Clinical Presentation

The clinical manifestations of heat-related illness vary according to severity (Table 1). Heat stroke is characterized by the triad of hyperthermia, neurologic abnormalities, and recent exposure to hot weather (classic), physical exertion (exertional), or both.12 Tachycardia, tachypnea, and hypotension are common. Sweating is typical of exertional heat stroke, whereas in cases of classic heat stroke, the skin is often hot and dry.12 A change in mental status (e.g., confusion or delirium) best differentiates heat stroke from heat exhaustion and other milder forms of heat-related illness.24 Early manifestations include behavioral changes, confusion, delirium, dizziness, weakness, agitation, combativeness, slurred speech, nausea, and vomiting.25 Seizures and sphincter incontinence may occur in severe cases.26 Heat stroke characteristically manifests in three phases: a hyperthermic–neurologic acute phase, a hematologic–enzymatic phase (characterized by inflammation and coagulopathy and peaking at 24 to 48 hours after onset), and a late hepatic–renal phase (characterized by organ failure and occurring 96 hours or longer after onset), all of which have been well described elsewhere.12

Evaluation and Diagnosis Table 2. Table 2. Medications and Drugs with Potential to Increase Risk of Heat-Related Illness.

Heat-related illness may occur in the absence of a heat wave, and a high index of suspicion is warranted in the context of suggestive symptoms. The initial evaluation should include prompt assessment of the patient’s preceding heat exposure and exertion (on the basis of history obtained from emergency medical services personnel, the patient, or other sources) and measurement of the core temperature, recognizing that this may have decreased by the time of assessment. Important historical information includes the patient’s occupation, degree of preceding physical exertion, home environment, coexisting conditions, and use of medications or other drugs that may increase the risk of heat-related illness (Table 2). The presenting symptoms of heat stroke can mimic many other illnesses, including sepsis, ischemic stroke, and toxicologic or endocrinologic emergencies, particularly if the core body temperature is not measured.12 These other conditions must be considered, but treatment for heat stroke should not be delayed, because rapid intervention is paramount to prevent serious complications and death.29 Routine assessment of patients presenting with presumed heat stroke includes a complete blood count, complete metabolic panel, urinalysis, urine drug screen, prothrombin time and partial thromboplastin time, creatine kinase level, an electrocardiogram, and a chest radiograph if signs of respiratory involvement are present.

Management Mild-to-Moderate Heat-Related Illness Figure 2. Figure 2. Algorithm for the Diagnosis and Management of Heat-Related Illness.

If cold-water immersion is not feasible, consider the use of intravascular cooling devices; a three-way Foley catheter for bladder irrigation; infusion of chilled fluids; placement of ice packs to axilla, groin, and neck; or misting of the patient with water and directly fanning. These methods may also be used to augment cold-water immersion if the core temperature is not decreasing at a rate of 0.20° to 0.35°C per minute.

Evidence to inform the treatment of mild or moderate heat-related illness is limited. Society-based and expert guidelines, largely based on clinical experience and observational studies, are summarized in Table 1, and an algorithm for the treatment of heat-related conditions is provided in Figure 2. Heat exhaustion exists on a continuum with heat stroke, and its treatment is dictated by the severity of symptoms. Mild cases can be managed with passive cooling and rehydration, whereas moderate cases typically warrant active management with convective cooling (i.e., with the use of fans), infusion of cold fluids, and close monitoring.10

