Heat Waves and Heat-Related Illness

Commentary

Global warming, climate change, and hot weather are topics of major and increasing interest. However, there is reason to believe that more than hot weather should be of concern, given the potential for extreme heat waves and their substantial health effects. Although heat waves have been relatively uncommon in temperate climates, and historically have been responsible for relatively low numbers of excess deaths, this is likely to change. For example, the 2003 severe heat wave experience in Lyon, France, where almost 15 000 people died of heatstroke¹ was more severe, more sudden, and inflicted substantially more morbidity and mortality than typical heat waves in temperate climates. This event may be a harbinger of what is to come in other similar geographic regions, and it is important for physicians, public health authorities, and communities to pay heed to lessons learned from this devastating experience.

Heatstroke, defined as a core temperature higher than 40.5°C and central nervous system dysfunction (delirium and coma),² is the most severe form of heat-related illness and is associated with a poor prognosis. Heat exhaustion is a less severe form of heat-related illness and is marked by nonspecific symptoms such as malaise, headache, and nausea. Why some patients progress from heat exhaustion to heatstroke is not well understood, but there is evidence that at some point a cascade of inflammatory events begins in the spectrum of heat-related illness and if uninterrupted, can lead eventually to multiorgan failure and death.² Prompt recognition, immediate cooling, hydration, and supportive measures are the central features of managing heat-related illness.

The study by Argaud and colleagues³ reported in the Archives of Internal Medicine is a welcome addition to the field of heat-related illness. This study, which involved patients who sustained heatstroke during the 2003 French heat wave, is one of the few prospective cohort studies that identified patients with heatstroke at the time of this severe heat wave and performed follow-up of the patients' clinical course over 2 years to study the long-term prognosis of heatstroke. The authors identified prognostic factors among 83 patients with heatstroke who were treated at a large inner-city university hospital in Lyon. The major factors associated with both short- and long-term outcomes were multiorgan system failure, higher body temperature, hypertensive medication use, residence in an institution, anuria, congestive heart failure, or comatose mental status on admission.

These poor outcomes were derived from a setting in which the hospital and community were not prepared for heat-related illness, a situation that probably is similar to most communities in temperate climates. It is likely these poor outcomes could be improved with better and more efficient methods of rapid core temperature cooling in the emergency department setting, better ambient temperature control in medical facilities, and better community-based preventive measures at the time of heat waves, particularly among those at high risk. In a separate study of this same heat wave, the presence of air conditioning in hospitals, particularly in the intensive care unit, was associated with better outcomes.⁴

Once heatstroke occurs, morbidity and mortality are high, with most deaths occurring in the home. As Arguad et al³ found, even individuals who survive to receive hospital care have a high mortality rate, reaching 58% at 28 days, and 71% by 2 years. Moreover, survivors of hospitalization continued to be severely functionally compromised up to 2 years after the index heat event.

Although ongoing research is examining innovative ways to treat heatstroke in hospital-based settings, the current dire prognosis indicates that prevention of heat-related illness would be more desirable and more effective in reducing morbidity and mortality. The health care system in Lyon was unprepared for this catastrophic heat wave, suggesting that it is possible that with more immediate and appropriate care in emergency departments the prognosis may be different than the Lyon experience. The local communities apparently were even less prepared, and unfortunately, even less research and resources are devoted to studying prevention of heat-related illness earlier in the course of illness or even in preclinical states among those at risk.

Identifying individuals at risk for heat-related illness in the preclinical setting is a challenge for the public health community. Studying heat wave–related illness is difficult because of the unpredictable nature of these phenomena, the high mortality rate in preclinical settings, and the lack of research infrastructure to study such health phenomena in community-based settings. In a related article in the Archives of Internal Medicine, Bouchama et al⁵ synthesized the observational data from case-control studies to estimate potential preclinical prognostic factors that predispose individuals to heatstroke mortality. The factors they identified included being confined to bed, not leaving home daily, inability to care for oneself, medical comorbidity (especially psychiatric, cardiovascular, and pulmonary illnesses), social contact, and access to environmental cooling (air conditioning, fans, baths, and showers). Ample evidence and modeling data support the likelihood that climate-related extreme weather changes are likely to increase in frequency and severity in the future.⁶ Given this likely reality, it is important to better understand the health-related effects of heat illness.

However, heat waves are not the only climactic phenomena that are likely to increase in frequency and severity and which threaten health and survival. Considering that many of these weather-related phenomena (such as hurricanes, tsunamis, droughts) are predictable and known modifiable factors affect outcomes, it is clear that better coordination is needed among public health communities, direct care facilities, and weather agencies. This requires warning systems, disaster policies, and relief efforts that systematically coordinate and integrate with government and nongovernmental organizations, as well as the local medical communities involved in responding to natural disasters. While some of this planning is already occurring in the United States,^{7 - 8} comprehensive municipal planning for heat waves is not adequate presently.⁹ Moreover, additional research funding is needed within clinical settings to perform the important research necessary to improve care for patients when such disasters occur.

Until then, intervening for severe heat waves and heat-related illness cannot wait for randomized controlled trials to test the effectiveness of interventions. A rational strategy would be to identify individuals at risk, particularly the vulnerable elderly (those with comorbidities, isolated, or infirm), and ensure that they have access to appropriate cool environments, preferably through activation of communities in coordination with weather agency predictions of impending heat waves. When heat-related illness is apparent, efforts should focus attention on rapid, early core temperature cooling with all reasonable means possible, followed by intensive supportive care as appropriate in hospital-based settings.