Extreme Temperatures Can Trigger Heart Failure Hospitalizations Within Days, Landmark Swedish Study Finds

Every time the mercury lurches toward an extreme — a deep freeze that settles over a region for days, or an unseasonal heat spike that turns city asphalt soft — the cardiovascular toll begins accumulating in hospital wards. A sweeping new analysis of nearly half a million hospital admissions in Sweden, published in JACC, the flagship journal of the American College of Cardiology, and presented simultaneously at ESC Congress 2026, has now charted that toll with unusual precision: short-term exposure to cold spells, to unusually low ambient temperatures, and to unusually high ambient temperatures all raise the risk that a person living with heart failure will end up in the hospital. Crucially, the risk is not spread evenly across time. Cold exposures set off a delayed wave of admissions that peaks three to five days after temperatures drop, while hot days strike almost immediately — a temporal asymmetry that researchers say could reshape how health systems issue warnings, plan staffing, and mobilize care for one of the world’s most burdensome chronic conditions.

“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,” said Wenli Ni, PhD, the study’s lead author and a postdoctoral research fellow at the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health. The gap she describes is more than an academic footnote. High-latitude nations such as Sweden are, in a sense, engineered around cold: buildings, infrastructure, daily routines, and even cardiovascular physiology have been shaped by generations of long winters, and cold exposure there is frequent rather than exceptional. Yet the same populations are now confronting an entirely different hazard — heat they have never acclimatized to — as global warming pushes summer temperatures beyond anything local health systems have previously had to plan for. “This represents a critical knowledge gap,” Ni noted, “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.”

To close that gap, the researchers turned to the Swedish National Patient Register, a nationwide repository of hospital care that captures the country’s entire population under its universal health system — one of the reasons Nordic registers are considered gold-standard infrastructure for epidemiology. From it, the team extracted 482,000 heart failure hospitalizations recorded between 2006 and 2021: sixteen consecutive years of admissions spanning brutal winters as well as several of Europe’s most punishing heat episodes. Temperature exposure was reconstructed at the municipal level, and the definitions of extreme events were deliberately local rather than absolute. A cold spell was counted when daily temperatures fell to or below the fifth percentile of a municipality’s own October-to-March distribution for at least two consecutive days, while heat waves were flagged when daily temperatures climbed to or above the ninety-fifth percentile of the local April-to-September distribution. This percentile-based, municipality-specific design matters because a winter afternoon that feels bitterly cold in the south of the country may be unremarkable in the far north. By anchoring thresholds to local norms, the approach implicitly accounts for how adapted each community is to its own climate, isolating the temperatures that are genuinely anomalous for a given place rather than merely impressive on a national map.

The pattern that emerged for cold was strikingly structured. Cold spells were associated with a higher risk of heart failure hospitalization within two to six days of exposure, with the signal reaching statistical significance between three and five days. Continuous exposure to lower ambient temperatures told the same story: elevated risk within three to six days, again significant between days three and five. That lag is where the physiology lives. When air temperature falls, the body defends its core by constricting blood vessels in the skin and extremities, a response that drives blood pressure upward and forces the heart to pump against greater resistance. Cold also thickens the blood, increasing viscosity and nudging clotting risk higher, while the metabolic cost of staying warm adds to the cardiac workload. For a heart that is already failing — one with little reserve capacity to begin with — this cascade can slowly push a compensated patient toward decompensation: fluid backs up into the lungs and legs, breathing deteriorates, and within days an admission becomes inevitable. The three-to-five-day delay between exposure and hospitalization is consistent with exactly that trajectory, a slow-burn physiological crisis rather than a sudden collapse.

