By Linda Carroll

(Reuters Health) – Heart attacks occur more often when temperatures plummet, a large new study suggests.

Based on more than a decade and a half of medical and weather data, researchers linked an increased incidence of heart attacks to lower air temperatures, lower atmospheric pressure, higher wind velocity and shorter durations of sunshine, according to the report in JAMA Cardiology.

What’s unique about the new study, said its senior author, is that “all heart attacks occurring in a whole country have been followed for 16 years with weather data for the day the heart attack occurred.”

Dr. David Erlinge, head of cardiology at Lund University and Skane University Hospital in Sweden, told Reuters Health, “We had data on more than 280,000 heart attacks and 3 million weather data points.”

Erlinge and his colleagues pored over records from the SWEDEHEART registry, which enrolls all consecutive patients in Sweden with symptoms suggestive of a heart attack who are admitted to a coronary intensive care unit or a coronary catheterization lab. The registry contains a wealth of health information on patients, including age, body mass, smoking status, echocardiogram findings, interventions, discharge medications and diagnoses.

For meteorological data, the researchers turned to the SMHI, a Swedish government agency that registers data from 132 weather stations across the nation.

Erlinge and colleagues analyzed the weather and heart attack data from 1998 through 2013 for 274,029 patients, half of whom were aged 71 or older.

While lower air temperature, lower atmospheric air pressure, higher wind velocity and shorter sunshine duration all were associated with statistically meaningful increased risk of heart attack, the most pronounced effect was from temperature.

The researchers found a higher incidence of heart attack on days with air temperatures below freezing. The rates of heart attack declined when temperatures rose to more than 3 to 4 degrees Celsius, or 37.4 to 39.2 degrees Fahrenheit.

Overall, each temperature increase of 7.4 degrees Celsius (about 13 degrees F) was tied to a 2.8 percent decrease in heart attack risk, the study authors calculated.

The new findings are “an association we talk about frequently and it’s been suggested in studies before this as well,” said Dr. Nisha Jhalani of the Center for Interventional Vascular Therapy at NewYork-Presbyterian/Columbia University Irving Medical Center in New York City.

“One thing that’s interesting about this study is that they didn’t just look at temperatures. They looked at a number of other factors, such as sunshine hours and wind velocity. It’s also a nationwide study with a lot of patients,” said Jhalani, who wasn’t involved in the research.

So, why would cold temperatures raise the risk of heart attack?

“Colder temperatures increase vasoconstriction in the arteries which causes them to clamp down,” she explained. “In someone with 70 to 80 percent blocked arteries – which might not be causing any symptoms normally – the arteries can be clamped down enough that the blood supply doesn’t match demand.”

Cold can also increase clotting, Jhalani added.

There are other factors related to winter that can increase the risk of heart attacks, such as shoveling snow, which raises blood pressure to levels that could disturb vulnerable plaques, Jhalani said. And caffeine has a similar clamping down effect on arteries, albeit a lot smaller.

“So the worst thing you can do is go out in subzero temperatures, shovel snow, and then come in and drink coffee to warm up,” Jhalani said. “That can be the perfect storm.”

The best strategy to minimize the increased risk brought on by cold weather is to “dress appropriately,” Erlinge said in an email. “If you are at high risk (of a heart attack), you may want to avoid going out in really cold, windy weather. Or maybe move to a warmer climate.”

SOURCE: JAMA Cardiology, online October 24, 2018.