Optimizing the Public Health Response to Heat Waves to Minimize Cardiovascular Risk

People living with cardiovascular diseases are at increased risk of mortality during heat waves. Social and environmental determinants of health play a dominant role in determining individual and collective risk. Cardiovascular clinicians and researchers are uniquely placed to help prevent heat-wave mortality by identifying those patients who are most at risk with education, referrals, and advocacy; contributing to planning of public health interventions; and informing research and development of knowledge. As our climate warms and heat-related risks increase, clinician leadership is essential to achieving population-level health objectives. Extreme heat events, in which temperature or humidity are sufficiently elevated to generate health effects at a population level, are an important consequence of the accelerating climate crisis in Canada. Heat-related mortality in Canada is significant and has increased over the past decade.1Hackett F. Pétrin-Desrosiers C. McGregor D. et al.The Lancet countdown on health and climate change: policy brief for Canada.Can Med Assoc Can Public Health Assoc. 2021; (Available at:) (Accessed December 19, 2022)https://policybase.cma.ca/viewer?file = %2Fmedia%2FPolicyPDF%2FPD22-01.pdf#page = 1Google Scholar Modelling studies predict a continued increase in heat-related mortality over the next decades, related to more hot days and longer heat waves. Patterns of heat-related illness, with mortality concentrated among specific populations—including the elderly, Indigenous communities, and people with underlying health conditions—are determined by individual, environmental, and social factors. Individual health status is an important marker of risk, given that most deaths during heat waves are caused by exacerbation of underlying chronic diseases, including cardiovascular and respiratory diseases, diabetes, and certain mental health conditions: notably, psychotic disorders. Data collected during the last 2 major heat waves in Montréal (2010 and 2018) clearly show that people with ischemic heart disease and hypertension are over-represented among those dying during heat waves, echoing findings of increased mortality risk for people living with cardiovascular diseases in studies elsewhere.2Khatana S.A.M. Werner R.M. Groeneveld P. Association of extreme heat and cardiovascular mortality in the United States: a county-level longitudinal analysis from 2008 to 2017.Circulation. 2022; 146: 249-261Crossref PubMed Scopus (11) Google Scholar Environmental inequities are an important determinant of heat-related mortality. People living in neighbourhoods with less vegetation, as well as those living on higher storeys of multistorey buildings, are more exposed to heat, especially at night. During the extreme heat event in Montréal in 2018, for example, persons living in intra-urban heat islands experienced double the mortality of those living in other parts of the city. A similar pattern was observed during the 2021 Heat Dome in British Columbia. Social isolation and poverty are also associated with increased mortality during extreme heat events, although the exact mechanisms at an individual level have not been thoroughly elucidated. The vast majority of individuals who died during the 2018 heat wave in Montréal lived alone, and data from 2 heat waves in Chicago in the 1990s showed that people who had poorer access to transportation and those who left their homes less often had higher mortality. Social isolation likely increases risk because people who live alone and do not have frequent social contacts are less likely to access care for insidious heat-related symptoms, especially during prolonged exposure to elevated temperatures at night. Unfavourable social and environmental conditions are also strongly linked. Lower-income people and households are more likely to live in hotter neighbourhoods and have lower access to air conditioning at home. The public health approach to reducing heat-wave mortality is multipronged and touches on all core public health functions. The foundation for public health intervention consists of medium- and long-term actions targeting these underlying determinants: for example, through community development; urban greening; investments in social and affordable housing, including funding for climate-resilient building strategies; and public policies aimed at eliminating poverty. Heat-response plans (HRPs)—organized approaches to preventing and mitigating the health effects of extreme heat—have been developed in most North American cities over the past 2 decades. A growing body of literature suggests that the implementation of such plans may reduce heat-related mortality in addition to reducing inequalities in risk among groups of the population.3Dwyer I.J. Barry S.J.E. Megiddo I. White C.J. Evaluations of heat action plans for reducing the health impacts of extreme heat: methodological developments (2012–2021) and remaining challenges.Int J Biometeorol. 2022; 66: 1915-1927Crossref PubMed Scopus (6) Google Scholar Postevent analyses of the 2018 heat wave in Montréal and comparison with the 2010 event suggest that the population mortality rate was lower in 2018, potentially indicating a benefit of the enhanced plan implemented after the 2010 heat wave. So, based on our experience of the past decade in Montréal and elsewhere in Canada, how can we do better? And what can clinicians and researchers do to reduce their patients’ risk of heat-related illness? First, clinicians, public health practitioners, and policy makers must recognize and approach heat-related illness as a complex social issue, rather than a question simply of exposure to heat or individual health status. Data showing that many of the populations most at risk from heat waves have also suffered the highest effects—direct and indirect—of the pandemic underline the importance of shared social and environmental factors. Although cardiovascular disease is undoubtedly a marker of increased risk, it is neither a necessary nor a sufficient factor to explain vulnerability to extreme heat. Although individual interventions tailored specifically to people living with cardiovascular disease—regarding, for example, medication management, water consumption, and cooling strategies—are unlikely to be game changers, clinicians have several important roles to play in reducing cardiovascular risk. The specific contribution of social isolation to overall risk cannot be overstated and