpyrexia > 40°C
• Myoglobinuria (dark urine)
• Tachycardia, Muscle stiffness

Differentials

• Light anesthesia and response to that
• Hypoventilation
• Thyroid storm
• Overheating
• Phaeochromocytoma

Investigations

• U&E : renal failure from dehydration of Myoglobinuria. K?? CK ??
• ABG : Metabolic acidosis/Respiratory acidosis with Increased CO₂ production
• ECG monitoring

Complications

• Acute kidney injury, Cardiac arrhythmias, Cardiac arrest
• Hyperkalaemia, Disseminated intravascular coagulation

Management: Avoid calcium channel blockers

• ABC. Potentially fatal if untreated as uncontrolled hypermetabolism leads to cellular hypoxia that is manifested by a progressive and worsening metabolic acidosis. Once diagnosed if intraoperative then Notify the surgeon.
• Stop causative drug immediately and halt surgery if possible and get help. Otherwise, maintain anaesthesia with intravenous agents such as propofol until surgery is completed. The surgeon may be able to give cavity ice water lavage if hyperthermia.
• Admit to ICU, central line and urine catheter. Look for and treat any hyperkalaemia if ECG changes and with Insulin/Dextrose and Calcium Gluconate and send off blood.
• Hyperventilate the patient with 100% O₂ at > 10 l/min via a clean breathing circuit. Do not waste time securing another anaesthetic machine - use an Ambu bag and an O₂ cylinder initially.
• Start Active Cooling management - cool IV fluids and ice packs. Can give cool IV fluids. Antipyretics do not work.
• Dantrolene: Initially 2-3 mg/kg, then 1 mg/kg, repeated if necessary; maximum 10 mg/kg per course. Cautions as extravasation can cause soft tissue injury should be given until the tachycardia, rise in CO₂ production and pyrexia start to subside
• IV Bicarbonate may be given for metabolic acidosis with pH < 7.2 or for hyperkalaemia.

References

• Malignant hyperthermia FRCA website
• Malignant hyperthermia