Short-term exposure to cold spells, lower temperatures, and higher temperatures was associated with a higher risk of HF hospitalization, with delayed cold effects and more immediate heat effects.
Key Findings
Results
Cold spells during the cold season were associated with an 8.5% higher cumulative odds of heart failure hospitalization over lag 2 to 6 days.
Cold spells were defined as ≥2 consecutive days with daily mean temperature ≤5th percentile during October to March using municipality-specific temperature distributions.
Cumulative association over lag 2 to 6 days: OR 1.085 (95% CI: 1.042-1.129).
Associations were statistically significant at lag 3 to 5 days specifically.
Study included 482,000 HF hospitalizations from the Swedish National Patient Register (2006-2021).
Analysis used a nationwide time-stratified case-crossover design with conditional logistic regression combined with distributed lag nonlinear models.
Results
Lower ambient temperatures during the cold season were associated with higher odds of heart failure hospitalization per 10-percentile-point decrease in temperature.
Cumulative association over lag 2 to 6 days: OR 1.008 (95% CI: 1.005-1.011) per 10-percentile-point decrease in temperature.
Significant associations were observed at lag 2 to 6 days, with significance specifically at lag 3 to 5 days.
The cold effect was delayed compared to the heat effect, which appeared more immediately.
Municipality-specific temperature percentile distributions were used to define temperature exposure.
Results
Higher ambient temperatures during the warm season were associated with higher odds of heart failure hospitalization with a more immediate onset.
Cumulative association over lag 0 to 3 days: OR 1.009 (95% CI: 1.006-1.013) per 10-percentile-point increase in temperature.
Associations were observed at lag 1 to 3 days with attenuation at longer lags.
The warm season was defined as April to September.
Heat effects appeared more rapidly than cold effects, which were delayed to lag 2-6 days.
Results
Heat waves showed nonsignificant positive associations with heart failure hospitalizations when a narrower, acute-exacerbation-focused HF definition was used, and no associations when a broader HF definition including chronic conditions was applied.
Heat waves were defined as ≥2 consecutive days with daily mean temperature ≥95th percentile during April to September using municipality-specific distributions.
The nonsignificant positive associations for heat waves were observed under the HF definition more likely to capture acute exacerbations.
No associations were observed using the broader HF definition that included chronic conditions.
This difference by HF definition suggests heat waves may differentially affect acute versus chronic presentations of HF.
Methods
The study was a nationwide time-stratified case-crossover study of 482,000 heart failure hospitalizations in Sweden over 15 years.
Data were drawn from the Swedish National Patient Register covering 2006 to 2021.
The time-stratified case-crossover design was used to control for time-invariant individual confounders.
Temperature exposure was assessed at the municipality level using municipality-specific percentile distributions for both cold and warm seasons.
Lag-specific associations were estimated from lag 0 to 6 days using distributed lag nonlinear models.
The study setting is a high-latitude country (Sweden), which the authors note is an understudied climate context for this research question.
Results
Both cold and heat exposures were independently associated with increased heart failure hospitalization risk, indicating a U- or J-shaped relationship between temperature and HF risk.
Cold effects were significant cumulatively over lag 2-6 days; heat effects were significant cumulatively over lag 0-3 days.
The authors note that 'extreme temperature events are projected to increase with climate change,' framing findings in a public health context.
Associations were observed for both discrete extreme events (cold spells, heat waves) and continuous temperature metrics.
The study characterized both the timing (lag structure) and magnitude of these associations.
What This Means
This research suggests that both cold and hot weather increase the risk of being hospitalized for heart failure, but in different ways and on different timelines. Using data from nearly half a million heart failure hospitalizations across Sweden between 2006 and 2021, researchers found that cold spells and lower temperatures during winter months were associated with an increased risk of hospitalization, but this risk peaked several days after the cold exposure — around 3 to 5 days later. In contrast, higher temperatures during summer months were linked to increased hospitalization risk more quickly, within 1 to 3 days. Extreme cold spells were associated with about an 8.5% higher cumulative odds of heart failure hospitalization compared to non-cold-spell periods.
For extreme heat waves, the picture was more nuanced: there were positive but statistically nonsignificant associations with heart failure hospitalizations when looking only at acute (sudden) heart failure cases, and no association at all when a broader definition including chronic heart failure was used. This suggests that heat waves may be more relevant to triggering sudden worsening of the condition rather than influencing chronic disease management visits.
This research matters because climate change is expected to increase both the frequency and severity of extreme temperature events, including heat waves and cold spells. These findings, drawn from a high-latitude country like Sweden that is often underrepresented in climate-health research, suggest that both ends of the temperature spectrum pose risks for people with heart failure. Understanding the timing of these risks — cold effects being delayed by several days and heat effects being more immediate — could help healthcare systems prepare for surges in hospitalizations during and after extreme weather events.
Ni W, Agewall S, Benson L, Breitner S, Nobile F, Savarese G, et al.. (2026). Cold Spells, Heat Waves, and Nonoptimal Air Temperature and Risk of Heart Failure Hospitalization in Sweden.. Journal of the American College of Cardiology. https://doi.org/10.1016/j.jacc.2026.07.008