Hyperthermia

OVERVIEW

Definitions

• Hyperthermia is when core temperature exceeds that normally maintained by homeostatic mechanisms
• Fever  or pyrexia is an elevation of body temperature above the normal range of 36.5–37.5 °C (97.7–99.5 °F) due to an increase in the temperature regulatory set point
• Uncontrolled hyperthermia differs from fever in that the body temperature is elevated above the thermoregulatory set point due to excessive heat production and/or insufficient heat dissipation

PATHOPHYSIOLOGY

• sensor-central controller effector mechanism involving autonomic nervous system
• temperature sensors -> hypothalamus -> cold defenses (vasoconstriction, piloerection, behavioural, shivering, sweating)

CAUSES

• endogenous
• exogenous

OR

• infections
• non-infectious

OR

• excessive heat production:
  exertional, MH, NMS, thyrotoxicosis, phaeochromocytoma, drug intoxication (sympathomimetic, serotonergic), seizures
• diminished heat dissipation:
  heat stroke, dehydration, autonomic dysfunction, NMS, anticholinergic poisoning (may be exacerbated by heart failure)
• hypothalamic dysfunction:
  CVA, encephalitis, trauma, granulomatous disease, NMS

EFFECTS

Fever

• increased Q and HR
• increased metabolic rate with O2 consumption and CO2 production

Heat stroke

• altered mental state, seizures, coma
• rhabdomyolysis
• DIC
• liver failure
• renal failure

MANAGEMENT

Goals

(1) seek and treat underlying cause
(2) lower temperature if required (aim to cool to 39C in most settings)

> 40 C

• potentially dangerous
• treat if > 41 C (adults) or > 39 C (< 3 years of age)

Mild to Moderate hyperthermia

• may be protective (e.g. in sepsis)
• normalisation of temperature only required to avoid potential harm (e.g. stroke, TBI, hypoxic brain injury)

Practical Aspects

• physical cooling:
  — surface (may cause shivering): take off clothes, tepid water sprays and fanning, ice packs (axillae, groin, neck), cooling garments and blanket
  — immersion is effective, but not suitable for sick patients and can cause vasoconstriction and impair central heat dissipation
  — cold IV fluids (see therapeutic hypothermia)
  — invasive: lavage (bladder, gastric, peritoneal, pleural), intravascular cooling catheters, RRT and ECMO/ bypass
• pharmacological:
  — paracetamol, aspirin, NSAIDs (no evidence of benefit in heat stroke)
  — neuromuscular blockade if toxicological cause (e.g. serotonin syndrome) or increased muscular activity (e.g. seizure)
  — dantrolene for MH (not beneficial for heat stroke)

Supportive care and monitoring

• oesophageal probe preferably in critically ill

Seek and treat underlying cause and complications

References and Links

LITFL

• CCC — Malignant hyperthermia
• CCC — Heat Stroke

Journal articles

• Grogan H, Hopkins PM. Heat stroke: implications for critical care and anaesthesia. Br J Anaesth. 2002 May;88(5):700-7. PMID: 12067009
• Hadad E, Cohen-Sivan Y, Heled Y, Epstein Y. Clinical review: Treatment of heat stroke: should dantrolene be considered? Crit Care. 2005 Feb;9(1):86-91. Epub 2004 Aug 11. PMC1065088.
• Leon LR, Helwig BG. Heat stroke: role of the systemic inflammatory response. J Appl Physiol. 2010 Dec;109(6):1980-8. PMID: 20522730.
• Musselman ME, Saely S. Diagnosis and treatment of drug-induced hyperthermia. Am J Health Syst Pharm. 2013 Jan 1;70(1):34-42. PMID: 23261898