Hyperthermia

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Hyperthermia

Defined as a core (oesophageal, tympanic) temperature above 40.5⁰ C. Hyperthermia may be an extreme form of pyrogen-induced fever associated with infection, inflammation, neoplasia or CVA

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Heat stroke

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Clinical features

CNS

• Oedema and petechial haemorrhages cause focal and generalised damage

Muscle

• Skeletal muscles show widespread degeneration of fibres
• Rhabdomyolysis releases myoglobin, K⁺, CPK and purines (which are metabolised into uric acid) into the circulation.

Lungs

• Non cardiogenic pulmonary ooedema

Kidneys

• Oliguric acute renal failure due to renal ischaemia, muscle breakdown products, DIC, hyperuricaemia and hypovolaemia.
• Renal failure occurs in up to 35%

Blood

• DIC (poor prognosis), thrombocytopaenia, leucocytosis.
• Thermal injury to endothelium releases thromboplastins which result in intravascular thrombosis and secondary fibrinolysis.

Metabolic

• Metabolic acidosis, respiratory alkalosis, hypoglycaemia, hyper- or hypo-kalaemia

Poor Prognostic Factors in Heat Stroke

• Core temperature >41.1° C
• AST >1000 IU in first 24 hours
• Hypotension not responsive to cooling and fluid replacement
• Renal failure or hyperkalaemia

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Malignant hyperthermia

Rare, autosomal dominant, drug-induced myopathy associated with a Ca⁺⁺ transfer defect in patients receiving volatile anaesthetics, muscle relaxants, antidepressants, alcohol or Ecstasy. Heat production is increased by muscle catabolism, spasm and peripheral vasoconstriction.

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The neuroleptic malignant syndrome

A drug induced hyperthermic syndrome secondary to antipsychotics - especially dopamine antagonists such as haloperidol, thioridazine and chlorpromazine. It is associated with muscle rigidity, extra pyramidal signs, dyskinesia, impaired consciousness and autonomic dysfunction and continues for 1-2 weeks.

Features NMS

• Sweating may or may not be present - hot dry skin was previously thought to be a feature of heat stroke but patients with heat stroke may still be sweating
• Skin surface may be cool due to peripheral vasoconstriction
• Tachycardia and hypotension. Arrhythmias. Hypotension results from a combination of hypovolaemia, peripheral vasodilatation and cardiac dysfunction.
• Tachypnoea is a universal finding in heat stroke
• Confusion, delirium and seizures
• Dilated pupils, oculogyric crises
• Coma with decerebrate posturing
• Cerebellar dysfunction
• Salt water depletion
• Rhabdomyolysis
• Disseminated intravascular coagulation. Purpura, conjunctival haemorrhages, melaena, haematocrit
• Heart failure with ST depression and T wave flattening

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Management of hyperthermia

Mortality from heat stroke approaches 80% if prompt, effective treatment is not undertaken

• Secure the airway and give high flow oxygen. Shivering and vasoconstriction may occur during rapid cooling and increase oxygen consumption and heat production.
• Rapid cooling should be instituted for patients with temperatures >41° C. (Outcome depends on the height and duration of temperature elevation).
• Clothing should be removed and patients nursed in a cool environment.
• Use surface cooling with a fan, tepid sponging, wet sheets, ice packs or a cool bath.
• Do not give anti-pyretics such as aspirin (risk DIC) or paracetamol (ineffective).
• Correct hypoglycaemia with dextrose 10%.
• Active cooling measures should be stopped when temperature is <38.5° C.
• Supportive treatment includes fluid replacement and seizure control.
  • Control fits with benzodiazepines.
  • Saline or 0.45% saline/5% dextrose may be used (depending on U&Es,  clinical assessment and haematocrit).
  • Up to 4L may be required over 6-12 hours but avoid over-rapid infusion, which may cause pulmonary/cerebral ooedema. Monitor CVP.
• Pressor agents may be indicated for hypotension but alpha adrenergic agents (noradrenaline, adrenaline) should be avoided since they impair heat dissipation by causing vasoconstriction. Consider low dose dopamine (2.5mcg/kg/min)
• Insert a urinary catheter. If myoglobinuria is present aim for urine output >50ml/h and consider bicarbonate and/or mannitol. If using bicarbonate 1-2mEq/Kg boluses should be given to maintain urinary pH>6.
• Phenothiazines may be used to reduce temperature and prevent shivering (not in neuroleptic malignant syndrome).
• Muscle relaxants with non depolarising agents should be used if the patient is ventilated/resistant to external cooling
• For malignant hyperthermia the offending drug should be stopped and dantrolene 1mg/kg given i.v. every 5 minutes to a maximum dose of 10mg/kg.
• Mechanical ventilation with a high FiO₂ and treatment of hyperkalaemia are required.
• The neuroleptic malignant syndrome is treated by stopping the offending drug, giving dantrolene, as above, and dopamine agonists (e.g. L-dopa or bromocriptine).
• Coagulopathy should be treated with FFP and platelets

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Differential Diagnosis of Heat Stroke

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Content by Dr Jon Dallimore, Dr Íomhar O' Sullivan 10/02/04. Reviewed by Dr ÍOS 10/02/05. Reviewed by Dr IOS 25/04/2007. Next review 25/04/2008