Heat behaved differently, and the difference is clinically consequential. Exposure to higher ambient temperatures was linked to an increased risk of heart failure hospitalization over zero to three days — meaning the danger arrived with the heat itself rather than trailing behind it. The mechanisms here operate on a faster clock. To shed heat, the body diverts a large share of blood flow toward the skin and depends on sweating, which depletes circulating volume and concentrates the blood; a failing heart must substantially increase its output to support that cooling response, even as dehydration reduces the volume it needs to do so. Fluid and electrolyte disturbances can further destabilize heart rhythm in this state. Yet in a finding that may appear counterintuitive, the study detected no association between heat waves, as formally defined, and heart failure hospitalization. One plausible reading is that when daily hot-temperature exposure already carries a robust signal, overlaying a discrete multi-day “event” definition adds little extra predictive information; Sweden’s comparatively short and mild summers may also mean the heat-wave threshold captures fewer truly oppressive episodes than its cold counterpart does. The data cannot fully settle that question, but the practical takeaway stands: for vulnerable hearts, the hottest day is the dangerous day.

That asymmetry — cold risk that smolders for days, heat risk that detonates within hours — is precisely what makes the findings actionable. According to the authors, the delayed risk of hospitalization after cold temperatures and the immediate risk seen with hot temperatures underscore the need for time-sensitive management. In practice, that could mean intensified monitoring in the days after a cold alert: home telemonitoring of weight and blood pressure, medication reviews, early adjustment of diuretics, and proactive outreach to patients whose hearts are quietly sliding toward decompensation during that three-to-five-day window. Heat, by contrast, demands urgency on the day itself — same-day warnings, cooling strategies, hydration guidance, and rapid clinical escalation for symptomatic patients. Hospital administrators could likewise use temperature forecasts as a capacity signal, anticipating surges of heart failure admissions the way emergency departments already anticipate injury waves around major holidays. Weather, in other words, becomes a modifiable cardiac risk factor — one that can be forecast, communicated, and prepared for rather than simply endured.

The study is accompanied by an editorial comment from Tiantian Li, PhD, deputy director of the National Institute of Environmental Health at the Chinese Center for Disease Control and Prevention in Beijing, whose framing is deliberately blunt. “We can no longer practice cardiovascular medicine in a meteorological vacuum,” Li writes. The commentary argues that the field has passed an inflection point: the evidence tying non-optimal temperatures to cardiovascular harm is no longer provisional, and continuing simply to accumulate epidemiological associations is no longer the mission. “The evidence linking non-optimal temperature to cardiovascular harm is now substantial,” Li said. “It is time to translate these robust data into an actionable early-warning capability.” That call effectively reimagines the weather forecast as a clinical instrument — a system in which meteorological data, vulnerability maps, and patient registries converge into alerts that reach clinicians and the millions of people living with weakened hearts before the damage begins.

The broader context lends the findings added weight. Heart failure is among the most common reasons for hospitalization in high-income countries, and its prevalence rises steeply with age, which means the population exposed to temperature extremes is expanding at both ends of the thermometer: more elderly patients, and more extreme days. The sixteen-year Swedish window analyzed in the study encompassed not only severe winters but a string of record-setting European heat episodes, and climate projections indicate that both tails of the temperature distribution will continue to stretch in the decades ahead. The corresponding presentation, “Cold spells, heat waves, and non-optimal air temperature and risk of heart failure hospitalization in Sweden,” is being delivered at ESC Congress 2026 in a moderated poster session on Saturday, August 29, from 5:15 p.m. to 6 p.m. CEST, and the wider topic of environmental stress on the heart takes center stage in a dedicated ESC and JACC joint session, “Environmental cardiology: from exposure to action,” scheduled for Monday, August 31, at 11:15 a.m. CEST — signals that weather-driven cardiovascular risk is moving from the periphery of cardiology toward its core.

For the scientists behind the work, the next frontier is translation. The Swedish registers that made this analysis possible offer a template for other nations with longitudinal health data, and the lag structure the study uncovered hands modelers a concrete target: predict not merely whether risk will rise, but exactly when it will peak. If that can be operationalized, a five-day forecast could one day carry more than a probability of rain — it could carry a probability of decompensation, calibrated to local climate norms and delivered to the patients who need it most. Until then, the message for people with heart failure, and for the clinicians who treat them, is simpler and more immediate: the weather outside is not background noise. It is a physiological event, one that a vulnerable heart registers within days — and that medicine, at last, is learning to read.