Study shows how temperature extremes can contribute to heart failure hospitalizations

Heart failure hospitalization risk increases after short-term exposure to cold spells, lower ambient temperatures and higher ambient temperatures, according to a study published in JACC, the flagship journal of the American College of Cardiology, and simultaneously presented at ESC Congress 2026. The increase in hospitalizations, seen in Sweden, shows how temperature extremes can contribute to sudden worsening of illness and the need for health systems to respond to and prepare for environmental stressors.

To the best of our knowledge, no previous study has examined the short-term associations between extreme temperature events (cold spells and heat waves) or continuous non-optimal ambient temperatures and heart failure hospitalizations within a Nordic population. This represents a critical knowledge gap, given that high-latitude settings such as Sweden are characterized by frequent cold exposure while simultaneously facing an emerging, unacclimatized vulnerability to heat as the global climate warms.“

Wenli Ni, PhD, lead author of the study and postdoctoral research fellow at the Center for Climate, Health, and the Global Environment, Harvard T.H. Chan School of Public Health

In this study, researchers looked at 482,000 heart failure hospitalizations in the Swedish National Patient Register between 2006-2021. Cold spells were defined as two or more consecutive days with daily temperatures less than or equal to the 5th percentile (October-March) and heat waves were defined as greater than or equal to the 95th percentile (April-September), using municipality-specific temperature distributions.

Cold spells were associated with a higher risk of heart failure hospitalization within two to six days, reaching statistical significance between three and five days. Lower ambient temperature exposure was associated with a higher risk of hospitalization within three to six days, reaching statistical significance between three to five days. Higher temperature exposure was linked to a higher risk of heart failure over zero to three days. There was no association between heat wave exposure and heart failure hospitalization.

According to the authors, the delayed risk of hospitalization after cold temperatures and immediate risk seen with hot temperatures underscores the need for time-sensitive management, including monitoring following cold alerts and more urgent response strategies during heat.

In an accompanying editorial comment, Tiantian Li, PhD, deputy director of the National Institute of Environmental Health, Chinese Center for Disease Control and Prevention in Beijing, said, “We can no longer practice cardiovascular medicine in a meteorological vacuum.”

“The evidence linking non-optimal temperature to cardiovascular harm is now substantial. The scientific mission can no longer rest on accumulating further epidemiological associations alone; it is time to translate these robust data into an actionable early-warning capability,” Li said.