presents a significant limitation to individual-level and population-level public health strategies: for example, those based on home-care services. The very fact that people are not connected to services at baseline means that they are unlikely to be reached during an extreme heat event. This also means that clinicians are in a unique position to contribute, to the extent that they are in direct contact with patients, some who may not have any other points of contact with social institutions. A patient encounter is, first of all, a powerful tool for generating social connection. Although it may be the cardiovascular symptoms or disease that brings patients into contact with care providers, the clinician has an opportunity to enquire about social and environmental conditions, identifying those patients who are most at risk, and tailoring interventions accordingly. In some jurisdictions, information regarding individual risk can also be fed into heat registries that allow for quicker and more efficient interventions, targeting those most at risk during a heat wave: for example, through automated phone warning systems. The clinician also has an important educational role: providing basic information regarding the physiology of heat including symptoms, warning signs, and when to access care. Physicians and care teams can help patients identify an individual HRP, including, for example, how to access a cool space during a heat wave: notably, during the night-time hours. This is relevant in a variety of clinical contexts, including inpatient (discharge planning) and outpatient (chronic disease follow-up, home care) settings. During heat waves, clinicians are an indispensable source of information regarding the health status of individuals and population groups. In Montréal, targeted interventions during heat waves are often based on notifications provided by physicians and paramedics. A notification from a clinician who sees a patient suffering from decompensated heart disease during a heat wave may be the starting point for a public health intervention in a collective living environment: for example, a seniors’ residence, social housing, or rooming house. Developing and implementing effective public health policies require establishing and potentiating collaborations among a diversity of actors. Heat response plans already provide a framework for collaboration, and clinicians are uniquely positioned to forge new links among patients, other care providers, and public health authorities. The frontline perspectives of clinicians are essential to adjusting and improving public health interventions. In addition, clinicians and care teams can help link patients to community resources, which are often an underused asset in the response to extreme heat. Finally, development of interventions must involve at-risk individuals at all stages: from planning to intervention, evaluation, and quality improvement. This is not only important for tailoring interventions to the needs of those most at risk but can also be a tool for generating social connection and reducing baseline social risk. Clinicians can solicit feedback from their patients with regard to their needs and understanding of public health measures and amplify patient voices in conversations with public health authorities and decision makers. Despite evidence of the effectiveness of HRPs, it is difficult to identify the specific interventions that have most impact, as HRPs are composed of many interventions involving multiple intersectoral partners. For example, in Montréal, the plan includes everything from ensuring that long-term care facilities have functional air conditioners to door-to-door canvassing in targeted sectors and opening cooling shelters during a heat wave. As the number of hot days and extreme heat events increase over the next decades, municipalities and public health units will need to adapt their HRPs, which tend to be resource intensive and designed to deal with relatively infrequent events. Evaluative research looking at the critical, highest-value components of HRPs is crucial to improving health outcomes in the context of accelerating heat. Despite several decades of experience with heat waves and a significant descriptive literature, we still have relatively few real-world data on the effectiveness of individual-level interventions. The messages put forward by public health units—including drinking water regularly, spending several hours a day in a cool place, and taking a cool bath or shower—are based on what we know about the physiology of heat-related illness and—to a lesser extent—on descriptive studies of extreme-heat events. A handful of experimental studies suggest that relatively simple interventions—including use of an electric fan, immersing feet in lukewarm water, and wearing a wet t-shirt—may have beneficial effects on core temperature and cardiovascular indicators, depending on temperature, humidity, and individual risk factors.4Cramer M. Huang M.U. Moralez G. Crandall C.G. Keeping older individuals cool in hot and moderately humid conditions: wetted clothing with and without an electric fan.J Appl Physiol. 2020; 128: 604-611Crossref PubMed Scopus (22) Google Scholar The impact of these interventions remains to be tested in real-world conditions. The true burden of mortality and morbidity related to extreme heat is difficult to quantify because of insufficient and fragmented surveillance systems. More work needs to be done to develop robust and integrated surveillance systems that respond to the needs of clinicians and public health practitioners. Finally, the effects of public policies targeting the underlying social, economic, and environmental determinants of heat-related morbidity and mortality must be better characterized and quantified. The most effective, durable, and equitable approach to reducing heat-related risk in people living with cardiovascular disease is one founded on improving the social and environmental conditions in which people live. Cardiovascular clinicians, care teams, and researchers can play crucial roles in protecting patients and reducing their vulnerability through risk assessment; education; referral; advocacy; nurturing collaborative relationships with patients, public health authorities, and communities; and developing evidence to inform action.5Andermann A. Taking action on the social determinants of health in clinical practice: a framework for health professionals.Can Med Assoc J. 2016; 188: 17-18Crossref PubMed Scopus (267) Google Scholar