It's hot and it's going to get hotter. Climate records show that 15 of the past 20 years have been the warmest on record.1 Since the late 19th century, there has been a worldwide inoculation of greenhouse gases leading to accelerated global warming.1,2 An increased number of heat waves, defined as air temperatures of 32.2°C (90°F) or higher for three or more consecutive days, is likely to accompany this trend. As a result, the Intergovernmental Panel on Climate Change predicts significant increases in heat-related mortality for urban areas of North America in the future.3-5
In the United States, heat-related deaths average approximately 400 per year and are the most frequent environmentally related cause of death.5 Over the past decade, cities throughout the world have experienced epidemic death rates during heat waves.3,5,6 This is true even though the pathophysiology of heatstroke (HS) is well understood and strategies for its treatment and prevention are well known.7
HS occurs when homeostatic thermoregulatory mechanisms fail, causing an elevation in core body temperature to higher than 40°C (104°F), CNS dysfunction such as confusion or seizures, and organ damage and death if not treated aggressively.7-9 Risk factors for HS include prolonged exposure to high environmental temperatures, lack of acclimatization to warm environments, poor physical condition, and wearing excessive clothing in high temperatures.7,9-11 People with preexisting cardiac, vascular, or respiratory disease; obesity; or alcoholism are at high risk for HS.9,11 Numerous medications are also associated with an increased risk for HS (see Table 1).9,11 Anhydrosis is no longer considered an essential element for the diagnosis of HS because some persons who develop HS while exercising continue to perspire.9
Classic and exertional heat stroke
HS is categorized as either classic (CHS) or exertional (EHS) (see Table 2).
Classic heatstroke This form of HS typically occurs during periods of high ambient temperatures; epidemics in temperate climates correlate with spring or early summer heat waves. The number of HS cases escalates about 24 hours after very hot days; abnormally high night temperatures particularly increase the risk for CHS. Persons older than 75 years or those who have preexisting medical conditions, children youn
ger than 5 years, and urban dwellers are most vulnerable.7-9,12 Access to an air-conditioned environment is the most important factor for preventing CHS.3,5-7
About half of heat-related deaths occur in adults older than 65 years, who are at increased risk for a number of reasons.9 When dehydrated, they tend to maintain blood volume via a compensatory loss of fluids from the intracellular compartment.13 They also have an altered response to heat stress, including reductions in sweat gland function, skin blood flow, and cardiac output.14
Young children produce more metabolic heat than do adults and have less efficient mechanisms for dissipating heat. Children are also less likely to reduce activity in extreme heat and can experience a faster rise in core temperature when dehydrated.9
Exertional heatstroke This form of HS occurs when the skin and lungs are unable to dissipate heat generated by working skeletal muscle.12,15-17 With heavy exercise, net heat production can rapidly result in extreme hyperthermia. The greatest predisposing factor to EHS is prolonged physical activity in high ambient temperature.15,16,18 Dehydration, insufficient caloric intake, and electrolyte imbalances can all exacerbate EHS.16 Normally, compensatory mechanisms such as sweating, increased respiration, changes in circulatory distribution, and renal function can moderate this heat gain.10 EHS occurs when these mechanisms fail.8
Those who are young and in apparent good health are the most common victims of EHS, most likely because this age-group often participates in strenuous activity in high temperatures.8,10,12,15-17 HS is the third leading cause of death among athletes in the United States.19,20 People can suffer EHS while participating in organized sports. Occupation may also increase risk: For example, foundry workers, firefighters, and military personnel may experience EHS.9 In 1995 alone, 700 deaths reportedly resulted from EHS.12 Among survivors, evidence of brain damage is reported to be as high as 14% to 20%.6,21,22
Physiology
The hypothalamus is the central integrator for maintaining temperature in a homeothermic range.7,9 It receives afferent signals from thermoreceptors in the skin and CNS and responds by activating elements of both the sympathetic and parasympathetic pathways.7 The body's initial reactions to heat stress include peripheral vasodilation, activation of sweat glands, and tachycardia. These mechanisms combine to transfer internal heat to the body surface, where it can dissipate under most conditions.7,9,14 Adaptation, which generally takes several days in a hot climate, includes increases in plasma volume, sweating capacity, and cardiovascular performance.7
Whether CHS or EHS is the cause, the underlying pathophysiology of HS is the same. It occurs when the body's thermoregulatory system fails in the face of heat stress, which develops from a combination of metabolic production plus environmental heat.9 Homeostatic mechanisms favor thermogenesis and heat retention. Humans have a limited capacity to dissipate excess body heat effectively, and the two primary mechanisms for heat dissipation—peripheral vasodilation and perspiration—are inefficient in excessive heat or humidity.
Heat transfer The rate of heat exchange increases when the temperature difference between substances increases and when the surface area available for heat exchange is augmented. Insulation of any kind interferes with heat transfer, so it is important to uncover a patient with HS as much as possible.
The four methods of heat transfer are radiation, evaporation, conduction, and convection.
Radiation involves the transfer of heat via electromagnetic waves. The quantity of heat transferred via radiation is directly related to the mass and proximity of the objects. The effects of radiation are felt in direct sunlight or when standing near a fire. While radiation can add to heat stress, it does not play a role in treating HS. In urban areas, buildings and asphalt create a “heat island effect” by absorbing heat during the day and radiating it throughout the night, contributing to increased HS risk in urban areas during heat waves.
Conduction involves the direct transfer of heat from one substance to another. There is minimal conductive heat transfer in warm, still air, although water is approximately 30 times more effective than air for conducting heat. As such, immersing a patient in cool water can be an extremely efficient method for treating HS in some settings, although monitoring and rapid access to the patient are limited. Immersion has been used effectively for treating athletes and soldiers.8,15,23