Heat Stroke

For patients with heat stroke, treatment begins with maintaining the airway, breathing, and circulation, immediately followed by rapid cooling. Because delays in cooling are associated with worse outcomes, initial management is focused on rapidly reducing the core body temperature to 38° to 39°C, ideally within 30 minutes after presentation.30 The most effective cooling methods are cold-water immersion and ice-water immersion31; a crossover study found no difference in cooling rates between these methods.32 A rapid rate of cooling, ideally 0.20° to 0.35°C per minute, with continuous monitoring of the core temperature, is safe and has been associated with a better prognosis than slower cooling in observational studies.12 In settings outside the hospital, where immersion is not available, a cooling rate of 0.10°C per minute can be achieved by pouring copious amounts of water over the victim and fanning.33 If airway compromise, ongoing cardiopulmonary resuscitation, or resource availability prevents cold-water immersion, treatment involves a mixture of evaporative and conductive cooling methods, including infusion of cold fluids; application of ice packs to the neck, groin, and axillae; and fanning.12 Intravascular cooling can also be used.13 Monitoring of the core temperature throughout the cooling process is imperative, with a goal of reaching normothermia. It is recommended that cooling measures be taken before and during transport to the hospital whenever possible.34

Antipyretic agents should not be used; they are ineffective in patients with heat stroke and may aggravate coagulopathy and end-organ damage.10 Dantrolene has been associated with reduced cooling time but not with improved rates of recovery; it is not used to treat heat stroke in practice.35 Benzodiazepines may be used to control agitation, discomfort, and shivering.

Patients who are successfully cooled and survive the hyperthermic–neurologic phase are at high risk for progression to the hematologic–enzymatic and late hepatic–renal phases. These patients are best treated in an intensive care setting with a multidisciplinary team.13

Risk Reduction Table 3. Table 3. Prevention Strategies for the General Population and for Specific Groups.

Because evidence from randomized trials is limited,36 strategies to prevent heat-related illness are guided largely by clinical experience and observational data. These data support the benefits of screening for risk and of the use of behavioral interventions, particularly those targeted to susceptible persons and their caregivers, athletes, and outdoor workers. Table 3 outlines strategies for risk reduction.

Expert guidelines recommend that before the warm season, clinicians should identify heat-vulnerable patients (Figure 1) and alert them and their caregivers to their potential risks, noting that a single day of extreme heat poses a threat; counsel them about the identification of high-risk heat conditions (Fig. S1 in the Supplementary Appendix, available with the full text of this article at NEJM.org) and the signs and symptoms of heat-related illness; and provide clear instructions about how to reduce risks and when to seek medical attention.27,38 A randomized trial of a preventive-messages tool for older adults in Australia showed significant uptake of behavioral strategies and a 63% lower risk of self-reported heat stress among those who received the intervention than among those who did not.36 General guidance to reduce risk includes keeping living spaces cool with fans, air conditioning, or misting (with the caveat that fans are likely to be ineffective at temperatures >99°F [>37.2°C] and may be detrimental in dry conditions40), locating cooling centers or other accessible places with cooling, limiting physical activity, increasing water intake, wearing lightweight clothing, self-dousing and taking cool showers, and monitoring for symptoms, including frequent safety checks from caregivers.39

Athletes and persons who manage athletic practices and events should be counseled that heat acclimatization, which involves short periods (1 to 2 hours) of heat-exposed exertion each day over a period of 10 to 14 days, has been shown to increase the ability of the body to tolerate and dissipate heat.10 In higher-risk heat conditions, practices and events should be scheduled at cooler times of the day and allow for more rest breaks.43 Clinicians can refer to region-specific exercise-modification tables for additional guidance43 and should encourage organizers of sporting events to have trained personnel on site and the capacity to provide cold-water immersion if needed.43

Preventive actions are challenging for many workers who perform heavy labor in hot conditions.44 Outdoor workers often have little control over their work environment and activities, which complicates normal behavioral responses to heat.45 According to data from the Centers for Disease Control and Prevention, crop workers die from heat stroke at a rate nearly 20 times that of U.S. civilian workers overall; most fatalities occur among adults 30 to 54 years of age, with a majority of victims being foreign-born workers.46 Factors contributing to the high risk of heat-related illness in persons who work outdoors include direct exposure to heat (indoor and outdoor), extreme physical exertion, heat-trapping protective work clothing, and job insecurity.47 Despite convincing evidence of the negative health effects of increased workplace heat, there is no dedicated U.S. standard that specifically addresses occupational heat exposure, although there are efforts under way to create one,48 and certain states (e.g., California and Washington) have standards and regulations in place.