This research analyzed data from 250,640 Swedish patients who died between 2006 and 2021, all of whom had been diagnosed with heart failure. The study used a time-stratified case-crossover design which linked individual health data with detailed daily temperature records across Sweden and measured at a 1 × 1-kilometer spatial resolution.
The study examined the effects of short-term exposure to low and high ambient temperatures on both all-cause and cardiovascular mortality. This research defined low and high temperatures using municipality-specific percentiles, where the coldest 2.5% and hottest 2.5% of daily temperatures were considered extreme. This approach accounted for regional climate variations and local adaptation.
The findings revealed a U-shaped association between temperature and mortality which indicates that both cold and heat increased the risk of death among patients with heart failure. For all-cause mortality, the odds of death rose by 13% during cold spells and by 5% during heatwaves. Similarly, for cardiovascular mortality, the risk increased by 16% in cold conditions and by 8% during hot conditions, particularly in the later study years.
The impact of high temperatures intensified between 2014 and 2021, suggesting that rising global temperatures and more frequent heat events are making heat exposure increasingly dangerous even in typically cool climates.
The study showed how mortality risks varied by temperature extremes and subgroups, where low temperatures had odds ratio (OR) around 1.130 for all-cause deaths and 1.160 for cardiovascular deaths. For high temperatures, the OR was around 1.054 for all-cause deaths and 1.084 for cardiovascular deaths (notably higher in recent years).
These results indicated greater cold sensitivity among men, diabetes patients, and diuretic users, while heat-related deaths were more frequent in patients with atrial fibrillation or flutter, especially in regions with higher ozone levels. The mean age of patients at death was 84.3 years, with nearly half (48.3%) being women. Overall, these findings warn that as temperatures continue to fluctuate more sharply due to climate change, heart failure patients in northern regions must be better protected.
Source:
Ni, W., Benson, L., Ljungman, P., Nobile, F., Breitner, S., Zhang, S., de Bont, J., Lund, L. H., Savarese, G., Schneider, A., & Agewall, S. (2025). Short-term exposure to low and high temperatures and mortality among patients with heart failure in Sweden. JAMA Cardiology. https://doi.org/10.1001/jamacardio.2025.3932
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