Big outdoor temperature swings tied to heart attack risk

Many people know that extreme cold can raise your chances of having a heart attack, but a new study suggests that wild swings in temperature may do the same.

The greater the temperature change during the course of a single day, the more people show up at the hospital in need of emergency surgery for a heart attack, the researchers discovered.

The risk of heart attack seems to increase by about 5 percent for every 9 degrees Fahrenheit of temperature swing on a given day, the study findings showed.

This risk appears to crop up mainly during warmer weather, with the most marked effect occurring on days with an average temperature of 86 degrees, said senior researcher and IHPI member, Dr. Hitinder Gurm, associate chief clinical officer at the University of Michigan.

"On cold days, there's not much of a difference," Gurm said. "One of the reasons for this might be that when you have really freezing temperatures, most people stay indoors and they're not really exposed to the outdoor temperature."

However, the study did not prove that big temperature changes can cause heart attacks, just that there's an association.

Gurm and his colleagues suspected that temperature changes could be linked to heart attacks, particularly given a large body of evidence linking cold weather to the highest risk.

Cold weather causes the blood vessels to constrict, restricting blood flow and making the heart pump harder to maintain oxygen delivery throughout the body, according to the British Heart Foundation. Your heart rate and blood pressure can increase as a result.

To further investigate this effect, the research team turned to a database that tracks all patients in Michigan who undergo an emergency procedure to open blocked arteries during a heart attack.

The investigators found just over 30,400 heart attacks treated at 45 hospitals between 2010 and 2016. They then referred to weather logs to find out the daily temperatures in each hospital's general area on the day of the event.

Analysis revealed that swings of more than 45 degrees Fahrenheit were associated with a greater increase in heart attack rates compared with swings of 18 to 45 degrees.

A swing of more than 30 degrees Fahrenheit in a single day appears to cause a more than 10 percent increase in the proportion of heart attacks that take place, compared to days when the temperature remains relatively stable, the researchers reported.

Such big temperature swings are rare, Gurm said.

Days with a more than 30-degree swing in temperature occurred only about 5 percent of the time during the study, the researchers said. About half the time, temperatures changed between 10 and 20 degrees during the course of a day.

There were more days in which the temperature swung from warm to cold than the other way around, Gurm said. Given that, one potential explanation for the apparent increased risk could be the effects of a sudden chill on the heart and blood vessels.

Global warming may affect this heart attack risk, but Gurm couldn't say whether it might help or hurt.

Overall, warmer days and nights could lead to a reduced difference between maximum and minimum temperature, which would decrease the risk of heart attack.

On the other hand, global warming might cause an increase in extreme weather events featuring wildly fluctuating temperatures, which might cause the risk to rise.

Dr. Martha Gulati, division chief of cardiology for the University of Arizona-Phoenix, called the study "very thought-provoking."

"We've seen lots of drama in our temperature fluctuations, both extreme cold and extreme heat, and we see that more and more," Gulati said.

Researchers should consider looking farther back into the historical record, to see if they can uncover similar effects on heart attack cases based on temperature changes in other places, Gulati suggested.

The study is scheduled for presentation on March 10 at the American College of Cardiology's annual meeting, in Orlando, Fla. Research presented at meetings should be considered preliminary until published in a peer-reviewed journal.