Heat Stress / Fever vs. Hyperthermia


  • Elevation of body temp due to the “resetting” of the hypothalamic set poin­t in response to endogenous or exogenous pyroxenes


  • Elevation of body temp above the hypothalamic set point due to the failure of the body’s heat dispersing mechanisms

Physiologic Response to Heat

Hypothalamus signals for:

  • Sweat production which causes evaporative heat loss
  • Peripheral vasodilatation which causes increased skin blood flow
    • this results in the removal of heat from core through convective heat loss

Heat exhaustion vs. Heat stroke

  • Important to think of heat exhaustion and heat stroke as two ends of a spectrum
  • The point at which heat exhaustion becomes heat stroke – when thermoregulatory mechanisms fail or are overwhelmed
  • Heat exhaustion can easily progress to heat stroke if not adequately treated
  • Thus early recognition and treatment essential!

Heat exhaustion

  • Two types classically described
  • Water depletion
  • Salt depletion

Water depletion heat exhaustion

  • Occurs in patients who do not drink enough fluids when exposed to heat stress
  • Salt depletion heat exhaustion
  • Occurs in patients who have sweated profusely in response to heat stress
  • These patients have attempted to hydrate themselves with water, but did not compensate for the salt lost in their sweat

Heat exhaustion-diagnosis

  • Very nonspecific symptoms
  • Dizziness, lightheadedness, headache
  • Nausea, vomiting, weakness, malaise
  • Mild disorientation, clumsiness

Heat Stroke

  • Total breakdown of body’s thermoregulatory system
  • Leads to multi-organ damage if left untreated
  • A true medical emergency
  • 2 forms described
  • Exertional
  • Non-exertional/Classical

Exertional Heat Stroke

  • History of increased endogenous heat production eg. Strenuous exercise
  • Heat dispersing mechanisms are intact but overwhelmed by the heat stress
  • Athletes, military personnel, miners & other labourers are at risk

Classical Heat Stroke

  • History of increased exogenous heat gain (eg. hot day with no air conditioning) combined with decreased heat dispersing ability (eg. history of cardiac disease)

Initial management for both Exertional and Classical heat stroke


  • The key to successful outcome in heat stroke
  • Prognosis in heat stroke is directly related to how quickly the body can be cooled down
  • Slowly cool – Goal is to cool by 0.1-0.2 degrees/min

In the field …

  • Move the patient to a cool, shady environment
  • Remove clothes
  • Keep the patient wet
  • Ice packs
  • During transport –> open windows
  • Immersion
  • Evaporation

Evaporative Cooling

  • Fans positioned beside an undressed patient while warm water is sprayed/sponged on
  • Patient kept continually wet for continued cooling
  • Can achieve cooling rates comparable to immersive techniques
  • For all methods, cooling should be discontinued when temp hits 39-40 degrees
  • The ABC’s…
  • Airway, Breathing
  • Cooling
  • Evaporative/Immersive +/- adjuncts
  • Circulation
  • Cautious rehydration


  • Altered mental state + hyperthermia = heat stroke until proven otherwise
  • ABC’s = Airway, Breathing, Cooling
  • Think of heat exhaustion and heat stroke as ends of a spectrum
  • Important to recognize heat exhaustion early – can easily turn into heat stroke!


  • Rosen’s 5th edition, pages 1997-2009
  • Tintinalli’s 5th edition, pages 1235-1242
  • Khosla et al, “Heat-Related Illnesses”, Critical Care Clinics, 15(2), 251-263
  • Tek et al, “Heat Illness”, Emergency Medicine Clinics of North America, 10(2), 299-309
  • Wexler, Randall K, “Evaluation and Treatment of Heat-Related Illnesses”, American Family Physician, 65(11), 2307-2313

Links for more information on Hyperthermic Conditions: 

WCB of BC:  additional information

NIOSH – National Institute of Occupational Safety & Health:   http://www.cdc.gov/niosh/topics/heatstress/