"Many environmental exposures are set to be amplified by climate change," Barrak Alahmad, MD, MPH, PhD, a research fellow at the Harvard T. H. Chan School of Public Health at Harvard University in Boston and faculty member at the College of Public Health at Kuwait University in Kuwait City, told theheart.org | Medscape Cardiology.
"Investigating the burden of extreme temperatures from now on will enable us to further understand what climate change might hold for cardiovascular risks," he said. "In this rapidly changing climate and unprecedented pace of warming, it is not the time to be asleep at the wheel."
No specific temperatures are considered "extreme," Alahmad noted. "Heat and cold are context- and location-specific. The impact on human health of a given extreme temperature event (for example, a 40° C day) can depend on where and when it occurs; 40° C in Kuwait is a typical summer day, whereas a 40° C in London resulted in widespread, incalculable damage."
The study, published online December 12 in Circulation, showed that for every 1000 all-cause cardiovascular disease (CVD) deaths, 2.2 excess deaths were attributed to extremely hot days and 9.1 CVD deaths to extremely cold days. Across cities, sample temperature ranges varied from −30° C in Helsinki, Finland, to 44° C in Kuwait City, Kuwait.
"I'm urging professional cardiology societies like the American College of Cardiology and the American Heart Association, among others, to commission scientific statements that can provide exact guidance for healthcare providers," Alahmad said. "Meanwhile, I would [tell patients] if you have heart disease, it is prudent to take special precautions during extreme weather to protect the health of your heart."
Relative Risks Rise
The researchers used data from the Multi-Country Multi-City Collaborative Network to create a database of daily counts of cardiovascular causes of death from 567 cities in 27 countries across five continents in overlapping periods ranging from 1979 to 2019.
The analyses included deaths from any cardiovascular cause (32,154,935); ischemic heart disease (11,745,880); stroke (9,351,312); heart failure (3,673,723); and arrhythmia (670,859).
City-specific daily ambient temperatures were obtained from weather stations and climate reanalysis models, the authors note.
Using various models and calculation strategies, the team compared extreme temperature percentiles with the minimum mortality temperature (MMT) ― the temperature associated with least mortality ― in each location.
Pooled overall temperature and CVD mortality relationships were nonlinear; there was increased risk for mortality with both hot and cold temperatures. The relative risk (RR) of death increased gradually for cold temperatures below the MMT. The slope for hot temperatures was slightly steeper, especially with heart failure, where the RRs appeared to escalate quickly.
The pooled RRs of death associated with extreme heat (99th percentile vs MMT) were 1.07 for ischemic heart disease; 1.10 for ischemic stroke; and 1.12 for heart failure.
The pooled RRs of death associated with extreme cold (1st percentile vs MMT) from ischemic heart disease, ischemic stroke, and heart failure were 1.33, 1.32, and 1.37, respectively.
The risk of dying from arrhythmias was associated with greater uncertainty and a smaller effect size estimate for extreme heat (1.05) and cold (1.19).
Overall, a range of extreme temperatures, defined as hot days above the 97.5th percentile and cold days below 2.5th percentile, accounted for 2.2 and 9.1 excess deaths for every 1000 cardiovascular deaths, respectively.
Heart failure was associated with the highest excess deaths proportion from extreme hot days (2.6 per 1000 heart failure deaths) and cold days (12.8).
Results persisted after adjustment for temperature variability, heat waves, long-term trends, relative humidity, and air pollutants (eg, ozone, nitrogen dioxide, particulate matter with aerodynamic diameter).
Study limitations include underrepresentation of some regions, such as South Asia, the Middle East, and Africa, and potential modifiers at the individual level, such as age, sex, and education.
Contain Carbon Footprint
The first step to potentially mitigate the health impact of extremes of hot and cold temperatures that are associated with climate change "is for all of us to lower our carbon footprint," American Heart Association expert volunteer Nieca Goldberg, MD, medical director of Atria New York and clinical associate professor of Medicine at NYU Grossman School of Medicine in New York City, told theheart.org | Medscape Cardiology.
"We are seeing more electric cars on the road, and the fashion industry is focused on sustainability," she noted. "The healthcare industry needs to fund more research on the health consequences of climate change.
"In the office, primary care doctors and cardiologists need to counsel patients on avoiding outdoor activity in extreme temperatures for those with or at risk of cardiovascular disease," advised Goldberg, who was not involved in the study. "Also, following a heart-healthy lifestyle of a plant-based Mediterranean-style diet and walking can help lower cardiovascular risk and our carbon footprint."
The study was supported by the Kuwait Foundation for the Advancement of Science; the US Environmental Protection Agency; the Harvard Chan National Institute of Environmental Health Sciences Center for Environmental Health; the UK Medical Research Council; the UK Natural Environment Research Council; the European Union's Horizon 2020 Project Exhaustion; the Australian National Health and Medical Research Council; the National Institute of Environmental Health Sciences–funded HERCULES Center; the MCIN/AEI/10.13039/501100011033; the Taiwanese Ministry of Science and Technology; the Environmental Restoration and Conservation Agency, Environment Research and Technology Development Fund; the São Paulo Research Foundation; and Fundação para a Ciência e a Tecnlogia. The authors have disclosed no relevant financial relationships.
Circulation. Published online December 12, 2022. Abstract