Malignant hyperthermia

Malignant hyperthermia

Describe the management of malignant hyperthermia

Malignant hyperthermia is an inherited disorder of skeletal muscle that can be pharmacologically triggered to produce a combination of hypermetabolism, muscle rigidity and muscle breakdown. The incidence of reported malignant hyperthermia reactions varies from 1:40000 to 1:100000 anaesthetics. Estimates of the population prevalence of genetic susceptibility are between 1:5000 and 1:10000.

The diagnosis of malignant hyperthermia may not be obvious at first. The primary features of malignant hyperthermia are a direct consequence of loss of skeletal muscle cell calcium homeostasis with a resulting increased intracellular calcium ion concentration. Some or all of the following signs may be present. If suspicious, insert a temperature probe immediately.

Signs

• There is high and rising end-tidal CO2.
• SpO2 falls as oxygen delivery is incapable of meeting the metabolic demands of stimulated muscle. Skeletal muscle constitutes 40% of body mass.
• Muscle rigidity (especially trismus) is due to continuous actin-myosin interaction: 1)Either following suxamethonium ( fasciculation) 2)or during the course of anaesthesia.
• There is a rapid rise in body temperature: 1°C every ten minutes.
• There may be sudden onset of unexplained tachycardia.
• Tachypnoea is a feature.
• Cardiac arrhythmias are due to hyperkalaemia secondary to rhabdomyolysis.
• Blood pressure is unstable.
• There may be unexplained cyanosis.
• Disseminated intravascular coagulation may occur as a result of the release of tissue clotting activators from muscle and through the resulting hyperthermia.
• Blood analysis – decreasing PaO2, increasing PaCO2, decreasing pH, decreasing (HCO3), increasing plasma K concentration. Plasma myoglobin concentrations are sufficient to cause renal tubular damage and acute renal failure.

Treatment

1. Abandon the procedure or terminate surgery as soon as possible.
2. Stop inhalational agents. Maintain anaesthesia with intravenous drugs whilst the surgery is concluded.
3. Give 100% oxygen. If possible, hyperventilate with 2–3 times the predicted minute volume. Vecuronium or atracurium may be used to facilitate ventilation but may not overcome muscle spasms.
4. Active cooling measures should be commenced, including infusion of cold intravenous solutions, application of ice in the axillae and groins and a cooling mattress.
5. Give dantrolene by rapid intravenous infusion. Dantrolene is the only drug effective in limiting the accumulation of calcium ions within muscle cells. Dantrolene 20mg is presented with mannitol 3g which is required to solubilise the dantrolene. Repeated doses of dantrolene (20mg in adults, 1mg/kg in children under 20kg) should be administered until pyrexia, tachycardia and rise in end-tidal CO2 subside. Repeat as necessary to a total of 10mg/kg (the average requirement is 3mg/kg). Avoid using calcium-channel blocking drugs as they can produce marked cardiac depression when used in combination with dantrolene.
6. Give a large dose of glucocorticoid (e.g. methylprednisolone 2g) to the average adult.
7. Acidosis is treated with bicarbonate and hyperkalaemia with insulin and Dextrose. Treatment is guided by regular blood gases and electrolyte measurements.
8. A diuresis of at least 2mL/kg per hour should be maintained to limit renal tubular damage by myoglobin.
9. Correct raised K with 50mL of 50% dextrose with 10 units of soluble insulin.
10. If dysrhythmias are present, follow the advanced life support (ALS) arrhythmia algorithm.
11. Inotrope to maintain cardiac output.
12. When the patient is adequately rehydrated (as assessed by CVP and haematocrit) maintain adequate urinary output (1mL/kg per hour) with a diuretic if necessary.
13. Body temperature may be unstable for 24–48 hours.
14. After the acute episode check for hypokalaemia, myoglobinuria and disseminated intravascular coagulation (DIC).
15. Admit to ICU.

Notes

• Each vial of dantrolene contains 20g dantrolene sodium and 3g mannitol and requires 60 mL of water to reconstitute (pH 9.5).
• If the episode has been controlled and surgery is essential, use a ‘safe’ technique:                

1. Regional block with local anaesthetic (plain bupivacaine)
2. Thiopentone, propofol, fentanyl, vecuronium, atracurium and pancuronium are all considered ‘safe’
3. Follow-up with patients and family.