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Heartwarming spring news from SWEDEHEART


As weather warms, heart attack risks may decline, says a Lund University study.

Heart attacks seem more common in winter, doctors have long observed, including in a nearly 100-year-old clinical paper. To see if a rigorous analysis supports this observation, Lund University researchers merged data from SWEDEHEART, the registry for acute cardiac care outcomes in Sweden, and government data from weather stations across the country. The scientists meticulously matched clinical information on each Swedish heart attack patient with detailed weather data for the patient’s cardiac care unit on the day of the event.

The raw data spanned 16 years of more than 800,000 cardiac patients and 3.5 million weather measurements. The final study population had nearly 281,000 patients with myocardial infarctions (heart attacks) and sufficient weather data.

Senior study author David Erlinge, head of the Lund University Department of Cardiology, says “The strength of the study was the nationwide data. In SWEDEHEART, every myocardial infarction in hospital care is counted, with continuous complete data gathering.”

Lead author Moman Mohammad took years to organize, align, and analyze the large clinical and meterological datasets. Rather than using summaries such as mean temperature for an area, the team matched each patient with precise, daily weather data. Erlinge says, “Using patient-level information, we could dissect out different parts of the weather. We didn’t lose any exactness.”

Keep warm

The results showed that cold air temperature had the strongest association with heart attacks. Low atmospheric pressure, high wind, and less sunshine were also linked. Rain and snow effects were not significant overall. Possible reasons for the strong effects of cold and wind are that they can cause blood vessel constriction, straining the heart and worsening existing heart disease. Other winter factors such as respiratory infections and changes in exercise, diet, or mood may contribute. The Erlinge group is interested in studying why and how weather affects cardiovascular events.

The results are useful for health systems considering how to allocate resources and patients seeking to reduce heart risks. The study process has lessons for researchers working with massive, disparate data sources: Allow lots of time to thoroughly organize and align the data and conduct multiple accuracy checks.

The next step

The Erlinge group has studied other environmental effects on the heart. They report in another paper that Christmas and midsummer holidays, but not two major Swedish sporting events, were linked to higher heart attack risks. The next step is finding mechanisms that link heart events and external factors. For example, holidays might increase stress, loneliness, or other risk factors, so the team plans to contact patients who had heart attacks at Christmas to learn about their experiences.


Mohammad MA, Koul S, Rylance R, Fröbert O, Alfredsson J, Sahlén A, Witt N, Jernberg T, Muller J, Erlinge D. Association of Weather With Day-to-Day Incidence of Myocardial Infarction A SWEDEHEART Nationwide Observational Study. JAMA Cardiol. 2018;3(11):1081-1089. doi: 10.1001/jamacardio.2018.3466