Mechanisms underlying a rise in temperature are poorly understood. It reflects the balance between heat loss and heat production. There may be inability to lose heat (e.g. high ambient temperature), 'thermostat' dysregulation within the hypothalamus or increased heat generation (e.g. due to mitochondrial uncoupling). There is some laboratory evidence that a raised temperature may be beneficial in terms of white cell response, heat shock protein activation and mitochondrial protection. Septic patients presenting with a low temperature have a poorer prognosis.

An excessive temperature may be unpleasant to the patient (e.g. rigors), will increase metabolic rate and therefore oxygen demand, may induce excessive vasodilatation and salt and water loss. At very high temperatures, biochemical function is disrupted with altered enzyme function and increased cell breakdown (e.g. rhabdomyolysis).

Causes Infection

The commonest cause in the ICU patient, though over-diagnosed. Main sites are chest and intravascular catheter sites. Urinary tract infections are difficult to diagnose in the presence of a urethral catheter. Similarly, the respiratory tract is routinely colonised with bacteria within a few days of ICU admission; differentiation between colonising and pathogenic bacteria is difficult. Seek malaria in patients who have visited endemic areas. Antibiotic therapy may itself be a cause of pyrexia.


Inflammation unrelated to infection will usually generate a pyrexic response, e.g. systemic inflammatory response syndrome, post-cardiac surgery, post-burns, post-myocardial infarction, vasculitis, glomerulonephritis, hepatitis, acalculous cholecystitis. Other than specific therapy, e.g. immnosuppression for vasculitis, management is generally symptom-orientated to include cooling.

Adverse drug reaction

Numerous drugs may induce an idiosyncratic pyrexia, including antibiotics, sedatives, paralysing agents, and amphetamines. Usually removal of the offending drug is sufficient but more active measures may have to be taken, including active cooling and dantrolene.

Adverse reaction to blood transfusion

This may be related to an immunological reaction to one of the cellular constituents, or to contamination with an organism, bacterial cell products or other pyrogen.

Ambient heating

Excessive heating or prevention of heat loss may cause pyrexia. Consider strong sunlight, excess temperature control settings on specialised beds or mattresses, and heat-retaining bed clothing.


Other causes of pyrexia include neoplasm and post-cerebral insult (e.g. head injury, cerebrovascular accident).


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