For the study, researchers analyzed data from more than 274,000 patients who had been admitted to a coronary care unit in Sweden for a heart attack between 1998 and 2013. Using meteorological data, they then looked to see what the weather was like on the days of those medical emergencies in the specific regions where the hospitals were located.
They found that the incidence of heart attack was higher on days with lower air temperature, lower atmospheric air pressure, higher wind velocity and fewer hours of sunshine.
The association was particularly strong when air temperatures fell below 32 degrees Fahrenheit.
When the researchers broke down the data by geographical region, they also found a “pronounced” association between heart attacks and snow and wind in Sweden’s northernmost latitudes.
“Our results not only suggest that weather is independently associated with the incidence of [heart attack] but also that the association may differ with regard to season,” the study’s authors write.
As background information in the study points out, the idea that heart attacks occur more often during winter has been reported in medical journals as far back as 1926, when researchers reported the phenomenon among residents of New England.
Most previous studies linking weather and heart attacks tended to focus only on cold temperatures and snow, however. And most did not use clinically validated diagnoses of heart attacks for their data.
The current study was designed to overcome those problems — and to include a very large number of participants.
Still, the study was observational, which means it can’t prove that wintry weather causes heart attacks. Also, the effects found in the study were modest. An increase of 45 degrees Fahrenheit was associated with a 2.8 percent decrease in the risk of heart attack.
Yet, given that 7.9 million Americans have heart attacks each year (about one every 40 seconds), even a 2.8 percent decline is significant, as it represents about 181,000 people.
As Gary Jennings, chief medical adviser of the National Heart Foundation of Australia, points out in a commentary for The Conversation, winter’s link with heart attacks has a plausible biological explanation:
There is a clear association between cold and artery function (the vessels that deliver oxygenated blood from the heart to other parts of the body). This can be illustrated by a common physiology lab manoeuvre known as the cold pressor test. People are asked to put their forearm into iced water. Blood pressure rises immediately because arteries constrict, presumably to maintain core body temperature at normal levels.
Simple hydrodynamics tells us constriction is more profound and impacts more on the flow through a tube – in this case a coronary artery – at points of obstruction. In a few people with coronary disease the cold pressor test is enough to cause the artery to spasm and for flow to cease until the artery relaxes again.
But other factors also make heart attacks more likely in winter.
“In many places, air pollution is more common, and evidence is accumulating that certain particles in the air are related to heart disease,” Jennings explains. “Winter is also flu season, which makes people already at risk of heart disease more vulnerable.”
Jennings also notes that during the sunnier, warmer days of summer, people tend to spend more time engaged in outdoor leisure activities, a factor that can improve blood pressure and other measures of heart health.
The higher rates of heart attack in winter may also be related, therefore, to increased sedentary behavior.
“So, by all means keep warm and comfortable in winter but get out and do something too,” he says.
“Look after your risk factors and see your doctor regularly for a heart check,” he adds.
FMI: You’ll find an abstract of the JAMA Cardiology study on the journal’s website, but the full study is behind a paywall.