"There is seasonal variation in the occurrence of heart attack, with incidence declining in summer and peaking in winter," author Moman A. Mohammad from the Department of Cardiology at Lund University said in a release issued by the university.
"It is unclear whether this is due to colder temperatures or behavioral changes," Mohammad added.
The authors used the Swedish myocardial infarction registry (SWEDEHEART) to track the 280,873 heart attacks that occurred between 1998 and 2013. The report also tracked meteorological data from the Swedish Meteorological and Hydrological Institute and calculated an average daily minimum temperature for the whole country as well as different health care regions.
The findings reveal that heart attacks are significantly more frequent during sub-zero temperatures across the country and across health care regions. This means that on a day when the average temperature is below 0 degrees Celsius, four more heart attacks are likely to happen than on a day where the temperature is above 10 degrees Celsius.
The incidence of heart attacks also increases with higher wind speeds and air humidity and shorter periods of sunshine.
The investigators also explored the relationship between heart attack frequency and seasonal variations in different groups: the elderly, people with diabetes mellitus and/or hypertension, as well as patients who previously suffered heart attacks or take certain medications. Even across all these different subgroups, the original findings stand.
According to Mohammad, "Our results consistently showed a higher occurrence of heart attacks in sub-zero temperatures. The findings were the same across a large range of patient subgroups, and at national as well as regional levels, suggesting that air temperature is a trigger for heart attack."
The researchers believe that the relationship between cold weather and heart attacks may be due to how the body's superficial blood vessels constrict in the cold, increasing arterial blood pressure.
"In the majority of healthy people these mechanisms are well tolerated," Mohammad said. "But in people with atherosclerotic plaques in their coronary arteries, they may trigger a heart attack."
"Respiratory tract infections and influenza are known risk factors for heart attack that have a clear seasonal variation. In addition, seasonal-dependent behaviors such as reduced physical activity and dietary changes could play a role in the increased occurrence of heart attack during colder weather," he explained.One limitation of the observational study, however, is that other factors like respiratory tract infections, which are more common during winter seasons, were not taken into account.
Another study published in the British Medical Journal in 2010 evaluated four years of hospital admissions from 15 hospitals across the UK, and also found that a daily temperature drop of 1 degree Celsius increased the prevalence of heart attacks.