Heat waves are sporadic events of high temperatures, which can have serious consequences on human life. More than 70,000 people died during the heat wave that hit Europe in 2003, and another 10,860 died during a heat wave in Russia in 2010. The criteria for defining a heat wave vary from country to country. In Canada, a heat wave occurs when it is 30°C or higher for at least three consecutive days. It has been estimated that the average temperature of our planet will increase by 1°C by 2100 if we reduce greenhouse gas (GHG) emissions or 3.7°C if we do not. In 2000, about 30% of the world’s population was exposed to heat waves for at least 20 days a year. By 2100, it is expected that this proportion will increase to about 48% if we drastically reduce GHG emissions and 74% if we continue to increase GHG emissions.

When it is very hot, humid or both, the excess heat absorbed by the body must be dissipated by the skin and the respiratory system in order to maintain body temperature at 37°C: this is the thermoregulation process. The hypothalamus initiates a cardiovascular response by dilating blood vessels to redistribute blood to the body surface (the skin) where heat can be dissipated into the environment. Sweating is activated, allowing heat to dissipate by evaporation (600 kcal/hour). When it is very hot and humid, the evaporation of sweat is greatly reduced and the body struggles to maintain an adequate temperature. Heat stroke is a serious and life-threatening condition, which is defined as a body temperature above 40°C, accompanied by neurological signs such as confusion, seizures or loss of consciousness. The main risk factors for heat stroke are shown in Table 1.

Table 1. Risk factors for heat stroke. From Yeo, 2004.

Cardiovascular disease
Extremes of age (younger than 15, older than 65)
Skin-altering conditions (psoriasis, eczema, burns)
Lack of air conditioning in home
Living in a multi-storey building
Low socioeconomic status
Occupations with prolonged exertion and environmental exposure to temperature extremes (e.g., athletes, military workers, miners, steel workers, firefighters, factory workers, rescue workers)
· Impaired thermoregulation (diuretics, beta blockers, anticholinergics, phenothiazines, alcohol, butyrophenones)
· Increased metabolic heat production (benzotropin, trifluoperazine, ephedra containing dietary supplements, diet pills, amphetamines, cocaine, ecstasy)
Previous history of heat-related illness
Prolonged sun exposure
Wearing heavy or excessive clothing

Physiological mechanisms
In a review of the literature on the causes of death during heat waves, 5 physiological mechanisms disrupting 7 vital organs have been identified (brain, heart, intestines, kidneys, liver, lungs, pancreas). The authors have identified 27 different ways in which heat-activated physiological mechanisms can lead to organ failure and ultimately death.

1- Ischemia.  When the human body is exposed to heat, the hypothalamus initiates a cardiovascular response by dilating the blood vessels to redistribute blood to the body surface (the skin) where heat can be dissipated into the environment. This compensatory process can lead to an insufficient supply of blood to the internal organs (ischemia) and consequently to a lack of oxygen (hypoxia).

2- Toxicity due to thermal shock.  High body temperature causes stress the body reacts to by producing stress proteins and free radicals that damage cells. This damage, combined with that caused by ischemia, affects the functioning of several organs.

3- Inflammatory response.  Erosion of the intestinal mucosa allows bacteria and endotoxins to enter the bloodstream, leading to sepsis and activation of a systemic inflammatory response. If hyperthermia persists, the exaggerated inflammatory response causes damage to various organs.

4- Disseminated intravascular coagulation.  Systemic inflammation and damage to the vascular endothelium caused by ischemia and heat shock can initiate this harmful mechanism. The proteins responsible for the control of coagulation become overactive and this can lead to the formation of clots that block the blood supply to vital organs. Depletion of blood clotting proteins can lead to subsequent bleeding (even in the absence of injury), which can be fatal.

5- Rhabdomyolysis.  This is the rapid degradation of skeletal muscle cells caused by heat shock and ischemia. Muscle proteins such as myoglobin are released into the bloodstream and are toxic to the kidneys and can lead to kidney failure.

The heart is hit hard
In the heart, the combination of ischemia, heat shock cytotoxicity, and hypokalemia (potassium deficiency caused by excessive sweating) can lead to cardiac muscle breakdown. This myocardial injury increases the risk of cardiac arrest due to loss of myofibrils and reduced efficiency of the body in controlling heart rate and blood pressure. Stress on the heart can be exacerbated by dehydration, which thickens the blood and causes vasoconstriction, increasing the risk of coronary thrombosis and stroke. In the pancreas, erosion of the endothelial lining allows leukocytes to infiltrate the tissue, exacerbating inflammation. In the brain, the permeability of the blood-brain barrier allows toxins and pathogens to enter, increasing the risk of neuronal damage. All these physiological responses are interconnected in such a way that the failure of one organ can lead to negative effects on others, initiating a vicious cycle of deterioration that often leads to permanent damage, long-term recovery, or death.

To prevent heat stroke (according to Peiris et al., JAMA, 2014):

  • Schedule outdoor activities during cool times of the day.
  • Drink plenty of fluids. Avoid drinks with too much sugar or alcohol, which can cause dehydration.
  • Wear loose-fitting, light-coloured clothing.
  • Acclimate to new hot environments, over many days if possible.
  • Be aware of medication side effects. If taking medications, be aware of those that may cause fluid losses, decrease sweating, or slow the heart rate. Common medications include those used for depression, blood pressure and heart disease, and coughs and colds.
  • Never leave an impaired adult or a child in a car unattended.

What to do if you suspect a heat stroke
Call 911 if you notice these signs of heat stroke: body temperature over 40°C; accelerated heart rate; accelerated breathing; hot and red skin; nausea or vomiting; change of mental state (confusion, headache, difficulty in articulating words, convulsions or coma).

What to do while you wait for help:

  • Move the individual out of the heat.
  • Remove clothing to promote cooling.
  • Position the person on his or her side to minimize aspiration.
  • Immerse the individual in cold water or apply cold, wet cloths or ice packs to the skin (neck, armpits, and groin areas, where large blood vessels are located) to lower the body temperature.
  • Continue cooling the individual until the body temperature reaches 38.4°C to 39°C (101°F to 102°F).
  • Do not give any fluids to the person because it is not safe to drink during an altered level of consciousness. If the person is alert and requests water, give small sips.
  • Avoid aspirin and acetaminophen; they do not help with cooling.
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